[GRASS-SVN] r33493 - grass/branches/develbranch_6/raster

[svn_grass at osgeo.org]

Author: epatton
Date: 2008-09-19 12:49:03 -0400 (Fri, 19 Sep 2008)
New Revision: 33493

Modified:
   grass/branches/develbranch_6/raster/rasterintro.html
Log:
Added resampling module details from Glynn (merge from trunk, r33492)

Modified: grass/branches/develbranch_6/raster/rasterintro.html
===================================================================
--- grass/branches/develbranch_6/raster/rasterintro.html	2008-09-19 16:34:41 UTC (rev 33492)
+++ grass/branches/develbranch_6/raster/rasterintro.html	2008-09-19 16:49:03 UTC (rev 33493)
@@ -51,8 +51,8 @@
 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. Always the full
-map is imported.
+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
@@ -91,6 +91,7 @@
 
 <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
@@ -98,14 +99,87 @@
 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.
+<p>
+
+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.
+<p>
+
+<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.
+<p>
+
+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 miniscule difference 
+in the position of the boundary results in the addition or subtraction of
+a sample weighted by a miniscule 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> and <a href="r.surf.idw2.html">r.surf.idw2</a>)</li>
+<li> Interpolating from contour lines (<a href="r.contour.html">r.contour</a>)</li>
+</ul>
+
+For Lidar and similar data, <a href="r.in.xyz.html">r.in.xyz</a> supports 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.
+<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>
+<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
@@ -137,41 +211,6 @@
 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>Raster map resampling and interpolation methods</h3>
-
-GRASS offers various raster resampling and interpolation methods. There are
-available modules for reinterpolation of "filled" raster maps (continuous data)
-to a different resolution:
-
-<ul>
-<li> Resampling with nearest neighbor, bilinear, and bicubic method
- (<a href="r.resamp.interp.html">r.resamp.interp</a>)</li>
-<li> Regularized spline with tension (RST) interpolation 2D
- (<a href="r.resamp.rst.html">r.resamp.rst</a>)</li>
-<li> Regularized spline with tension (RST) interpolation 2D for hole filling (e.g., SRTM DEM)
- (<a href="r.fillnulls.html">r.fillnulls</a>)</li>
-</ul>
-
-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> and <a href="r.surf.idw2.html">r.surf.idw2</a>)</li>
-<li> Interpolating from contour lines (<a href="r.contour.html">r.contour</a>)</li>
-</ul>
-
-For Lidar and similar data, <a href="r.in.xyz.html">r.in.xyz</a> supports 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.
-<P>
-
-Otherwise, for interpolation of scattered data, use the <EM>v.surf.*</EM> set of
-modules.
-
-
 <h3>Hydrologic modeling toolbox</h3>
 
 Watershed modeling related modules are