I am trying to wrap my ahead around the meaning of the M-value. I read ESRI's dictionary definition here: https://support.esri.com/en/other-resources/gis-dictionary/term/m-value

and was not able to get a good grasp of the concept.

What does the M-Value in GIS data represent and can you provide an example of using it or how its useful?

  • 3
    It's actually an arbitrary value, as is the Z, technically. It can represent whatever you need it to, on a per-vertex basis (provided you can work with a scaled floating-point value) – Vince Jun 5 '18 at 23:47
  • It's technically not arbitrary, but also technically not super-useful - and with no danger in using it arbitrarily. ;) The main problem is "why just XYZM"? You have nowhere left to go. – mdsumner Jun 6 '18 at 0:18

It is a measured distance along a route. A mile marker along a highway that is nominally 10.1 miles from some start (such as where the route crosses into a county) may not be exactly 10.1 miles. Someone surveys where certain landmarks, mile markers, or buildings are along the route exactly. These route events get an M value representing their measured location. A route feature has both its line and its measured locations, and other events, along it. A measured distance along a line has many uses, such as route planning.

I need to vamoose for the day and I can't get the ESRI link to load up. I may be able to explain this better tomorrow.

(Added). Measured distance points can also be used to improve linear referencing. Some points that have been located exactly can be given higher weight, so that they have more influence on where new geocoded points are placed along a line. This is very important on long lonesome stretches of highway where there is not much to geocode from.

  • This is because shape.length is a 2d measurement, the real world feature that the shape represents is 3d, so as you said a line that is 10.1 miles by geometry would have a real world (survey) distance considerably different to 10.1 miles.. – Michael Stimson Jun 6 '18 at 0:37
  • To add to this you could have a look at linear referencing (also called dynamic segmentation) these concept made full use of M value – J.R Jun 6 '18 at 8:46
  • Agreed, measurements also differ because no one moves mile markers when a road is reoriented; at the DOT where I interned, there were some sections of the highway, usually at interchanges, with 0 measured length to adjust for changes in actual highway miles, i.e., so the mile markers were closer to the real distances from the start of the route.. – danak Jun 6 '18 at 16:53

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