/*! \page vlibFormat Vector format description

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

\tableofcontents

\section vlibDirectoryStructure Directory structure

Vector map is stored in a number of data files. Vector map directory
structure and file names were changed in GRASS 6 with respect to
previous GRASS versions. All vector files for one vector map are
stored in one directory:

\verbatim
$MAPSET/vector/vector_name/
\endverbatim

This directory contains these files:

- <b>coor</b> - binary file, coordinates [former dig/ file] (see \ref vlibCoorFileFormat)
- <b>topo</b> - binary file, topology [former dig_plus/ file] (see \ref vlibTopoFileFormat)
- <b>sidx</b> - binary file, spatial index (see \ref vlibSidxFileFormat)
- <b>cidx</b> - binary file, category index (see \ref vlibCidxFileFormat)
- <b>head</b> - text file, header information [former part of dig/ file] (see \ref vlibHeadFileFormat)
- <b>dbln</b> - text file, link(s) to attribute table(s) (see \ref vlibDblnFileFormat)
- <b>hist</b> - text file, vector map change history
- <b>frmt</b> - text file, format description (external formats only)
- <b>fidx</b> - binary file, feature index (OGR format only)

\section vlibHeadFileFormat Header file format specification

The header contains meta information, a description of the
vector map and many other information. The file is an unordered list
of key/value entries. The <i>key</i> is a string separated from
<i>value</i> by a colon and optional whitespace.

Keywords are:

- ORGANIZATION - organization that digitized the data
- DIGIT DATE - date the data was digitized
- DIGIT NAME - person who digitized the data
- MAP NAME - title of the original source map
- MAP DATE - date of the original source map
- MAP SCALE - scale of the original source map
- OTHER INFO - other comments about the map
- ZONE - zone of the map (e.g., UTM zone)
- MAP THRESH - digitizing threshold

This information holds \ref dig_head data structure.


\section vlibDblnFileFormat DB link file format specification

Each vector maps has its own DBMI settings stored in the
'$MAPSET/vector/vector_name/dbln' text file. For each pair <em>vector map +
layer</em>, all of <em>table, key column, database, driver</em> must be
defined in a new row. This definition must be written to
'$MAPSET/vector/vector_name/dbln' text file. Each row in the 'dbln'
file contains names separated by spaces in following order ([ ] -
optional):

\verbatim
map[@mapset] layer table [key [database [driver]]]
\endverbatim

If key, database or driver are omitted (on second and higher row only)
the last definition is used. When reading a vector map from another
mapset (if mapset is specified along with map name), definitions in
the related "dbln" file may overwrite the DBMI definition in the
current mapset. This means that the map-wise definition is always
"stronger".

Wild cards <b>*</b> and <b>?</b> may be used in map and mapset names.

Variables $GISDBASE, $LOCATION_NAME, $MAPSET, and $MAP may be used in
table, key, database and driver names (function
Vect_subst_var()). Note that $MAPSET is not the current mapset but
mapset of the map the rule is defined for.

Note that vector features in GRASS vector maps may have attributes in
different tables or may be without attributes. Boundaries form areas
but it may happen that some boundaries are not closed (such boundaries
would not appear in polygon layer).  Boundaries may have
attributes. All types may be mixed in one vector map.

The link to the table is permanent and it is stored in 'dbln' file in
vector directory. Tables are considered to be a part of the vector and
the command <tt>g.remove</tt>, for example, deletes linked tables of
the vector. Attributes must be joined with geometry.

Information about database links holds \ref dblinks data structure.

<b>Examples:</b>

Examples are written mostly for the DBF driver, where database is full
path to the directory with dbf files and table name is the name of dbf
file without .dbf extension:

\verbatim
* 1 mytable id $GISDBASE/$LOCATION_NAME/$MAPSET/vector/$MAP dbf
\endverbatim

This definition says that entities with category of layer 1 are linked
to dbf tables with names "mytable.dbf" saved in vector directories of
each map. The attribute column containing the category numbers is
called "id".

\verbatim
* 1 $MAP id $GISDBASE/$LOCATION_NAME/$MAPSET/dbf dbf
\endverbatim 

Similar as above but all dbf files are in one directory dbf/ in mapset
and names of dbf files are $MAP.dbf

\verbatim
water* 1 rivers id /home/grass/dbf dbf
water* 2 lakes lakeid /home/guser/mydb
trans* 1 roads key basedb odbc
trans* 5 rails
\endverbatim

These definitions define more layers (called "field" in the API) for
one vector map i.e. in one vector map may be more features linked to
more attribute tables. Definitions on first 2 rows are applied for
example on maps water1, water2, ... so that more maps may share one
table.

