Geografic-Information-System/lib/gis/gislib.dox

/*! \page gislib GRASS GIS Library
<!-- doxygenized from "GRASS 5 Programmer's Manual" 
     by M. Neteler 2/2004, 2005, 2006
  -->

by GRASS Development Team (http://grass.osgeo.org)

\tableofcontents

\par Subpages

- \subpage gislib_cmdline_parsing

This chapter is divided as follows:

- \subpage gislibintro
 - \subpage init
 - \subpage diag
 - \subpage envir

- \subpage dbaseaccess
 - \subpage Fully_Qualified_File_Names
 - \subpage finding
 - \subpage Legal_File_Names
 - \subpage Opening_an_Existing_Database_File_for_Reading
 - \subpage Opening_an_Existing_Database_File_for_Update
 - \subpage Creating_and_Opening_a_New_Database_File
 - \subpage Database_File_Management

- \subpage Region
 - \subpage Database_Region
 - \subpage Active_Module_Region

- \subpage Projection_Information
 - \subpage Latitude_Longitude_Databases
 - \subpage Coordinates
 - \subpage Global_Wraparound
 - \subpage Miscellaneous

- \subpage Calculations
 - \subpage Raster_Area_Calculations
 - \subpage Polygonal_Area_Calculations
 - \subpage Distance_Calculations

- \subpage General_Plotting_Routines

- \subpage Temporary_Files

- \subpage String_Manipulation_Functions

- \subpage Enhanced_UNIX_Routines
 - \subpage Running_in_the_Background
 - \subpage Partially_Interruptible_System_Call
 - \subpage ENDIAN_test
 - \subpage Miscellaneous

- \subpage GIS_Library_Data_Structures
 - \subpage struct_Cell_head
 - \subpage struct_Categories
 - \subpage struct_Colors
 - \subpage struct_History
 - \subpage struct_Range

- \subpage Loading_the_GIS_Library

- \subpage TimeStamp_functions

- \subpage Memory_Allocation

\section gislibintro Introduction

The <em>GIS Library</em> is the primary programming library provided
with the GRASS system. <b>Programs must use this libary to access the
GIS database.</b> It contains the routines which locate, create, open,
rename, and remove GRASS database files. It contains the routines
which read and write raster files. It contains routines which
interface the user to the database, including prompting the user,
listing available files, validating user access, etc. It also has
some general purpose routines (string manipulation, user information,
etc.) which are not tied directly to database processing.


It is assumed that the reader has read \ref location for a general
description of GRASS databases, \ref Raster_File_Processing for
details about raster map layers in GRASS, and \ref Region_and_Mask
(<b>???</b>) which discusses regions and masks. The routines in the
<em>GIS Library</em> are presented in functional groupings, rather
than in alphabetical order. The order of presentation will, it is
hoped, provide a better understanding of how the library is to be
used, as well as show the interrelationships among the various
routines. Note that a good way to understand how to use these routines
is to look at the source code for GRASS modules which use them. Most
routines in this library require that the header file <b>grass/gis.h</b> be
included in any code using these routines. Therefore, programmers
should always include this file when writing code using routines from
this library:

\code
#include <grass/gis.h>
\endcode

<b>Note:</b> All routines and global variables in this library,
documented or undocumented, start with the prefix <b>G_</b>. To avoid
name conflicts, programmers should not create variables or routines in
their own modules which use this prefix.

\subsection init Library Initialization

It is <B>mandatory</B> that the system be initialized before any other 
library routines are called.

G_gisinit() initialize GIS library.

This routine reads the user's GRASS environment file into memory and
makes sure that the user has selected a valid database and mapset. It
also initializes hidden variables used by other routines. If the
user's database information is invalid, an error message is printed
and the module exits.  The <b>program_name</b> is stored for later
recall by G_program_name(). It is recommended that argv[0] be
used for the program_name:

\code
int main (int argc, char **argv)
{
  G_gisinit(argv[0]);
}
\endcode

\subsection diag Diagnostic Messages

The following routines are used by other routines in the library to
report warning and error messages. They may also be used directly by
GRASS programs.

 - G_fatal_error() print error message and exit

 - G_debug() print debug message

 - G_warning() print warning message and continue

These routines report errors to the user. The normal mode is to write
the <em>message</em> to the screen (on the standard error
output). G_warning() will return and G_fatal_error() will exit.

If the standard error output is not a tty device, then the message is
mailed to the user instead.

If the file GIS_ERROR_LOG exists (with write permission), in either
the user's home directory or in the <tt>$GISBASE</tt> directory, the
messages will also be logged to this file.

While most applications will find the normal error reporting quite
adequate, there will be times when different handling is needed. For
example, graphics modules may want the messages displayed graphically
instead of on the standard error output. If the programmer wants to
handle the error messages differently, the following routines can be
used to modify the error handling:

 - G_set_error_routine() change error handling

This routine provides a different error handler for G_fatal_error()
and G_warning(). The <em>handler</em> routine must be defined as
follows:

\code
int handler(char *message, int fatal)
\endcode

where <i>message</i> is the message to be handled and <i>fatal</i>
indicates the type of error:

 - 1 (fatal error)
 - or 0 (warning).

<b>Note:</b> The handler only provides a way to send the message
somewhere other than to the error output. If the error is fatal, the
module will exit after the handler returns.

 - G_unset_error_routine() reset normal error handling

This routine resets the error handling for G_fatal_error() and
G_warning() back to the default action.

 - G_sleep_on_error() sleep on error

 - G_suppress_warnings() suppress warnings

\subsection envir Environment and Database Information

The following routines return information about the current database
selected by the user. Some of this information is retrieved from the
user's GRASS environment file. Some of it comes from files in the
database itself.  See \ref Environment_Variables for a discussion of
the GRASS environment.

The following four routines can be used freely by the programmer:

 - G_location() current location name

Returns the name of the current database location. This routine should
be used by modules that need to display the current location to the
user. See \ref Locations for an explanation of locations.

