Analytical hillshading is a technique for producing shaded relief maps automatically. Relief shading is used to visually enhance the terrain features by simulating the appearance of the effects of sunlight falling across the surface of the land. Hill Shading estimates surface reflectance from the sun at any altitude and any azimuth. The reflectance is calculated in a range from 0 to 100. The equation for the sun in the northwest sky with a 45 degree altitude is as follows

Reflectance = 1/2 + (p'/2) / SQR(po2 + p'2) * 100

where:
p' = (po * dEW + qo * d NS) / SQR(po2 + qo2)
po = 1 / SQR(2)
qo = -po

fig. 1 hillshade map of Bellarine Peninsula with light source coming from south

If you create a hill shade map with the light coming from the 'south' or bottom of the screen the result is not so easily interpreted (see Fig. 1). It seems that even for people living in the northern hemisphere their 'natural' interpretation of terrain is with the light from 'north' (as in Fig. 2)).

fig. 2 hillshade map of Bellarine Peninsula with light source coming from north