reference where to find equations used in the module added

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@35022 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.slope.aspect/r.slope.aspect.html

@@ -138,7 +138,7 @@ layer. These cells are assigned a "zero slope" value (category 0) in both
 the slope and aspect raster map layers.
 
 <p>
-Horn's formula is used to find the derivatives in x and y directions.
+Horn's formula is used to find the first order derivatives in x and y directions.
 
 <p>
 Only when using integer elevation models, the aspect is biased in 0,
@@ -159,6 +159,9 @@ aspect is computed. The aspect for all cells with slope &lt;
 
 Horn, B. K. P. (1981). Hill Shading and the Reflectance Map, Proceedings
 of the IEEE, 69(1):14-47.
+Hofierka, J., Mitasova, H., Neteler, M., 2008. Geomorphometry in GRASS GIS. 
+In: Hengl, T. and Reuter, H.I. (Eds), Geomorphometry: Concepts, Software, Applications. 
+Developments in Soil Science, vol. 33, Elsevier, 387-410 pp.
 
 <h2>SEE ALSO</h2>
 <em><a href="r.mapcalc.html">r.mapcalc</a></em>,