Global Mapper v25.0

analyzing / merging contour lines from multiple sources

I have a project where I am analyzing contour lines generated by different systems, datasets, etc... Some have contour lines with a vertex every 4 metres while others have detailed contours with vertexes at 0.20 intervals. Though more detailed, they represent the same thing and wander back and forth over the more general line. But another dataset may have a detailed line which is seemingly offset more one way than the other.

I am trying to figure out a way to measure and report the differences between lines.

One thought is to use one line as a base and then extract just the vertices from another line. Does GM have a way to calculate the offset for each resulting point? Then I could create a histogram of the offsets.

Another thought is to buffer the base line and see how many points from the other line are inside the resulting polygon. I like this less because it doesnt provide as much information.

At the end of the day, I may conclude that one line is best, or I may want to create a new "average" line that perhaps is a best-fit that goes through all of the points. Any idea how to do this?


  • Hi Ron,

    That's a pretty detailed analysis you are undertaking right there!

    What I've done in the past as a simpler analysis was to create DTM's (i.e. Elevation Grids) using the contours from the multiple sources and then create a "heat map" of the differences between them using the "Analysis-> Combine/Compare Terrain Layers-> Subtraction (Difference) - Signed" tool.

    You could perhaps use this cruder method as a screening tool to highlight areas of difference above a certain threshold and then use something like the more detailed method you are suggesting just in those areas?

    Anyhow, food for thought...



  • Thanks for the suggestion @CarrickCon I have done that before as well but now need to have a laser focus on one particular line and the effects on it. I have done some more research on this topic and found that I should have included the key phrase "proximity analysis". I have some other tools that I use and am looking at them as well.

  • I discovered the station and offset tool which looks promising, but it reports all offset distances as positive. There is no way to distinguish between points on one side of the baseline or the other. 🤨