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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. | 
