What exactly does hill shade or Shaded relief , if performed on SRTM DEM will output?

Question

Two questions actually. Does shaded relief or hill shade outputs mean "expelling of shadows"?

For a vast country like India, how to prepare a map from SRTM DEM, which does not have any shadow effects?

Answer 1

Hillshade computes the local illumination from a light source located at infinity (like the sun). Basically, it yields the cosinus of the normal to the face of the terrain and the light ray.

This can be used for :

• visualisation and cartography : the light source is then located near the north, which then gives a nice picture of the relief.

• analysis : the light source is then located at the position of the sun (computed based on latitude and time of the day). This gives a value proportional to the amount of direct solar energy that reach the pixel.

Note that there is an option to take the shadows from high neighbouring objects into account. In this case, there is a model to account for the interception of light ray by obstacles and the direct illumination in the shadows is set to zero.