Global Mapper v25.0

I am using GM 22.1 64bit +Lidar. Laying out a pipeline route and using the Vertex Feature List.

Datum WGS 84 UTM Z30N. Between vertex 5 and 6 the given heading in the list is 224° 09' 34.7". The heading computing using the UTM coords gives a heading of 210 09 14 and using the measurement tool is given as 210 20 10.5. which is near enough for a gross error check. The convergence at the location is about 00:24:41. My question is: Is there any reasone why the given heading is about 14 degrees different or am I doing something wrong?


Best Answer

  • bmg_mike
    bmg_mike Global Mapper Guru Moderator, Trusted User
    Answer ✓

    Ah this confused me for a second as well. What happens when you use 'Grid North' for the bearing calculation is that the Heading is calculated using the grid/projection system that is NATIVE to the layer, rather than the display projection. So because your data layer is natively lat/lon (Geographic), the Heading is calculated using the simple bearing of the lat/lon values, NOT the projected UTM values. Those of course get very distorted as you get further from the equator.

    If you convert your data layer to a new file that is natively in UTM Z30N WGS84, then the Grid North headings will be based on the UTM values as that would be the native 'grid' system for that feature. When I do this, I get values in line with what you would expect.

    It might make sense for us to add a setting somewhere allowing users to instead calculate Grid North bearings using the currently displayed grid/projection rather than the native system of the layer.

    This of course does not affect Headings in True North/Magnetic North as those are along the ellipsoid.

    Thanks,

    Mike

    Global Mapper Guru

Answers

  • bmg_mike
    bmg_mike Global Mapper Guru Moderator, Trusted User

    The reported heading on the Feature Vertex List dialog will be calculated based on the Bearing Options settings on the Measure / Units tab of the Configuration dialog. So you will get different results depending on what your bearing is calculated relative to. If you are hand-calculating from the UTM coordinates, then 'Grid North' bearing is likely what you would want to compare to.

    Thanks,

    Mike

    Global Mapper Guru

  • Hi Mike, Thanks for that. I am set up to show Grid North and grid distances as below:

    However as convergence is about 00° 24' 41" I was trying to work out why the difference would be 14 degrees. Computing the other coords gives similar varying differences. I can forward a spreadsheed.

    I can forward the spreadsheet if you like.

  • Thanks Mike,

    I think it was the value of the differences that caused me concern, as the convergence at the location is only about 25 minutes. But now you have explained and looking back on the original data, this is a mercator chart backgrond referred to WGS84. which would explain the higher difference in heading.

    I then selected UTM 3W as the working projection as it is more practical for engineering use.

    I will do a check once I have the usual Monday rush out of the way. A note on the origin of the heading values in the menu would be useful.

    BTW when using the Vertx editing feature in conjuction with the COGO input you can input the lines as grid or true bearings and the heading is output in true or grid bearings depending as above, but when you input a horizontal curve you input the chord distance and radii but then GM asks you for the Quadrant Bearing, when you are already using grid or true bearings in all the other bearing functionality! Okay I just use a spreadsheet to provide this as I have never used quadrants at work, except in college as an academic exercise 40 years ago. As a surveyor it is Grid plus convergence every time.

    Thanks for your help.

    Cheers

    RGB