 - G_mapset() current mapset name

Returns the name of the current mapset in the current location. This
routine is often used when accessing files in the current mapset. See
\ref Mapsets for an explanation of mapsets.

 - G_myname() location title

Returns a one line title for the database location. This title is read
from the file MYNAME in the PERMANENT mapset. See also \ref
Permanent_Mapset for a discussion of the PERMANENT mapset.

 - G_gisbase() top level module directory 

Returns the full path name of the top level directory for GRASS
programs.  This directory will have subdirectories which will contain
modules and files required for the running of the system. Some of
these directories are:

\verbatim
bin  commands run by the user
etc  modules and data files used by GRASS commands
html help files
\endverbatim

The use of G_gisbase() to find these subdirectories enables GRASS modules 
to be written independently of where the GRASS system is actually installed 
on the machine. For example, to run the module <i>sroff</i> in the GRASS 
<tt>etc</tt> directory: 

\code
char command[200];
sprintf(command, "%s/etc/sroff", G_gisbase());
G_spawn(command, "sroff", NULL);
\endcode

The following two routines return full path UNIX directory names. They
should be used only in special cases. They are used by other routines
in the library to build full UNIX file names for database
files. <B>The programmer should not use the next two routines to
bypass the normal database access routines.</B>

 - G_gisdbase() top level database directory

Returns the full UNIX path name of the directory which holds the
database locations. See \ref GISDBASE for a full explanation of this
directory.

 - G_location_path() current location directory

Returns the full UNIX path name of the current database location. For
example, if the user is working in location <I>spearfish</I> in the
<tt>/home/user/grassdata</tt> database directory, this routine will
return a string which looks like
<tt>/home/user/grassdata/spearfish</tt>.

These next routines provide the low-level management of the
information in the user's GRASS environment file. <B>They should not
be used in place of the higher level interface routines described
above.</B>

 - G_getenv()

 - G__getenv()

These routines look up the variable <em>name</em> in the GRASS
environment and return its value (which is a character string). If
<em>name</em> is not set, G_getenv() issues an error message and calls
exit(). G__setenv() just returns the NULL pointer.

 - G_setenv()

 - G__setenv()
  
These routines set the the GRASS environment variable <em>name</em> to
<em>value</em>. If <em>value</em> is NULL, the <em>name</em> is unset.

Both routines set the value in module memory, but only G_setenv()
writes the new value to the user's GRASS environment file.


\section dbaseaccess Fundamental Database Access Routines

The routines described in this section provide the low-level interface
to the GRASS database. They search the database for files, prompt the
user for file names, open files for reading or writing, etc. The
programmer should never bypass this level of database interface. These
routines must be used to access the GRASS database unless there <b>are
other higher level library routines which perform the same
function</b>. For example, routines to process raster files (see \ref
Raster_File_Processing), vector files (see \ref
Vector_File_Processing), etc., should be used instead.

In the descriptions below, the term database <i>element</i> is used.
Elements are subdirectories within a mapset and are associated with a
specific GRASS data type. For example, raster files live in the "cell"
and "fcell" element. See \ref Elements for more details.


\subsection Fully_Qualified_File_Names Fully Qualified File Names

All GRASS routines which access database files must be given both the
file name and the mapset where the file resides. Often the name and
the mapset are 2 distinct character strings. However, there is a need
for a single character string which contains both the name and the
mapset (e.g., for interactive interfacing to command-line
programs). This form of the name is known as the <em>fully qualified
file name</em> and is built by the following routine:

 - G_fully_qualified_name()
  
Returns a fully qualified name for the file <i>name</i> in
<i>mapset</i> Currently this string is in the form <i>name@mapset</i>,
but the programmer should pretend not to know this and always call
this routine to get the fully qualified name.

The following example shows how an interactive version of <tt>d.rast</tt>
interfaces with the command-line version of <tt>d.rast</tt>:

\code
#include <stdlib.h>
#include <grass/gis.h>

int main(int argc, char *argv[])
{
    char name[GNAME_MAX], *mapset, *fqn;
    char command[1024];

    G_gisinit(argv[0]);
    mapset = G_ask_cell_old("", name);

    if (mapset == NULL)
        exit(EXIT_SUCCESS);

    fqn = G_fully_qualified_name(name, mapset);
    sprintf(command, "d.rast map='%s'", fqn);
    G_spawn(command, "d.rast", NULL);
    exit(EXIT_SUCCESS);
}
\endcode

\subsection  finding Finding Files in the Database

Command line driven module requires a database file name as one of the
command arguments. In this case, the programmer must search the
database to find the mapset where the file resides.

The following routines search the database for files:

 - G_find_file()
  
Look for the file <i>name</i> under the specified <i>element</i> in
the database. The <i>mapset</i> parameter can either be the empty
string "", which means search all the mapsets in the user's current
mapset search path, or it can be a specific mapset, which means. Look
for the file only in this one mapset (for example, in the current
mapset).

If found, the mapset where the file lives is returned. If not found,
the NULL pointer is returned.

If the user specifies a fully qualified file name, (i.e, a name that
also contains the mapset; see \ref Fully_Qualified_File_Names) then
G_find_file() modifies <em>name</em> by eliminating the mapset from
the <em>name</em>.

For example, to find a "paint/labels" file anywhere in the database:

\code
char name[GNAME_MAX];
char *mapset;

if ((mapset = G_find_file("paint/labels", name, "")) == NULL)

/* not found */
\endcode

To check that the file exists in the current mapset:

\code
char name[GNAME_MAX];

if (G_find_file("paint/labels", name, G_mapset()) == NULL)

/* not found */
\endcode

\subsection Legal_File_Names Legal File Names

Not all names that a user may enter will be legal files for the GRASS
databases. The routines which create new files require that the new
file have a legal name. If the name is obtained from the command line,
the programmer must check that the name is legal. The following
routine checks for legal file names:

 - G_legal_filename()
  
\subsection Opening_an_Existing_Database_File_for_Reading Opening an Existing Database File for Reading

The following routines open the file <i>name</i> in <i>mapset</i> from
the specified database <i>element</i> for <b>reading</b> (but not for
writing). The file <i>name</i> and <i>mapset</i> can be obtained using
G_find_file().

