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GM scale bar is not correct ??

chuckchuck Global Mapper UserPosts: 82Trusted User
edited April 2011 in Technical Support
scale test copmparison.pdfscale test.pdfFolks:

I am working on a simple project -- a dxf file. I have loaded the file and then noticed the scale bar provided with the drawing was not matching the map scale shown in the GM workspace. I have used the measurement tool and verified the drawing's scale bar is indeed 1,000 ft.; yet, when viewing the GM map scale it appears to be too long. I have attached pdf's of both a GM output pdf print and a screen capture to illustrate the issue I am facing.

GM v12.01
Build Time Jan 20 2011 22:46:02

Please advise.

Thank you in advance.

-Chuck

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited April 2011
    Chuck,

    This is because of the projection that you are viewing the data in. The Geographic (lat/lon) projection is extremely distorted compared to ground truth in the X direction if you are far from the equator, so any measurements along the X direction will not match the distance scale. Measurements in the Y direction will though. To get a more consistent scale in both directions you would need to use a better projection for smaller areas of the earth, like UTM, on the Projection tab of the Configuration dialog.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • chuckchuck Global Mapper User Posts: 82Trusted User
    edited April 2011
    Thank you, Mike...I understand the projection distortion. Does the GM scale bar always default to a great circle? Is it, or will it be, possible to show and X & Y scale bar to assist and show the map distortion?

    Thanks again and keep up the good work.

    -Chuck
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited April 2011
    Chuck,

    The scale bar is based on the Y direction scale as that tends to be much less distorted in most projections. Note that a perfectly correct scale bar isn't possible for any 2D view of the earth as the actual true scale will vary depending on where you are on the map, so the scale say at the bottom of your map won't necessarily be the same as the scale at the top, especially if a large area is covered. The best you can do is use a projected system appropriate for your area to minimize distortion.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
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