update GARI formula

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@58389 15284696-431f-4ddb-bdfa-cd5b030d7da7

imagery/i.vi/gari.c

@@ -7,7 +7,7 @@ double ga_ri(double redchan,double nirchan,double bluechan,double greenchan)
 {
     double result;
     {
-        result = (nirchan - (greenchan - (bluechan - redchan)))/(nirchan - (greenchan + (bluechan - redchan)));
+        result = (nirchan - (greenchan - (bluechan - redchan)))/(nirchan + (greenchan - (bluechan - redchan)));
     }
     return result;
 }

imagery/i.vi/i.vi.html

@@ -122,10 +122,16 @@ evi2( redchan, nirchan )
 
 <p>
 GARI: green atmospherically resistant vegetation index
+<p>
+The formula was actually defined:
+Gitelson, Anatoly A.; Kaufman, Yoram J.; Merzlyak, Mark N. (1996)
+Use of a green channel in remote sensing of global vegetation from EOS-
+MODIS, Remote Sensing of Environment 58 (3), 289-298.
+<a href="http://dx.doi.org/10.1016/s0034-4257(96)00072-7">doi:10.1016/s0034-4257(96)00072-7</a>
 
 <div class="code"><pre>
 GARI = ( nirchan - (greenchan - (bluechan - redchan))) / 
-( nirchan - (greenchan + (bluechan - redchan)))
+( nirchan + (greenchan - (bluechan - redchan)))
 gari( redchan, nirchan, bluechan, greenchan )
 </pre></div>