The database file <i>name</i> under the <i>element</i> in the
specified <i>mapset</i> is opened for reading (but not for writing).

 - G_open_old()

The UNIX open() routine is used to open the file. If the file does not
exist, -1 is returned. Otherwise the file descriptor from the open()
is returned.

 - G_fopen_old()

The UNIX fopen() routine, with "r" read mode, is used to open the
file. If the file does not exist, the NULL pointer is
returned. Otherwise the file descriptor from the fopen() is returned.

\subsection Opening_an_Existing_Database_File_for_Update Opening an Existing Database File for Update

The following routines open the file <i>name</i> in the current mapset
from the specified database <i>element</i> for <b>writing</b>. The
file must exist. Its <i>name</i> can be obtained using G_find_file().

The database file <i>name</i> under the <i>element</i> in the current
mapset is opened for reading and writing.

 - G_open_update()

The UNIX open() routine is used to open the file. If the file does not
exist, -1 is returned. Otherwise the file is positioned at the end of
the file and the file descriptor from the open() is returned.

- G_fopen_append()

The UNIX fopen() routine, with "a" append mode, is used to open the
file. If the file does not exist, the NULL pointer is
returned. Otherwise the file is positioned at the end of the file and
the file descriptor from the fopen() is returned.


\subsection Creating_and_Opening_a_New_Database_File Creating and Opening a New Database File

The following routines create the new file <i>name</i> in the current
mapset (GRASS does not allow files to be created outside the current
mapset; see \ref Database_Access_Rules) under the specified database
<i>element</i> and open it for writing. The database <i>element</i> is
created, if it does not already exist.

The file <i>name</i> is obtained noninteractively (e.g., from the
command line), G_legal_filename() should be called first to make sure
that <i>name</i> is a valid GRASS file name. <b>Warning:</b> It is not
an error for <i>name</i> to already exist. However, the file will be
removed and recreated empty. G_find_file() could be used to see if
<i>name</i> exists.

The database file <i>name</i> under the <i>element</i> in the current
mapset is created and opened for writing (but not reading).

 - G_open_new()
  
The UNIX open() routine is used to open the file. If the file does not
exist, -1 is returned. Otherwise the file is positioned at the end of
the file and the file descriptor from the open() is returned.

 - G_fopen_new()
  
The UNIX fopen() routine, with "w" write mode, is used to open the
file.  If the file does not exist, the NULL pointer is
returned. Otherwise the file is positioned at the end of the file and
the file descriptor from the fopen() is returned.


\subsection Database_File_Management Database File Management

The following routines allow the renaming and removal of database
files in the current mapset (These functions only apply to the current
mapset since GRASS does permit users to modify things in mapsets other
than the current mapset; see \ref Database_Access_Rules).

 - G_rename() rename a database file
  
 - G_remove() remove a database file

<b>Note:</b> These functions only apply to the specific <i>element</i>
and not to other "related" elements. For example, if <i>element</i> is
"cell", then the specified raster file will be removed (or renamed),
but the other support files, such as "cellhd" or "cats", will not. To
remove these other files as well, specific calls must be made for each
related <i>element</i>.

\section Region Region

The region concept is explained in \ref Region. It can be thought of as a
two-dimensional matrix with known boundaries and rectangular cells.

There are logically two different regions. The first is the database
region that the user has set in the current mapset. The other is the
region that is active in the module. This active module region is what
controls reading and writing of raster file data. The vector map
export does not take care for the active region settings.

The routines described below use a GRASS data structure
<tt>Cell_head</tt> to hold region information. This structure is
defined in the "gis.h" header file. It is discussed in detail under
\ref GIS_Library_Data_Structures.


\subsection Database_Region Database Region

Reading and writing the user's database region are done by the
following routines:

Note: Previous versions of GRASS called this the "window". Due to
overuse of this term (database window, graphics window, etc.), the
term was changed to "region". However, to maintain compatibility with
existing programs, library routine names were not changed - hence the
term "window" is used in the routine name (where "region" should
probably be used instead).

 - G_get_window()

Reads the database region as stored in the WIND file in the user's
current mapset into region.

An error message is printed and exit() is called if there is a problem
reading the region.

<b>Note:</b> GRASS applications that read or write raster files should
not use this routine since its use implies that the active module
region will not be used. Programs that read or write raster file data
(or vector data) can query the active module region using
G_window_rows() and G_window_cols().

 - G_put_window()

Writes the database region file (WIND) in the user's current mapset
from region.

<b>Warning:</b> Since this routine actually changes the database
region, it should only be called by modules which the user knows will
change the region. It is probably fair to say that only the
<tt>g.region</tt> should call this routine.

There is another database region. This region is the default region
for the location. The default region provides the user with a
"starting" region, i.e., a region to begin with and return to as a
reference point. The GRASS modules <tt>g.region</tt> allow the user to
set their database region from the default region (see \ref
Permanent_Mapset for a discussion of the default region).  The
following routine reads this region:

 - G_get_default_window()

\subsection Active_Module_Region Active Module Region

The <em>active module region</em> is the one that is used when reading
and writing raster file data. This region determines the resampling
when reading raster data. It also determines the extent and resolution
of new raster files.

Initially the active module region and the user's database region are
the same, but the programmer can make them different. The following
routines manage the active module region.

