hint for handling irregular time intervals for regression

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@37384 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.series/r.series.html

@@ -49,6 +49,10 @@ ignored by most aggregates, or will cause the result to be NULL if -n is given).
 The <em>low,high</em> thresholds are floating point, so use <em>-inf</em> or
 <em>inf</em> for a single threshold (e.g., <em>range=0,inf</em> to ignore
 negative values, or <em>range=-inf,-200.4</em> to ignore values above -200.4).
+<p>
+Linear regression (slope, offset, coefficient of determination) assumes equal time intervals.
+If the data have irregular time intervals, NULL raster maps can be inserted into time series
+to make time intervals equal (see example).
 
 <h2>EXAMPLES</h2>
 
@@ -59,9 +63,15 @@ r.series input="`g.mlist pattern='insitu_data.*' sep=,`"
     output=insitu_data.stddev method=stddev
 </pre></div>
 <p>
-
-Note the <em>g.mlist</em> script also supports regular expressions for 
+Note the <em>g.mlist</em> script also supports regular expressions for
 selecting map names.
+<p>
+Using <em>r.series</em> with NULL raster maps:
+<br>
+<div class="code"><pre>
+r.mapcalc "dummy=null()"
+r.series in=map2001,map2002,dummy,dummy,map2005,map2006,dummy,map2008 out=res_slope,res_offset,res_coeff meth=slope,offset,detcoeff
+</pre></div>
 
 <p>
 Example for multiple aggregates to be computed in one run (3 resulting aggregates from two input maps):