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As far as I understand, the m value "is a measured distance along a route" (similar answers here), thus similar to the mileage of a car driving over a highway. The m value defines the point on a line as the distance from a set starting point. So the m-value, even if not explicitely set, is somehow inherent in the very geometry of the line feature, defined as the distance from the first vertex of the geometry. Thus, every point of the line (not only vertices) has a well defined m value - comparable to an x and y coordinate, even if these are explicitely defined only for the vertices, not the points on the segments between vertices.

However, with a look at the QGIS documentation, I am confused. The image to explain the Filter vertices by M value function there shows a line with 5 vertices, with m-values in this order: 0, 11, 5, 12, 8. This would correspond to the mileage going up and down, so something is wrong.

How is this possible if the route is measured from the start point of the line? Or did I misunterstand the concept and the m value relates only to the neighboring vertex, not to the start point of the geometry?

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    I always thought I could stuff any value I liked in there, there is an assumption I can interpolate along the line segments between point values but I don't think that can be enforced. – Ian Turton Jan 27 at 16:12
  • M for Mesure ? And store a value information in the coordinates without attributes table. Maybe. – J. Monticolo Jan 27 at 17:17
  • Everything is fine with the QGIS example. Related: postgis.net/workshops/postgis-intro/3d.html you can look at the points composing your line as POINTM ;) – s.k Jan 28 at 1:28
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Those explanations are just what ArcGIS chooses to use to store in M values in specific contexts. They are not a standard or consensus. The other answers on the questions you linked also highlight that.

While I am not sure where it originates from and how to prove my statement: m stands for measured and can be anything that is measured at that vertex. Temperature, time, distance, density, whatever.

You can see M values mentioned in the original Shapefile whitepaper as "measures". The Simple Feature standard says "The ordinates x, y and z are spatial, and the ordinate m is a measure." For a definitive answer you would probably have to dive into old textbooks and papers. But just don't take a random ArcGIS documentation page as spatial canon. ;)

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    So the history of the m value still has to be written ;-) Who knows, maybe I will do it one day. – Babel Jan 27 at 21:54

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