Geografic-Information-System/raster/rasterintro.html

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<h3>Raster maps in general</h3>

A "raster map" is a data layer consisting of a gridded array of cells.
It has a certain number of rows and columns, with a data point (or null
value indicator) in each cell. These may exist as a 2D grid or as a 3D
cube made up of many smaller cubes, i.e. a stack of 2D grids.
<p>
The geographic boundaries of the raster map are described by the north,
south, east, and west fields. These values describe the lines which bound
the map at its edges. These lines do NOT pass through the center of the
grid cells at the edge of the map, but along the edge of the map itself.
i.e. the geographic extent of the map is described by the outer bounds of
all cells within the map.

<p>
As a general rule in GRASS GIS:
<ol>
<li> Raster output maps have their bounds and resolution equal to those
of the current computational region.
<li> Raster input maps are automatically cropped/padded and rescaled
(using nearest-neighbour resampling) to match the current region.
<li> Raster input maps are automatically masked if a raster map named
     MASK exists. The MASK is only applied when <i>reading</i> maps
     from the disk.
</ol>

There are a few exceptions to this:

<tt>r.in.*</tt> programs read the data cell-for-cell, with no resampling. When
reading non-georeferenced data, the imported map will usually have its
lower-left corner at (0,0) in the location's coordinate system; the user
needs to use <a href="r.region.html">r.region</a> to "place" the imported map.
<p>
Some programs which need to perform specific types of resampling (e.g.
<a href="r.resamp.rst.html">r.resamp.rst</a>) read the input maps at
their original resolution then do the resampling themselves.
<p>
<a href="r.proj.html">r.proj</a> has to deal with two regions (source
and destination) simultaneously; both will have an impact upon the
final result.

<h3>Raster import and export</h3>

The module <a href="r.in.gdal.html">r.in.gdal</a> offers a common
interface for many different raster formats. Additionally, it also
offers options such as on-the-fly location creation or extension of
the default region to match the extent of the imported raster map.
For special cases, other import modules are available. The full map
is always imported.
<p>
For importing scanned maps, the user will need to create a
x,y-location, scan the map in the desired resolution and save it into
an appropriate raster format (e.g. tiff, jpeg, png, pbm) and then use
<a href="r.in.gdal.html">r.in.gdal</a> to import it. Based on
reference points the scanned map can be recified to obtain geocoded
data.
<p>
Raster maps are exported with <a href="r.out.gdal.html">r.out.gdal</a>
into common formats. Also <a href="r.out.bin.html">r.out.bin</a>,
<a href="r.out.vtk.html">r.out.vtk</a>, <a href="r.out.ascii.html">r.out.ascii</a>
and other export modules are available. They export the data according
to the current region settings. If those differ from the original map,
the map is resampled on the fly (nearest neighbor algorithm). In other
words, the output will have as many rows and columns as the current region.
To export maps with various grid spacings (e.g, 500x500 or 200x500), you
can just change the region resolution with <a href="g.region.html">g.region</a>
and then export the map. The resampling is done with nearest neighbor
algorithm in this case. If you want some other form of resampling, 
first change the region, then explicitly resample the map with e.g.
<a href="r.resamp.interp.html">r.resamp.interp</a> or
<a href="r.resamp.stats.html">r.resamp.stats</a>, then export the 
resampled map.
<p>
GRASS GIS raster map exchange between different locations (same projection)
can be done in a lossless way using the <a href="r.pack.html">r.pack</a>
and <a href="r.unpack.html">r.unpack</a> modules.

<h3>Metadata</h3>

The <a href="r.info.html">r.info</a> module displays general information
about a map such as region extent, data range, data type, creation history,
and other metadata.
Metadata such as map title, units, vertical datum etc. can be updated
with <a href="r.support.html">r.support</a>. Timestamps are managed
with <a href="r.timestamp.html">r.timestamp</a>. Region extent and
resolution are mangaged with <a href="r.region.html">r.region</a>.