 - G_window_rows() number of rows in active region

 - G_window_cols() number of columns in active region 

These routines return the number of rows and columns (respectively) in
the active module region. Before raster files can be read or written,
it is necessary to known how many rows and columns are in the active
region. For example:

\code
int nrows, cols;
int row, col;

nrows = G_window_rows();
ncols = G_window_cols();
for (row = 0; row < nrows; row++) {
    /* read row ... */
    for (col = 0; col < ncols; col++) {
        /* process col ... */
    }
}
\endcode

 - G_set_window() set the active region 

This routine sets the active region from given region. Setting the
active region does not change the WIND file in the database. It simply
changes the region for the duration of the module.

However, the new region setting is not retained across the UNIX exec()
call. This implies that G_set_window() cannot be used to set the
region for a module to be executed using the system() or popen()
routines.

<b>Note:</b> This routine overrides the region as set by the user. Its
use should be very limited since it changes what the user normally
expects to happen. If this routine is not called, then the active
region will be the same as what is in the user's WIND file.

<b>Warning:</b> Calling this routine with already opened raster files
has some side effects. If there are raster files which are open for
reading, they will be read into the newly set region, not the region
that was active when they were opened. However, CELL buffers allocated
for reading the raster files are not automatically reallocated. The
module must reallocate them explicitly. Also, this routine does not
change the region for raster files which are open for writing. The
region that was active when the open occurred still applies to these
files.

 - G_get_set_window() get the active region

Gets the values of the currently active region into region. If
G_set_window() has been called, then the values set by that call are
retrieved. Otherwise the user's database region is retrieved.

<b>Note:</b> For modules that read or write raster data, and really
need the full region information, this routine is preferred over
G_get_window(). However, since G_window_rows() and G_window_cols()
return the number of rows and columns in the active region, the
programmer should consider whether or not the full region information
is really needed before using this routine.

 - G_align_window() align two regions

Modifies the input region to align to the reference region. The
resolutions in region are set to match those in refefence region and
the region edges (north, south, east, west) are modified to align with
the grid of the reference region.

The <i>region</i> may be enlarged if necessary to achieve the
alignment. The north is rounded northward, the south southward, the
east eastward and the west westward.

 - G_col_to_easting()

Converts a column relative to a region to an easting.

<b>Note:</b> col is a double: col+0.5 will return the easting for the
center of the column; col+0.0 will return the easting for the western
edge of the column; and col+1.0 will return the easting for the
eastern edge of the column.

 - G_row_to_northing()

Converts a row relative to a region to a northing.

<b>Note:</b> row is a double: row+0.5 will return the northing for the
center of the row; row+0.0 will return the northing for the northern
edge of the row; and row+1.0 will return the northing for the southern
edge of the row.

 - G_easting_to_col()

Converts an easting relative to a region to a column.

<b>Note:</b> The result is a double. Casting it to an integer will
give the column number.

 - G_northing_to_row()

Converts a northing relative to a region to a row.

<b>Note:</b> the result is a double. Casting it to an integer will
give the row number.

\section Projection_Information Projection Information

The following routines return information about the cartographic
projection and zone. See \ref Region for more information about these
values.

 - G_projection()

This routine returns a code indicating the projection for the active
region. The current values are:

 - PROJECTION_XY    - unreferenced x,y (imagery data)
 - PROJECTION_UTM   - UTM
 - PROJECTION_SP    - State Plane
 - PROJECTION_LL    - Latitude-Longitude
 - PROJECTION_OTHER - Other (more than 121 projections are supported)

 - G_database_projection_name()

Returns a pointer to a string which is a printable name for projection
code (as returned by G_projection()).

 - G_database_unit_name()

Returns a string describing the database grid units.

 - G_database_units_to_meters_factor()

Returns a factor which converts the grid unit to meters (by
multiplication).  If the database is not metric (eg. imagery) then 0.0
is returned.

 - G_zone()

This routine returns the zone for the active region. The meaning for
the zone depends on the projection. For example zone 18 for projection
type 1 would be UTM zone 18.


\subsection Latitude_Longitude_Databases Latitude-Longitude GIS Databases

GRASS supports databases in a longitude-latitude grid using a
projection where the x coordinate is the longitude and the y
coordinate is the latitude. This projection is called the Equidistant
Cylindrical Projection (also known as Plate Carree). ECP has the
property that <em>where am I</em> and <em>row-column</em> calculations
are identical to those in planimetric grids (like UTM, Universal
Transverse Mercator Projection). This implies that normal GRASS
registration and overlay functions will work without any special
considerations or modifications to existing code. However, the
projection is not planimetric. This means that distance and area
calculations are no longed Euclidean.

Also, since the world is round, maps may not have edges in the
east-west direction, especially for global databases. Maps may have
the same longitude at both the east and west edges of the
display. This feature, called global wraparound, must be accounted for
by GRASS modules (particularly vector based functions, like plotting).
What follows is a description of the GIS Library routines that are
available to support latitude-longitude databases.

\subsection Coordinates Coordinates

Latitudes and longitudes are specified in degrees. Northern latitudes
range from 0 to 90 degrees, and southern latitudes from 0 to
-90. Longitudes have no limits since longitudes ±360 degrees are
equivalent.

Coordinates are represented in ASCII using the format
<tt>dd:mm:ssN</tt> or <tt>dd:mm:ssS</tt> for latitudes,
<tt>ddd:mm:ssE</tt> or <tt>ddd.mm.ssW</tt> for longitudes, and
<tt>dd.mm.ss</tt> for grid resolution. For example, 80:30:24N
represents a northern latitude of 80 degrees, 30 minutes, and 24
seconds. 120:15W represents a longitude 120 degrees and 15 minutes
west of the prime meridian. 30:15 represents a resolution of 30
degrees and 15 minutes. These next routines convert between ASCII
representations and the machine representation for a coordinate. They
work both with latitude-longitude projections and planimetric
projections.

<b>Note:</b> In each subroutine, the programmer must specify the
projection number. If the projection number is PROJECTION_LL (defined
in "gis.h"), then latitude-longitude ASCII format is invoked.
Otherwise, a standard floating-point to ASCII conversion is made.

