Global Mapper Pro

I don't get the same Bearing when using other software

When viewing the same .shp (including .prj) file in GM and obtaining the bearing of a line, it does not match the same bearing when measured in ACAD Map, I verified this by doing the same in ACAD Civil and both ACADs give the same result.

My process for measuring the bearing in ACAD was to draw a vertical line (North) and measure the angle in DDMMSS between the vertical line and the line I require the bearing for, all in N DDMMSS E. The difference is roughly 1D46'15".

Another check was to draw a line at a specific bearing in ACAD and then measure it in GM, the bearings are different.

Where is my error occurring?


KB

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Answers

  • bmg_mike
    bmg_mike Global Mapper Guru Moderator, Trusted User

    The value of a bearing depends on settings on the General -> Measure / Units command. The Bearing Options section there lets you control whether bearings are True North, Magnetic North, or Grid North. For example, a vertical line in some projected system would have a bearing of 0 degrees in Grid North, but could be quite different in True North or Magnetic North.

  • Ok, that got me closer bmg_mike.

    I've imported a test .dwg into GM that has a vertical 0 degree line (True North) and a line 45 degrees and one 90 degrees, all from the same origin all measured clockwise from the 0 degree line.

    I have GM set to measure bearings relative to True North and it gives me a bearing that is roughly 1d46' counter clockwise off of 0 degrees. I'd assume this is correct if we had the bearings set relative to Grid North as I'm using UTM 18 N projection.

    When I set GM to measure bearings relative to Grid North, all of my bearings on the test file are correct, my vertical 0 degree line measures 00D00'00" and all other line bearings measure correct.

    GM ver. 20.1

    I'm still missing something, it seems the bearing measurement is reversed? I'm still missing something.

  • bmg_mike
    bmg_mike Global Mapper Guru Moderator, Trusted User

    The True North setting is for lines that are vertical compared to the lat/lon grid (i.e. Geographic projection). So the longitudes would be the same at the start and end of the line.

    A line that is vertical in UTM will not have the same longitude at each endpoint unless it is at the very center of the UTM zone.

    The Grid North setting would give you 0 as that just means the line is vertical in the current display projection.

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