i.vi: corrected MSAVI2 equation according to https://doi.org/10.1016/0034-4257(94)90134-1, see https://trac.osgeo.org/grass/ticket/3385

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72549 15284696-431f-4ddb-bdfa-cd5b030d7da7

imagery/i.vi/i.vi.html

@@ -179,7 +179,7 @@ IPVI = nirchan/(nirchan+redchan)
 <div class="code"><pre>
 msavi2( redchan, nirchan )
 
-MSAVI2 = (1/2)*(2(NIR+1)-sqrt((2*NIR+1)^2-8(NIR-red)))
+MSAVI2 = (1/2)*(2*NIR+1-sqrt((2*NIR+1)^2-8*(NIR-red)))
 </pre></div>
 
 <p>

imagery/i.vi/msavi2.c

@@ -15,7 +15,7 @@ double msa_vi2(double redchan, double nirchan)
     }
     else 
     {
-	result=0.5*(2*(nirchan+1)-sqrt((2*nirchan+1)*(2*nirchan+1)-8*(nirchan-redchan)));
+	result=0.5*(2*nirchan+1-sqrt((2*nirchan+1)*(2*nirchan+1)-8*(nirchan-redchan)));
     }
     return result;
 }