 - G_format_easting() easting to ASCII

 - G_format_northing() northing to ASCII

Converts the double representation of the given coordinate to its
ASCII representation.

 - G_format_resolution()

Converts the double representation of the resolution to its
ASCII representation.

 - G_scan_easting() ASCII easting to double

 - G_scan_northing() ASCII northing to double

Converts the ASCII coordinate string in to its double representation.

 - G_scan_resolution() ASCII resolution to double

Converts the ASCII "resolution" string to its double representation
(into resolution).

The following are examples of how these routines are used.

\code
double north;
char buf[50];

G_scan_northing(buf, north, G_projection());   /* ASCII to double */

G_format_northing(north, buf, G_projection()); /* double to ASCII */

G_format_northing(north, buf, -1);             /* double to ASCII */

/* This last example forces floating-point ASCII format */
\endcode

\subsection Global_Wraparound Global Wraparound


These next routines provide a mechanism for determining the relative
position of a pair of longitudes. Since longitudes of ±360 are
equivalent, but GRASS requires the east to be bigger than the west,
some adjustment of coordinates is necessary.

 - G_adjust_easting()
 
Returns east larger than west. If the region projection is
PROJECTION_LL, then this routine returns an equivalent <i>east</i>
that is larger, but no more than 360 degrees larger, than the
coordinate for the western edge of the region. Otherwise no adjustment
is made and the original <i>east</i> is returned.

 - G_adjust_east_longitude()

This routine returns an equivalent <i>east</i> that is larger, but no
more than 360 larger than the <i>west</i> coordinate.

This routine should be used only with latitude-longitude coordinates.

 - G_shortest_way()

Returns shortest way between eastings. 

\subsection Miscellaneous Miscellaneous


 - G_ellipsoid_name()

This routine returns a pointer to a string containing the name for the
ellipsoid in the GRASS ellipsoid table. It can be used as follows:

\code
int n;
char *name;

for(n = 0; name = G_ellipsoid_name(n); n++)
    fprintf(stdout, "%s\n", name);
\endcode

 - G_get_ellipsoid_by_name()

This routine returns the semi-major axis (in meters) and
eccentricity squared for the named ellipsoid.

 - G_get_ellipsoid_parameters()

This routine returns the semi-major axis (in meters) and the
eccentricity squared for the ellipsoid associated with the
database. If there is no ellipsoid explicitly associated with the
database, it returns the values for the WGS 84 ellipsoid.

- G_meridional_radius_of_curvature()

Returns the meridional radius of curvature at a given longitude:

\f$
\rho = \frac{a (1-e^2)}{(1-e^2\sin^2 lon)^{3/2}}
\f$

 - G_transverse_radius_of_curvature()

Returns the transverse radius of curvature at a given longitude:

\f$
\nu = \frac{a}{(1-e^2\sin^2 lon)^{1/2}}
\f$

 - G_radius_of_conformal_tangent_sphere()

Returns the radius of the conformal sphere tangent to ellipsoid at a
given longitude:

\f$
r = \frac{a (1-e^2)^{1/2}}{(1-e^2\sin^2 lon)}
\f$

 - G_pole_in_polygon() 

For latitude-longitude coordinates, this routine determines if the
polygon contains one of the poles.


\section Calculations Calculations

\subsection Raster_Area_Calculations Raster Area Calculations

The following routines perform area calculations for raster maps.
They are based on the fact that while the latitude-longitude grid is
not planimetric, the size of the grid cell at a given latitude is
constant. The first routines work in any projection.

 - G_begin_cell_area_calculations()

This routine must be called once before any call to
G_area_of_cell_at_row(). It can be used in either planimetric
projections or the latitude-longitude projection.

 - G_area_of_cell_at_row()

This routine returns the area in square meters of a cell in the
specified row. This value is constant for planimetric grids and varies
with the row if the projection is latitude-longitude.

 - G_begin_zone_area_on_ellipsoid()

Initializes raster area calculations for an ellipsoid.

 - G_area_for_zone_on_ellipsoid()

Returns the area between latitudes scaled by the factor passed to
G_begin_zone_area_on_ellipsoid().

 - G_begin_zone_area_on_sphere()

Initializes raster area calculations for a sphere. 

 - G_area_for_zone_on_sphere()

Returns the area between latitudes.

\subsection Polygonal_Area_Calculations Polygonal Area Calculations

These next routines provide area calculations for polygons. Some of
the routines are specifically for latitude-longitude, while others
will function for all projections.

However, there is an issue for latitude-longitude that does not occur
with planimetric grids. Vector/polygon data is described as a series
of x,y coordinates. The lines connecting the points are not stored but
are inferred.  This is a simple, straight-forward process for
planimetric grids, but it is not simple for latitude-longitude. What
is the shape of the line that connects two points on the surface of a
globe?

One choice (among many) is the shortest path from <tt>x1,y1</tt> to
<tt>x2,y2</tt>, known as the geodesic. Another is a straight line on
the grid. The area routines described below assume the
latter. Routines to work with the former have not yet been developed.

 - G_begin_polygon_area_calculations()

This initializes the polygon area calculation routines. It is used
both for planimetric and latitude-longitude projections.

 - G_area_of_polygon()


Returns the area in square meters of the polygon. It is used both for
planimetric and latitude-longitude projections.

<B>Note.</B> If the database is planimetric with the non-meter grid,
this routine performs the required unit conversion to produce square
meters. 

 - G_planimetric_polygon_area()

Return the area in map units of the polygon,

 - G_begin_ellipsoid_polygon_area()

This initializes the polygon area calculations for the ellipsoid.

 - G_ellipsoid_polygon_area()

Returns the area in square meters of the polygon for latitude-longitude
grids.

<b>Note:</b> This routine assumes grid lines on the connecting the
vertices (as opposed to geodesics).


\subsection  Distance_Calculations Distance Calculations

Two routines perform distance calculations for any projection.

