Global Mapper Pro

grid vs geog distance

flintgeo
flintgeo Global Mapper User
edited December 2009 in Bug Report
I have a problem with correct representation of grid (projected) distance of line data - imported either form a DWG or TEXT file. This is best reproduced by import of a simple north-south line of 200 km from a DWG file - into a UTM projection in GM 11 - Feature Information returns a length of 199.68 km ????
Have I missed something ?


Thanks

James

Comments

  • global_mapper
    global_mapper Administrator
    edited December 2009
    James,

    How are you calculating the distance of the line? Global Mapper will always calculate the distance along the ellipsoidal surface of the earth using a very accurate calculation method. This will be much more accurate than a distance based on grid units.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • tjhb
    tjhb Global Mapper User Trusted User
    edited December 2009
    Are you using Vincenty Mike? Useful.

    Could someone get a simple grid distance (though inferior) if they needed one? (E.g. for standardisation with other measurements.)
  • flintgeo
    flintgeo Global Mapper User
    edited December 2009
    Hi Mike

    That is the answer -- thanks.

    For engineering we prefer to calculate everything on the projection. This is to allow the use of straight lines and rulers over long distances (submarine cable / pipeline routes for example) - and to simplify / reduce the chance of error when representing distance / direction. Eventually we need the true ellipsoidal distance of course (when ordering 200 km of cable or pipeline !!)

    Can GM be asked to default to expecting and using grid units unless specifically asked for ellipsoidal distance / direction. ?
    Also can GM default to showing 3 decimal km units ?

    many thanks for your excellent support !!

    James
  • global_mapper
    global_mapper Administrator
    edited December 2009
    Yes, GM uses Vincenty for distance calculations. There really isn't any way to get a grid distance out of GM.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • global_mapper
    global_mapper Administrator
    edited December 2009
    James,

    I went ahead and added a new right-click option to the Measure Tool to allow using the grid distance rather than the great circle distance. If you check that option you should get what you want.

    I have placed a new build at http://www.globalmapper.com/global_mapper11.zip with the change for you to try. Simply download that file and extract the contents into your existing v11.xx installation folder to give it a try. If you are using the 64-bit version, there is a new build available at http://www.globalmapper.com/global_mapper11_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
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