A Method for Assessing the Planimetric Accuracy of Historical Maps: The Case of the Colorado-Green River System*

The position of linear elements on historical maps can be digitized and numerically compared with similar features on modern maps. This allows for the evaluation of amounts and rates of improvement in cartographic accuracy and of the primary sources used for the generation of subsequent maps. A case study of the Colorado-Green River system illustrates the method and documents the pace of improvement in the cartographic depiction of the river system as a function primarily of the historical pressures of westward expansion.