 - G_begin_distance_calculations()

Initializes the distance calculations. It is used both for the
planimetric and latitude-longitude projections.

 - G_distance()

This routine computes the distance between two points in meters. 

 - G_begin_geodesic_distance()

Initializes the distance calculations for the ellipsoid. It is used
only for the latitude-longitude projection.

 - G_geodesic_distance() geodesic distance

Calculates the geodesic distance between two points in meters.

The calculation of the geodesic distance is fairly costly. These next
three routines provide a mechanism for calculating distance with two
fixed latitudes and varying longitude separation.

 - G_set_geodesic_distance_lat1() set the first latitude.

 - G_set_geodesic_distance_lat2() set the second latitude.

 - G_geodesic_distance_lon_to_lon()

Calculates the geodesic distance between two points set by
G_set_geodesic_distance_latl() and G_set_geodesic_distance_lat2().


\section General_Plotting_Routines General Plotting Routines


The following routines form the foundation of a general purpose line
and polygon plotting capability.

 - G_bresenham_line()

Draws a line from one point to another using Bresenham's
algorithm. A routine to plot points must be provided, as is defined
as: <tt>point(x, y)</tt> plot a point at x,y.

This routine does not require a previous call to G_setup_plot() to 
function correctly, and is independent of all following routines.

 - G_setup_plot()

Initializes the plotting capability. This routine must be called once
before calling the G_plot_*() routines described below.

 - G_plot_line()

Plots line between latlon coordinates. This routine handles global
wrap-around for latitude-longitude databases. See G_setup_plot() for
the required coordinate initialization procedure.

 - G_plot_polygon()

Plots filled polygon with n vertices. See G_setup_plot() for the
required coordinate initialization procedure.

 - G_plot_area()

Plots multiple polygons. Like G_plot_polygon(), except it takes a set
of polygons, each with n vertices, where the number of polygons is
specified with the <i>rings</i> argument. It is especially useful for
plotting vector areas with interior islands.

 - G_plot_where_en()

The pixel coordinates <i>x,y</i> are converted to map coordinates
<i>east,north</i>. See G_setup_plot() for the required coordinate
initialization procedure.

 - G_plot_where_xy()

The map coordinates <i>east,north</i> are converted to pixel
coordinates <i>x,y</i>. See G_setup_plot() for the required
coordinate initialization procedure.

 - G_plot_fx()

\section Temporary_Files Temporary Files


Often it is necessary for modules to use temporary files to store
information that is only useful during the module run. After the
module finishes, the information in the temporary file is no longer
needed and the file is removed. Commonly it is required that
temporary file names be unique from invocation to invocation of the
module. It would not be good for a fixed name like "/tmp/mytempfile"
to be used. If the module were run by two users at the same time, they
would use the same temporary file. In addition systematic use of the
/tmp directory could leave the system vulnerable to symlink attacks.
The following routine generates temporary file names which are unique
within the module and across all GRASS programs.

 - G_tempfile()

This routine returns a pointer to a string containing a unique file
name that can be used as a temporary file within the
module. Successive calls to G_tempfile() will generate new names.

Only the file name is generated. The file itself is not created. To
create the file, the module must use standard UNIX functions which
create and open files, e.g., creat() or fopen().

The programmer should take reasonable care to remove (unlink) the file
before the module exits. However, GRASS database management will
eventually remove all temporary files created by G_tempfile() that
have been left behind by the modules which created them.

<b>Note:</b> The temporary files are created in the GRASS database
rather than under <tt>/tmp</tt>. This is done for two reasons. The
first is to increase the likelihood that enough disk is available for
large temporary files since /tmp may be a very small file system. The
second is so that abandoned temporary files can be automatically
removed (but see the warning below).

<b>Warning:</b> The temporary files are named, in part, using the
process id of the module. GRASS database management will remove these
files only if the module which created them is no longer
running. However, this feature has a subtle trap. Programs which
create child processes (using the UNIX fork(), see also G_fork()
routine) should let the child call G_tempfile(). If the parent does it
and then exits, the child may find that GRASS has removed the
temporary file since the process which created it is no longer
running.


\section String_Manipulation_Functions String Manipulation Functions


This section describes some routines which perform string manipulation. 
Strings have the usual C meaning: a NULL terminated array of characters.

These next 3 routines remove unwanted white space from a single string.

 - G_squeeze()

Leading and trailing white space is removed from the string and
internal white space which is more than one character is reduced to a
single space character. White space here means spaces, tabs,
linefeeds, newlines, and formfeeds.

 - G_strip()

Leading and trailing white space is removed from the string.  White
space here means only spaces and tabs. There is no return value.

 - G_chop()

Chop leading and trailing white spaces: space, &#92;f, &#92;n, &#92;r,
  &#92;t, &#92;v.

The next routines replaces character(s) from string.

 - G_strchg()

Replace all occurencies of character in string with new.

This next routine copies a string to allocated memory.

 - G_store()

This routine allocates enough memory to hold the string, and returns a
pointer to the allocated memory.

The next 2 routines convert between upper and lower case.

 - G_tolcase()

Upper case letters in the string are converted to their lower case
equivalent.

 - G_toucase()

Lower case letters in the string are converted to their upper case
equivalent.

 - G_trim_decimal()

This routine remove trailing zeros from decimal number for example:
23.45000 would come back as 23.45.

 - G_index()

 - G_rindex()
 
Get position of delimiter.

 - G_strcasecmp()

 - G_strncasecmp()

String compare ignoring case (upper or lower).

 - G_strcasestr()

Return a pointer to the first occurrence of subString in mainString,
or NULL if no occurrences are found.

 - G_strdup()

Returns a pointer to a string that is a duplicate of the string given
to G_strdup.  The duplicate is created using malloc. If unable to
allocate the required space, NULL is returned.

