Until recent years, State Highways were inventoried using a distance measuring instrument (DMI) to determine the route's length and the milepost location of physical features along the route. Because this method did not take into account the highway's longitude and latitude, it limited the value of the information for mapping purposes.

As part of an effort to improve the accuracy of the Department's Linear Referencing (milepost) System (LRS), data points for all State Highways are now being collected using the Global Positioning System (GPS). Once completed, these data points will locate the precise position of the roadway on the

Earth's surface, as well as allow a GPS baseline to be established in both directions for each highway. This baseline will allow such roadside features as guardrails, culverts and signs to be inventoried and located with sub-meter accuracy.

The Transportation Data Office is sharing the knowledge and experience it has gained in GPS mapping and data collection by offering a GPS Mapping Grade Training Program. This program is intended for personnel in WSDOT, other State agencies, and County and City governments who collect GPS data for mapping purposes.