[GRASS-SVN] r34448 - grass/trunk/lib/gis

[svn_grass at osgeo.org]

Author: neteler
Date: 2008-11-23 02:45:32 -0500 (Sun, 23 Nov 2008)
New Revision: 34448

Modified:
   grass/trunk/lib/gis/gisrasterlib.dox
Log:
null file explanations from Glynn added

Modified: grass/trunk/lib/gis/gisrasterlib.dox
===================================================================
--- grass/trunk/lib/gis/gisrasterlib.dox	2008-11-23 07:45:12 UTC (rev 34447)
+++ grass/trunk/lib/gis/gisrasterlib.dox	2008-11-23 07:45:32 UTC (rev 34448)
@@ -1643,6 +1643,8 @@
 
 \section Null_no_data NULL (no data) handling
 
+The <TT>null</TT> file is stored in <TT>cell_misc/name/null file</TT>.
+
 -2^31 (= 0x80000000 = -2147483648) is the null value
 for the CELL type, so you'll never see that value in a map.
 
@@ -1661,6 +1663,18 @@
 generate NaNs doesn't have to explicitly convert them to the GRASS
 null value.
 
+<b>Presence or absence of <TT>null</TT> file:</b>
+For an integer map, any cells which were null will become zero, but
+any zeroes (cells which were previously either null or zero) will be
+treated as nulls (this is for compatibility with GRASS 4.x, which
+didn't have a <TT>null</TT> file, but typically used zero to indicate a null
+value).
+
+For a floating-point map, any cells which were null will become zero
+(when writing FP data, a null has a zero written to the fcell/<map>
+file, and the corresponding bit is set in the <TT>null</TT>
+file).
+
 \section Color_Functions Color Functions (new and upgraded)
 
 \subsection Upgraded_Colors_structures Upgraded Colors structures