Accuracy of determination of stream lengths from topographic maps

Two methods are used to measure stream lengths from topographic maps: (1) using streams printed in blue, or (2) inserting streams into the drainage net wherever there are V‐shaped contours. The accuracy of these two methods was tested by comparing measurements taken from maps with the streams as measured in the field. In basins with area less than one half square mile, a paired t ‐test between field measurements and the contour method showed no significant difference while a paired t ‐test between field measurements and streams marked in blue did show a significant difference. In basins with area varying from 0.5 to 2.68 sq mi, total stream lengths and drainage densities obtained using streams in blue are always smaller than those obtained by field measurement. This indicates that a significant difference exists. A paired t ‐test showed no significant difference between values of field measurement and the contour method. This statistical study indicates that, at least in very small watersheds, streams should not be measured from topographic maps using the streams printed in blue. The contour method may be used.