Should I use my DEM or ortho file to create my Hillshade layer?

Question

I've just finished processing my DEM and ortho files, and have been asked to make my hillshade layer. I'm not sure whether to use my DEM or ortho file with this tool. What is the difference between those two files, and which one should be used with the hillshade tool?

Answer 1

Short answer would be : have a try, you'll see what happen ;)

To be more explicit : you need to understand what is hillshading. Hillshading (as in shade from a hill) is a way to recreate some impression of shade on a 3d looking like raster like on the image below. For this, you need altimetric values.

"Hillshade function obtains the hypothetical illumination of a surface by determining illumination values for each cell in a raster. It does this by setting a position for a hypothetical light source and calculating the illumination values of each cell in relation to neighboring cells. It can greatly enhance the visualization of a surface for analysis or graphical display, especially when using transparency." (Source)

Ortho-images are essentially RGB values on 3 bands, like a classic picture you could take with your camera. Doing a hillshade on this kind of file would have no sense.

DEM represents altimetric values on one band. A high value on this type of data will cast a longer shade than a low value, as in real life a mountain casts a bigger shade then a little bump.

You could hillshade some other kind of monoband values, like high concentration of a parameter, but it's more unusual.