The best way to estimate accuracy is to compare the mapped and scanned information with independently collected data of higher accuracy. Two types of accuracy are important: thematic and spatial. Thematic accuracy refers to feature identification and labeling during the mapping or digital encoding process. In the case of historical map data, independent sources of highly accurate thematic information can be difficult to acquire or non-existent. This is the case with the T-sheets. And since the T-sheets are simply being encoded as images for subsequent visual interpretation (i.e., no additional attribution is being assigned to boundaries or shades in the scanned images) no additional thematic error is introduced through the digitizing process. Therefore, an assessment of their thematic accuracy will not be described here.

Spatial accuracy refers to the amount of error associated with location positioning. Sources of spatial error include the following:

1) boundary line placement during aerial photographic visual interpretation;
2) photogrammetric processing of source airborne imagery;
3) expansion/contraction of paper map products;
4) scanner induced distortions;
5) imprecise determination of row / column locations for map identifiable points on the T-Sheets during rubber sheeting;
6) imprecise determination of corresponding map or orthorectified image points during rubber sheeting.