Global Mapper v25.0

DWG Exporting with Incorrect Length

GeoBob
GeoBob Global Mapper UserTrusted User
edited November 2012 in Technical Support
I have recently encountered an odd situation where there is a difference in line length when I export a line to a shapefile or as a DWG. Both files are in UTM Zone 11 NAD83. The shapefile length is 61.434 km which is correct (i.e matches the attributes independently stored for that line). However the DWG line is 61.114 km long. I am using GM v14 (Sep 12 Build)

Any ideas? Files attached.

Thanx

Bob Saunders
Calgary, AB

Comments

  • global_mapper
    global_mapper Administrator
    edited November 2012
    Bob,

    It looks like the projection is 10TM/NAD83, not UTM zone 11. When I load the DWG with that projection the lengths match. Note that the lengths are by default calculated using great circle distance, so the actual location on the earth will make a difference in the calculation as great circle is more accurate than grid distance as any 2D projected distance is inherently inaccurate since the earth is not a flat plane and no 2D projection system can accurately represent it.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • GeoBob
    GeoBob Global Mapper User Trusted User
    edited November 2012
    Mike

    That makes very good sense but it has raised an interesting observation. The original dataset was likely incorrectly projected as UTM 11 when in fact it was 10TM. What is so weird is that when you extract and save a piece (like I did in this test) the correct projection is re-assigned. (Is that even possible?)

    Anyway, problem is definitely one of projection and not an issue with GM.

    Thanks!
  • global_mapper
    global_mapper Administrator
    edited November 2012
    When you load just the test.zip because the projection of the Shapefile is 10TM and it happens to get loaded first, the view/export projection is set to 10TM by default. So if you export to DWG it would be in 10TM. However the DWG itself would be incorrectly placed if it came in as UTM zone 11 and was exported as 10TM as they UTM coordinates would get reprojected to 10TM rather than just reinterpreted as such, so they would be wrong.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com