Considerations when measuring from a single photograph: positional uncertainty of digital monoplotting

Abstract

Digital photo coordinates captured off a single image – regardless of film size or camera orientation – can produce reliable measurements. A digital monoplotter, consisting of PC-based software and a standard digitizing tablet, was used with four examples to explore positional uncertainty associated with large- and small-format, vertical and oblique photography. To measure off a single photograph, the instrument determined elevation (Z) of a given (X, Y) point by querying a digital elevation model (DEM). Photo measurements were compared to map and field measurements using different DEMs: DEMs imported from an external source and created internally from control points. Results indicate positional error is random and systematic.