\section Enhanced_UNIX_Routines Enhanced UNIX Routines


A number of useful UNIX library routines have side effects which are
sometimes undesirable. The routines here provide the same functions as
their corresponding UNIX routine, but with different side effects.

\subsection Running_in_the_Background Running in the Background


The standard UNIX <em>fork()</em> routine creates a child process which is a
copy of the parent process. The fork() routine is useful for placing a
module into the background. For example, a module that gathers input
from the user interactively, but knows that the processing will take a
long time, might want to run in the background after gathering all the
input. It would fork() to create a child process, the parent would
exit() allowing the child to continue in the background, and the user
could then do other processing.

However, there is a subtle problem with this logic. The fork() routine does not
protect child processes from keyboard interrupts even if the parent is no longer
running. Keyboard interrupts will also kill background processes that do not
protect themselves.

<i>Note:</i> Programmers who use Bash know that programs run in the
background (using & on the command line) are not automatically
protected from keyboard interrupts. To protect a command that is run
in the background, Bash users must do nohup command &. Programmers
who use the C Shell (or other variants) do not know, or forget that
the C-shell automatically protects background processes from keyboard
interrupts.

Thus a module which puts itself in the background may never finish if
the user interrupts another module which is running at the keyboard.

\subsection Partially_Interruptible_System_Call Partially Interruptible System Call

The UNIX <em>system()</em> call allows one program, the parent, to
execute another UNIX command or module as a child process, wait for
that process to complete, and then continue. The problem addressed
here concerns interrupts. During the standard system() call, the child
process inherits its responses to interrupts from the parent. This
means that if the parent is ignoring interrupts, the child will ignore
them as well. If the parent is terminated by an interrupt, the child
will be also.

Note that code which uses system() or popen() tends to be error-prone,
particularly on Windows. We strongly discourage anyone from using
either system() or popen() in the GRASS code. If you don't need to
communicate with the child process via a pipe, you can use G_spawn()
or G_spawn_ex(). These provide similar functionality to system() but
without the shell getting in the way (using the shell creates problems
if filenames or other arguments contain spaces or other characters
which are significant to the shell). If you need to communicate via a
pipe, you can either use G_spawn_ex() and manage the pipe yourself, or
use popen(). See also G_popen_read() and G_popen_write() which do most of
the work for you. Alternatively, you can use a temporary file rather
than a pipe for communicating with the child process.

\subsection ENDIAN_test ENDIAN test


To test if the user's machine is little or big ENDIAN, the following
function is provided:

 - G_is_little_endian()

Test if machine is little or big endian.

\subsection Miscellaneous Miscellaneous

A number of general purpose routines have been provided.

 - G_date()

Returns a pointer to a string which is the current date and time. The
format is the same as that produced by the UNIX <tt>date</tt> command.

 - G_home()

Returns a pointer to a string which is the full path name of the
user's home directory.

 - G_percent()

This routine prints a percentage complete message to stderr. Example:

\code
#include<grass/gis.h>
#include<grass/glocale.h>

int row;
int nrows;

nrows = 1352; /* 1352 is not a special value - example only */

G_message(_("Percent complete:"));

for (row = 0; row < nrows; row++)
  G_percent(row, nrows, 10);
\endcode

This will print completion messages at 10% increments; i.e., 10%, 20%,
30%, etc., up to 100%. Each message does not appear on a new line, but
rather erases the previous message. After 100%, a new line is printed.

 - G_program_name()

Routine returns the name of the module as set by the call to
G_gisinit().

 - G_whoami()

Returns a pointer to a string which is the user's login name.

\section GIS_Library_Data_Structures GIS Library Data Structures


Some of the data structures, defined in the "gis.h" header file and used 
by routines in this library, are described in the sections below.

\subsection struct_Cell_head struct Cell_head


The raster header data structure is used for two purposes. It is used
for raster header information for raster map. It also used to hold
region values.

See documentation of Cell_head structure for details.

The <tt>format</tt> and <tt>compressed</tt> fields apply only to
raster headers. The <tt>format</tt> field describes the number of
bytes per raster data value and the <tt>compressed</tt> field
indicates if the raster file is compressed or not. The other fields
apply both to raster headers and regions. The geographic boundaries
are described by <tt>north, south, east</tt> and <tt>west</tt>. The
grid resolution is described by <tt>ew_res</tt> and
<tt>ns_res</tt>. The cartographic projection is described by
<tt>proj</tt> and the related zone for the projection by
<tt>zone</tt>. The <tt>rows</tt> and <tt>cols</tt> indicate the number
of rows and columns in the raster map, or in the region. See \ref
Raster_Header_File for more information about raster headers, and \ref
Region for more information about regions.

The routines described in \ref Raster_Header_File use this structure.

\subsection struct_Categories struct Categories

The <i>Categories</i> structure contains a title for the map layer,
the largest category in the map layer, an automatic label generation
rule for missing labels, and a list of category labels.

\code
struct Categories
{
    CELL ncats;			/* total number of categories              */
    CELL num;			/* the highest cell values. Only exists    
				   for backwards compatibility = (CELL)
				   max_fp_values in quant rules            */
    char *title;		/* name of data layer                      */
    char *fmt;			/* printf-like format to generate labels   */
    float m1;			/* multiplication coefficient 1            */
    float a1;			/* addition coefficient 1                  */
    float m2;			/* multiplication coefficient 2            */
    float a2;			/* addition coefficient 2                  */
    struct Quant q;		/* rules mapping cell values to index in
				   list of labels                          */
    char **labels;		/* array of labels of size num             */
    int *marks;			/* was the value with this label was used? */
    int nalloc;
    int last_marked_rule;
};
\endcode

This structure should be accessed using the routines described in
\ref Raster_Category_File.

\subsection struct_Colors struct Colors

The color data structure holds red, green, and blue color intensities
for raster categories. The structure has become so complicated that it
will not be described in this manual.