\verbatim
water@PERMANENT 1 myrivers id /home/guser/mydbf dbf
\endverbatim

This definion overwrites the definition saved in PERMANENT/VAR and
links the water map from PERMANENT mapset to the user's table.

Modules should be written so that connections to databases for each
vector layer are independent. It should be possible to read attributes
of an input vector map from one database and write to some other and
even with some other driver (should not be a problem).

There are open questions, however. For one, how does one distinguish when
new tables should be written and when not? For example, definitions:

\verbatim
river 1 river id water odbc
river.backup* 1 NONE
\endverbatim

could be used to say that tables should not be copied for backups of
map river because table is stored in a reliable RDBMS.


\section vlibCoorFileFormat Coor file format specification

In the coor file the following is stored: 'line' (element) type,
number of attributes and layer number for each category. Coordinates
in binary file are stored as double (8 bytes). See \ref Coor_info data
structure.

\subsection vlibCoorFileHead Header

<table border="1" style="border-collapse: collapse" cellpadding="5">
<tr><td><b>Name</b></td><td><b>Type</b></td><td><b>Number</b></td><td><b>Description</b></td></tr>
<tr><td>Version_Major</td> <td>C</td> <td>1</td> <td>file version (major)</td></tr>
<tr><td>Version_Minor</td> <td>C</td> <td>1</td> <td>file version (minor)</td></tr>
<tr><td>Back_Major</td>    <td>C</td> <td>1</td> <td>supported from GRASS version (major)</td></tr>
<tr><td>Back_Minor</td>    <td>C</td> <td>1</td> <td>supported from GRASS version (minor)</td></tr>
<tr><td>byte_order</td>    <td>C</td> <td>1</td> <td>little or big endian flag</td></tr>
<tr><td>head_size</td>     <td>L</td> <td>1</td> <td>header size of coor file</td></tr>
<tr><td>with_z</td>        <td>C</td> <td>1</td> <td>2D or 3D flag; zero for 2D</td></tr>
<tr><td>size</td>          <td>L</td> <td>1</td> <td>coor file size</td></tr>
</table>

\subsection vlibCoorFileBody Body

The body consists of line records:

<table border="1" style="border-collapse: collapse" cellpadding="5">
<tr><td><b>Name</b></td><td><b>Type</b></td><td><b>Number</b></td><td><b>Description</b></td></tr>
<tr><td>record header</td><td>C</td><td>1</td><td>
 - 0. bit: 1 - alive, 0 - dead line
 - 1. bit: 1 - categories, 0 - no categories
 - 2.-3. bit: type - one of: GV_POINT, GV_LINE, GV_BOUNDARY, GV_CENTROID, GV_FACE, GV_KERNEL
 - 4.-7. bit: reserved, not used 
</td></tr>

<tr><td>ncats</td><td>I</td><td>1</td><td>number of categories 
    (written only if categories exist) </td></tr>

<tr><td>field</td><td>I</td><td>ncats</td><td>field identifier,
    distinguishes between more categories append to one feature (written
    only if categories exist; field is called "layer" at user
    level)</td></tr>

<tr><td>cat</td><td>I</td><td>ncats</td><td>category value (written
    only if categories exist)</td></tr>

<tr><td>ncoor</td><td>I</td><td>1</td><td>written for GV_LINES and GV_BOUNDARIES 
    only</td></tr>

<tr><td>x</td><td>D</td><td>ncoor</td><td>x coordinate</td></tr>

<tr><td>y</td><td>D</td><td>ncoor</td><td>y coordinate</td></tr>

<tr><td>z</td><td>D</td><td>ncoor</td><td>z coordinate; present if
    with_z in head is set to 1</td></tr> </table>

Types used in coor file:

<table border="1" style="border-collapse: collapse" cellpadding="5">
<tr><td><b>Type</b></td><td><b>Name</b></td><td><b>Size in Bytes</b></td></tr>
<tr><td>D</td><td>Double</td><td>8</td></tr>
<tr><td>L</td><td>Long  </td><td>4</td></tr>
<tr><td>I</td><td>Int   </td><td>4</td></tr>
<tr><td>S</td><td>Short </td><td>4</td></tr>
<tr><td>C</td><td>Char  </td><td>1</td></tr>
</table>



*/