• FormU

FormU

FormU is so far the best way to reduce the uncertainty of an SPC model to a single number. It combines the pointing and overlap uncertainties from RESIDUALS.TXT that are extracted by get80.sh.

Typically you will get FormU from the maplets with the best GSD, but sometimes it is worthwhile to calculate it for all maplets. Part of the Class B certification for the OSIRIS-REx mission was to determine FormU during various stages of shape model (i.e. global DTM) generation of testing model Shape 3. We found FormU was always within a factor of 2 of the actual error. The actual error of an SPC model is usually never known, but in this case we had a digital truth model for comparison (Weirich et al., 2022). This sort of testing was never performed for a regional DTM, but from a theoretical perspective there's no reason this factor of 2 relation wouldn't apply to a regional DTM.