\code
struct Colors
{
    int version;		/* set by read_colors: -1=old, 1=new */
    DCELL shift;
    int invert;
    int is_float;		/* defined on floating point raster data? */
    int null_set;		/* the colors for null are set? */
    unsigned char null_red;
    unsigned char null_grn;
    unsigned char null_blu;
    int undef_set;		/* the colors for cells not in range are set? */
    unsigned char undef_red;
    unsigned char undef_grn;
    unsigned char undef_blu;
    struct _Color_Info_ fixed;
    struct _Color_Info_ modular;
    DCELL cmin;
    DCELL cmax;
    int organizing;
};
\endcode

The routines described in \ref Raster_Color_Table must be used 
to store and retrieve color information using this structure.

\subsection struct_History struct History

The History structure is used to document raster files. The
information contained here is for the user. It is not used in any
operational way by GRASS. 

See documentation of History structure for details.

The <i>mapid</i> and <i>mapset</i> are the raster file name and
mapset, <i>title</i> is the raster file title, <i>creator</i> is the
user who created the file, <i>maptype</i> is the map type (which
should always be "raster"), <i>datasrc_1</i> and <i>datasrc_2</i>
describe the original data source, <i>keywrd</i> is a one-line data
description and <i>edhist</i> contains <i>edlinecnt</i> lines of user
comments.

The routines described in \ref Raster_History_File use this structure.
However, there is very little support for manipulating the contents of
this structure. The programmer must manipulate the contents directly.

<b>Note:</b> Some of the information in this structure is not
meaningful. For example, if the raster file is renamed, or copied
into another mapset, the <i>mapid</i> and <i>mapset</i> will no longer
be correct. Also the <i>title</i> does not reflect the true raster
file title. The true title is maintained in the category file.

<b>Warning:</b> This structure has remained unchanged since the inception of
GRASS. There is a good possibility that it will be changed or eliminated in
future releases.

\subsection struct_Range struct Range

The <i>Range</i> (<i>FPRange</i> for floating point raster maps)
structure contains the minimum and maximum values which occur in a
raster file.

\code
struct Range
{
    CELL min;
    CELL max;
    int first_time;		/* whether or not range was updated */
};

struct FPRange
{
    DCELL min;
    DCELL max;
    int first_time;		/* whether or not range was updated */
};
\endcode

The routines described in \ref Raster_Range_File should be used to access
this structure.

\section Loading_the_GIS_Library Loading the GIS Library

The library is loaded by specifying $(GISLIB) in the Makefile. The
following example is a complete Makefile which compiles code that uses
this library:

<b>Makefile for $(GISLIB)</b>

\verbatim
MODULE_TOPDIR = ../..

PGM = r.info

LIBES = $(GISLIB)
DEPENDENCIES = $(GISDEP)

include $(MODULE_TOPDIR)/include/Make/Module.make

default: cmd
\endverbatim

See \ref Compiling_and_Installing_GRASS_Modules for a complete
discussion of Makefiles.

\section TimeStamp_functions Timestamp functions

This structure is defined in "gis.h", but there should be no reason to access its
elements directly:

\code
struct TimeStamp {
 DateTime dt[2];   /* two datetimes */
 int count;
};
\endcode

Using the G_*_timestamp() routines reads/writes a timestamp file in 
the cell_misc/rastername mapset element.

A TimeStamp can be one DateTime, or two DateTimes representing a 
range. When preparing to write a TimeStamp, the programmer should
use one of:

 - G_set_timestamp() to set a single DateTime

 - G_set_timestamp_range() to set two DateTimes

 - G_read_raster_timestamp to read raster timestamp

 - G_read_vector_timestamp to read vector timestamp

 - G_get_timestamps() to copy TimeStamp into Datetimes

Use to copy the TimeStamp information into Datetimes, so the members
of struct TimeStamp shouldn't be accessed directly.

 - count=0  means no datetimes were copied
 - count=1  means 1 datetime was copied into dt1
 - count=2  means 2 datetimes were copied

 - G_init_timestamp()

Sets ts-&gt;count = 0, to indicate no valid DateTimes are in TimeStamp.

 - G_set_timestamp()

Copies a single DateTime to a TimeStamp in preparation for writing
(overwrites any existing information in TimeStamp).

 - G_set_timestamp_range()

Copies two DateTimes (a range) to a TimeStamp in preparation for
writing (overwrites any existing information in TimeStamp).

 - G_write_raster_timestamp()

 - G_write_vector_timestamp()

 - G_write_grid3_timestamp()

 - G_format_timestamp()

 - G_scan_timestamp()

 - G_remove_raster_timestamp()

 - G_remove_vector_timestamp()

 - G_remove_grid3_timestamp()

Only timestamp files in current mapset can be removed.

 - G_read_grid3_timestamp()

See \ref DateTime_Library for a complete discussion of GRASS datetime
routines.

\section Memory_Allocation Memory Allocation

The following routines provide memory allocation capability. They are
simply calls to the UNIX suite of memory allocation routines malloc(),
realloc() and calloc(), except that if there is not enough memory,
they print a diagnostic message to that effect and then call exit().

 - G_malloc ()

Allocates a block of memory at least <i>size</i> bytes which is
aligned properly for all data types. A pointer to the aligned block is
returned.

 - G_realloc()

Changes the <i>size</i> of a previously allocated block of memory at
<i>ptr</i> and returns a pointer to the new block of memory. The
<i>size</i> may be larger or smaller than the original size. If the
original block cannot be extended "in place", then a new block is
allocated and the original block copied to the new block.

<b>Note:</b> If <i>ptr</i> is NULL, then this routine simply allocates
a block of <i>size</i> bytes. This routine works around broken
realloc() routines, which do not handle a NULL <i>ptr</i>.

 - G_calloc()

Allocates a properly aligned block of memory <i>n</i>*<i>size</i>
bytes in length, initializes the allocated memory to zero, and returns
a pointer to the allocated block of memory.

 - G_free()

Use the G_free() routine to release memory allocated by
these routines.

*/