Accuracy Assessment of Interstate Highway Length Using Digital Elevation Model

Abstract

Road length is part of the geometry of the roadway network. Its measurement is critical to all road inventory databases. One approach to obtaining it is to drive cars equipped with a distance measurement instrument (DMI) along roads to measure mileages. This method provides accurate measurements, but it is expensive and time consuming. This paper proposes an alternative way to acquire actual road length, which is currently under consideration by the geographic information system (GIS) unit of the North Carolina Department of Transportation. The emphasis of this study was to determine the accuracy of the proposed approach. The proposed approach employs GIS application programs written in ARC macro language to calculate the actual length (surface length) along the sloped surface of highway centerlines based on elevation data and the road network geometry. This was done for all interstate highways in North Carolina. The calculated GIS results were compared with DMI measurements, which is the most accurate approach presently available to NCDOT. Three filters were applied to remove suspect road segments that have significant errors which are irrelevant to the proposed approach. All remaining segments were grouped by slope and length to evaluate the impact of slope and length on the accuracy. Frequency analysis and root mean square error were determined for all groups. It was found that the proposed method is a technically feasible method with reasonable accuracy. The study also revealed that where there are errors, they occur primarily for road segments with relatively high slopes, short lengths, or both. The meaning of this is that GIS and digital elevation model technologies can be combined and used in lieu of DMI measurements, thus reducing resource demands.