Shadow accuracy

Posted on Jan 12, 2012 By : Unknown

A number of users have commented on occasional differences between computed shadow volumes and observed shadows.
There are a couple of possibilities that could account for the differences observed in the shadow model output and the real world shadow.
Please consider the following:

Data discrepancy between your building model and the actual building, or your surface model and the actual surface. Coarse resolution elevation data can introduce a significant variation from the observed proof points.

Make sure your building models represent the actual buildings and increase the resolution of your terrain data when comparing shadows. In ArcScene: Layer properties -> Base Heights -> Raster Surface Resolution.
Data offset of Sun point feature caused by the transformation of the Sun's position to the spatial reference of the input feature class. The Sun point is initially calculated in GCS WGS 1984, then projected into the spatial reference of the input feature. If the datum of the input feature is different than WGS 1984, then a default transformation is applied. This would typically be the first available option unless a particular transformation is specified in the environment setting. An improper datum transformation can result in an offset of up to 30 feet, depending on the locale and the method being used.

Please check the spatial reference of your input building feature and see if it necessitates the specification of a datum transformation. This help article offers a guide for determining the appropriate transformation.

Also the impact of such an offset on the final shadow output is reduced if the output Sun distance is increased. For example increase the distance from default 2500 to 10000.
Our tools use a point source, whereas the shadow measured out in the field is generated by a sun which has a significant angular diameter (0.5 degrees): soft vs hard shadows.
Another factor might be refraction, which is not considered in the create SunSkyMap script. Refraction causes the sun to appear higher than the theoretical position value, meaning that the shadow will be calculated shorter.