= 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.