<h3>Raster map operations</h3>

<h4>Resampling methods and interpolation methods</h4> 
GRASS raster map processing is always performed in the current region
settings (see <a href="g.region.html">g.region</a>), i.e. the current
region extent and current raster resolution is used. If the resolution
differs from that of the input raster map(s), on-the-fly resampling is
performed (nearest neighbor resampling). If this is not desired, the
input map(s) has/have to be resampled beforehand with one of the dedicated
modules.
<p>

The built-in nearest-neighbour resampling of raster data calculates
the centre of each region cell, and takes the value of the raster cell
in which that point falls.
<p>

If the point falls exactly upon a grid line, the exact result will be
determined by the direction of any rounding error. One consequence of 
this is that downsampling by a factor which is an even integer will 
always sample exactly on the boundary between cells, meaning that the 
result is ill-defined.
<p>

The following modules are available for reinterpolation of "filled" 
raster maps (continuous data) to a different resolution:
<ul>
<li><a href="r.resample.html">r.resample</a> uses the built-in resampling, 
so it should produce identical results as the on-the-fly resampling done
via the raster import modules.</li>
<li><a href="r.resamp.interp.html">r.resamp.interp</a> Resampling with 
nearest neighbor, bilinear, and bicubic method: <b>method=nearest</b> uses the 
same algorithm as <a href="r.resample.html">r.resample</a>, but not the same 
code, so it may not produce identical results in cases which are decided 
by the rounding of floating-point numbers.
<br>
For <a href="r.resamp.interp.html">r.resamp.interp</a> <b>method=bilinear</b>
and <b>method=bicubic</b>, the raster values are treated as samples at each
raster cell's centre, defining a piecewise-continuous surface. The resulting 
raster values are obtained by sampling the surface at each region cell's centre.
As the algorithm only interpolates, and doesn't extrapolate, a margin of 0.5 
(for bilinear) or 1.5 (for bicubic) cells is lost from the extent of the original 
raster. Any samples taken within this margin will be null.</li>

<li><a href="r.resamp.rst.html">r.resamp.rst</a> Regularized Spline with Tension 
(RST) interpolation 2D: Behaves similarly, i.e. it computes a surface assuming 
that the values are samples at each raster cell's centre, and samples the surface
at each region cell's centre.</li>
<li><a href="r.resamp.bspline.html">r.resamp.bspline</a> Bicubic or bilinear 
spline interpolation with Tykhonov regularization.</li>
<li>For <a href="r.resamp.stats.html">r.resamp.stats</a> without <b>-w</b>, the value of
each region cell is the chosen aggregate of the values from all of the raster
cells whose centres fall within the bounds of the region cell.
<br>
With <b>-w</b>, the samples are weighted according to the proportion of the
raster cell which falls within the bounds of the region cell, so the
result is normally unaffected by rounding error (a minuscule difference 
in the position of the boundary results in the addition or subtraction of
a sample weighted by a minuscule factor; also, The min and max aggregates 
can't use weights, so <b>-w</b> has no effect for those).</li>

<li> <a href="r.fillnulls.html">r.fillnulls</a> for Regularized Spline with Tension (RST) 
interpolation 2D for hole filling (e.g., SRTM DEM)</li>
</ul>
<p>

Furthermore, there are modules available for reinterpolation of "sparse"
(scattered points or lines) maps:

<ul>
<li> Inverse distance weighted average (IDW) interpolation
 (<a href="r.surf.idw.html">r.surf.idw</a>)</li>
<li> Interpolating from contour lines (<a href="r.contour.html">r.contour</a>)</li>
<li> Various vector modules for interpolation</li>
</ul>

For Lidar and similar data, <a href="r.in.lidar.html">r.in.lidar</a> and <a href="r.in.xyz.html">r.in.xyz</a>
support loading and binning of ungridded x,y,z ASCII data into a new raster map.
The user may choose from a variety of statistical methods in creating the new raster map.
<p>

Otherwise, for interpolation of scattered data, use the <em>v.surf.*</em> set of
modules.

