Farallon Helps Caltrans Improve Accuracy of States County Route Postmile System



The California Department of Transportation (“Caltrans”) locates points along California’s highways using a linear referencing system based on county, route (highway number) and postmile (distance along the highway centerline from the county boundary). Caltrans wanted to improve the accuracy of the county route postmile system by accurately determining locations where highways cross county lines, especially in areas where the highway repeatedly crosses county lines over a short distance or where the county line is coincident with the roadway.


Farallon helped Caltrans to determine locations of the intersection of highway centerline and county boundary crossings for selected sites, within 5 meters of true. Farallon used three different technical approaches to determining highway centerlines.

The most economical means of generating the highway centerline data was to extract these data from existing maps and survey records already owned by Caltrans. When photogrammetric methods were not suitable or the necessary data was not available for a specific highway/county boundary, Farallon’s team collected highway centerline information using global positioning system (GPS) field surveys. To ensure minimize disruption to traffic flow, closely spaced GPS data was collected from a moving vehicle. The antenna for the GPS receiver was mounted on the driver-side of a truck and the highway in both directions in order to derive centerline geometry. Where GPS reception was problematic, such as areas with tall trees and/or canyons, GPS control points were set adjacent to the study area and a traditional traverse survey was performed to establish the highway centerline.

To ensure that Caltrans used accurate spatial representations of the necessary county boundaries, Farallon’s team obtained the relevant legal records defining county boundaries and known control points, sometimes at variance between adjacent counties, or referring to outdated landscape elements (e.g., a damned lake where a river channel was the original designator).

Farallon used this data to identify the intersection points between each of the project highway centerlines and the appropriate county boundary. The GIS data included the county boundary linework in the vicinity of each project site, as well as linear geometries that represent the highway centerlines near each location.