Trouble with Hillshading

[RobertBlomquist]

I am doing a project using the real elevation and azumith of the sun. Many of the elevations are negative, this seems to be a problem with hillshade. Is there a way to convert negative values to positive?

I got my data from the NOAA solar position calculator at:http://www.srrb.noaa.gov/highlights/sunrise/azel.html .