MileCheckMile markers on the highway don't move. That's the whole trick.
Every state maintains a list of them: a route, a mile number, a latitude and longitude. MileCheck bundles that list into the app itself, once per state. No live connection is required to find your position on it.
Between two signs
As you drive, the app takes your GPS coordinates and asks one question: which two mileposts on your current route is this closest to? It measures the distance to both, then places you between them. Closer to mile 34 than mile 35, and a little past the midpoint, and you get "34.4."
The signs themselves aren't always where a database would put them. Some of Oregon's physical mile markers sit at mile 304.9 for the sign that reads "MP 305." MileCheck rounds and interpolates so the number on your screen matches what's stamped in the ground, not a straight-line average between two database entries.
Where two highways share the road
The harder problem isn't a single road. It's an interchange. Two highways can run on the same pavement for miles, and the moment your GPS ticks near an off-ramp, a naive search will flip between them, over and over, mid-drive.
MileCheck scans each state's data once, ahead of time, for stretches where two routes verifiably share pavement, confirmed against several consecutive mileposts in a row rather than one close call. Once you're on one of those stretches, the app holds your route steady instead of re-guessing at every GPS update.
Why "offline" isn't a trick
None of this needs a signal. The milepost list ships inside the app. The math runs on the phone. That's the offline part: not a trick, just a list that was already correct before you left town.
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