MileCheckWhat's the difference between a surveyed milepost and a calculated one?
A surveyed milepost is where the sign actually stands, as the state recorded it. A calculated one is a point placed along the state's route line at that mile. They are often close. They are not the same.
Why they differ
A sign goes where it fits, such as past a guardrail, before a bridge or clear of a ramp. The route line says exactly where the mile falls. So a surveyed sign can sit some distance from its calculated point.
Route lines also change when a road is rebuilt. If the state remeasures the line but leaves the old signs up, the two drift apart until the signs are renumbered.
Which one to use
For reading out "you are at mile 147", the sign position matches what a driver sees out the window. For measuring distance along the road, the route line is the better reference.
How the MileCheck dataset marks them
Every row has a position_type, set by state from how that state's data was built. physical means the state published the point as a sign location. derived means it was placed along the state's route line or the federal HPMS line. 24 states are physical, 19 derived, and 7 a mix. All positions are approximate, so don't treat a point as a surveyed sign unless the state's own documentation says so.
Mile marker data for all 50 states
396,842 marker positions as CSV and GeoJSON, one layout for every state, with a list of what changed in each release. Download the Utah sample with no form.
Questions, answered
Is a calculated milepost wrong?
No. It is correct for the route's measure. It may just not sit exactly where the sign is.
Which states are which?
The dataset's manifest lists the source and position type for each state.