Sun angle is very important to our mapping and ortho projects. Long shadows are usually not desirable and most large projects will specify minimum (and sometimes maximum) sun angles that we must translate into time parameters. The time of year and latitude of the photo area determine the angles for a given day and time. We use a web site from the US Naval Observatory to quickly gather the information as part of our flight planning. It is also a good tool for our customers to use to see when the photo season is appropriate and roughly how long our working day can be. Some projects are sensitive to “hot spot” where the angle of the sun is high enough that the “down sun” area is within the image area. This will be a visible light, low contrast area where no shadows exist. This effect can be troublesome to visual products such as an orthophoto or interpretation where shadows help define the terrain.

Common specifications for sun angle are a minimum of 30 degrees for mapping (which is compromised in the winter) and 40, 45 or even 50 degrees for forestry, depending on tree type, canopy cover and the final uses for the photography. To avoid hot spot, a project might avoid sun angles above 50 degrees. Other factors like direction of flight, focal length of camera, and latitude of project enter into the hot spot issue. All these factors need to be considered, but as photographers we need to point out that excessive restrictions will add multiple days to a job, potentially adding trips for the airplane and surveyor if AGPS is used.