Line segment length is determined by speed limit zones. Data were collected using a differential GPS in 1997. The data set was developed using GPS to replace an existing road coverage which was digitized from USGS 15-minute maps. The old data set contained known errors. All arcs have valid attributes.
All arcs have valid attributes.
Service and residential roads are not included in this layer. DRGs comply to the standards associated with USGS 7.5 minute maps. PDOP maximum was set to 6, and data were only collected when at least 4 satellites were readable. After differential correction of the GPS data the horizontal accuracy is estimated to be within 5 m. Accuracy changes with distance from base station by about a meter per 100 km.