Linear referencing
Locating things along a route rather than on the earth: measures and M values, postmiles and equations, dynamic segmentation, and moving events between networks.
A road is a line, and the things on it are located by how far along they are: postmile 49.64, milepost 233.7, chainage 12 400. It reads like a legacy convention and it is not. An asset inventory outlives every digitisation of the centreline it was drawn against, so a measure against a route register keeps finding the right bridge long after the coordinates have moved.
The cost is an arithmetic of its own. Measures are not lengths — a route is calibrated, and treating a postmile as a fraction of the line misplaces events by tens of feet. Numbering restarts at county lines, jumps where a bypass shortened the road, and repeats under a prefix where a curve was realigned, so the measure alone is never the key. The M ordinate that the standard provides for all this is the least supported part of the geometry model: reprojection drops it and most drivers refuse to write it.
The concepts below fix the vocabulary — route, measure, event, calibration, equation, conflation; the how-to guides read a real state highway network with its postmiles intact and locate, place, cut, overlay, aggregate and transfer events against it; and the fixes are the five ways the numbers come out wrong.
🧭 Start with the concepts
The ideas the how-to guides below assume you already have.
⚙️ Then do the task
Step-by-step walkthroughs with code you can copy and adapt.
- How to read a linear referencing system in Python
- How to find the measure of a point on a route
- How to find the point at a given measure on a route
- How to extract the part of a route between two measures
- How to overlay linear event tables in Python
- How to aggregate events to route segments
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🔧 When it breaks
The errors this topic produces, with the cause explained rather than just the command.
Learning paths through this topic
A curated route rather than a list — the guides in the order that builds on itself.
Related topics
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