<h4>Raster MASKs</h4>
If a raster map named "MASK" exists, most GRASS raster modules will operate
only on data falling inside the masked area, and treat any data falling
outside of the mask as if its value were NULL. The mask is only applied
when <em>reading</em> an existing GRASS raster map, for example when used
in a module as an input map.
<p>
The mask is read as an integer map. If MASK is actually a
floating-point map, the values will be converted to integers using the
map's quantisation rules (this defaults to round-to-nearest, but can
be changed with r.quant).
<p>
(see <a href="r.mask.html">r.mask</a>)

<h3>Raster map statistics</h3>

A couple of commands are available to calculate local statistics
(<a href="r.neighbors.html">r.neighbors</a>), and global statistics
(<a href="r.statistics.html">r.statistics</a>, <a href="r.surf.area.html">r.surf.area</a>).
Profiles and transects can be generated
(<a href="d.profile.html">d.profile</a>, <a href="r.profile.html">r.profile</a>,
<a href="r.transect.html">r.transect</a>) as well as histograms
(<a href="d.histogram.html">d.histogram</a>) and polar diagrams
(<a href="d.polar.html">d.polar</a>).

Univariate statistics (<a href="r.univar.html">r.univar</a>) and
reports are also available (<a href="r.report.html">r.report</a>,<a
href="r.stats.html">r.stats</a>, <a href="r.volume.html">r.volume</a>).

<h3>Raster map algebra and aggregation</h3>

The <a href="r.mapcalc.html">r.mapcalc</a> command provides raster map
algebra methods.
The <a href="r.resamp.stats.html">r.resamp.stats</a> command resamples raster
map layers using various aggregation methods, the <a href="r.statistics.html">r.statistics</a>
command aggregates one map based on a second map. 
<a href="r.resamp.interp.html">r.resamp.interp</a> resamples raster map 
layers using interpolation.

<h3>Regression analysis</h3>

Both linear (<a href="r.regression.line.html">r.regression.line</a>) and
multiple regression (<a href="r.regression.multi.html">r.regression.multi</a>)
are supported.

<h3>Hydrologic modeling toolbox</h3>

Watershed modeling related modules are
<a href="r.basins.fill.html">r.basins.fill</a>,
<a href="r.water.outlet.html">r.water.outlet</a>,
<a href="r.watershed.html">r.watershed</a>, and
<a href="r.terraflow.html">r.terraflow</a>.

Water flow related modules are
<a href="r.carve.html">r.carve</a>,
<a href="r.drain.html">r.drain</a>,
<a href="r.fill.dir.html">r.fill.dir</a>,
<a href="r.fillnulls.html">r.fillnulls</a>,
<a href="r.flow.html">r.flow</a>, and
<a href="r.topidx.html">r.topidx</a>.

Flooding can be simulated with <a href="r.lake.html">r.lake</a>.

Hydrologic simulation model are available as
<a href="r.sim.sediment.html">r.sim.sediment</a>,
<a href="r.sim.water.html">r.sim.water</a>, and
<a href="r.topmodel.html">r.topmodel</a>.

<h3>Raster format</h3>

In GRASS GIS, raster data can be stored as 2D or 3D grids.

<h4>2D raster maps</h4>

2D rasters support three data types (for technical details, please refer
to the Wiki article
<a href="https://grasswiki.osgeo.org/wiki/GRASS_raster_semantics">GRASS raster semantics</a>):

<ul>
<li>32bit signed integer (CELL),</li>
<li>single-precision floating-point (FCELL), and</li>
<li>double-precision floating-point (DCELL).
</ul>

In most GRASS GIS resources, 2D raster maps are usually called "raster" maps.

<h4>3D raster maps</h4>

The 3D raster map type is usually called "3D raster" but other names like
"RASTER3D", "voxel", "volume", "GRID3D" or "3d cell" are yet common.

3D rasters support only single- and double-precision floating-point.
3D raster's single-precision data type is most often called "float",
and the double-precision one "double".

<h4>No-data management and data portability</h4>

GRASS GIS distinguishes NULL and zero. When working with NULL data, it
is important to know that operations on NULL cells lead to NULL cells.

<p>
The GRASS GIS raster format is architecture independent and portable between
32bit and 64bit machines.

<h3>Raster compression</h3>
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All GRASS GIS raster map types are by default ZLIB compressed, i.e. using
ZLIB's deflate algorithm. Through the environment variable
<tt>GRASS_COMPRESSOR</tt> the compression method can be set to RLE, ZLIB,
LZ4, or BZIP2.
<p>
Important: the NULL file compression must be explicitly turned on with 
<tt>export GRASS_COMPRESS_NULLS=1</tt> - such raster maps can then only 
be opened with GRASS GIS 7.2.0 or later. NULL file compression can be 
managed with <b>r.null -z</b>.
<p>
Integer (CELL type) raster maps can be compressed with RLE if
the environment variable <tt>GRASS_INT_ZLIB</tt> exists and is set to value 
0. However, this is not recommended.
<p>
Floating point (FCELL, DCELL) raster maps never use RLE compression;
they are either compressed with ZLIB, LZ4, BZIP2 or are uncompressed.

<dl>
<dt><strong>RLE</strong></dt>
<dd><b>DEPRECATED</b> Run-Length Encoding, poor compression ratio but 
fast. It is kept for backwards compatibility to read raster maps 
created with GRASS 6. It is only used for raster maps of type CELL.
FCELL and DCELL maps are never and have never been compressed with RLE.
</dd>
<dt><strong>ZLIB</strong></dt>
<dd>ZLIB's deflate is the default compression method for all raster 
maps. GRASS GIS 7 uses by default 1 as ZLIB compression level which is the 
best compromise between speed and compression ratio, also when 
compared to other available compression methods. Valid levels are in 
the range [1, 9] and can be set with the environment variable 
<tt>GRASS_ZLIB_LEVEL</tt>.</dd>
<dt><strong>LZ4</strong></dt>
<dd>LZ4 is a very fast compression method, about as fast as no 
compression. Decompression is also very fast. The compression ratio is 
generally higher than for RLE but worse than for ZLIB. LZ4 is 
recommended if disk space is not a limiting factor.</dd>
<dt><strong>BZIP2</strong></dt>
<dd>BZIP2 can provide compression ratios much higher than the other 
methods, but only for large raster maps (&gt; 10000 columns). For large 
raster maps, disk space consumption can be reduced by 30 - 50% when 
using BZIP2 instead of ZLIB's deflate. BZIP2 is the slowest compression 
and decompression method. However, if reading from / writing to a 
storage device is the limiting factor, BZIP2 compression can speed up 
raster map processing. Be aware that for smaller raster maps, BZIP2 
compression ratio can be worse than other compression methods.</dd> 
</dl>

<p>
In the internal cellhd file, the value for "compressed" is 1 for RLE, 2 
for ZLIB, 3 for LZ4, and 4 for BZIP2.
<p>
Obviously, decompression is controlled by the raster map's compression,
not the environment variable.

<h3>See also</h3>

<ul>
  <li><a href="raster3dintro.html">Introduction into 3D raster data (voxel) processing</a></li>
  <li><a href="vectorintro.html">Introduction into vector data processing</a></li>
  <li><a href="imageryintro.html">Introduction into image processing</a></li>
  <li><a href="temporalintro.html">Introduction into temporal data processing</a></li>
  <li><a href="databaseintro.html">Database management</a></li>
  <li><a href="projectionintro.html">Projections and spatial transformations</a></li>
</ul>