georeference vectors from unknown projection

benkelmanbenkelman Global Mapper UserPosts: 12
edited November 2008 in Vector Data
I have vector data from an unknown projection - these are CAD files using a local, non-mapping coordinate system. I'm trying to georeference them.

Method 1: is there any method similar to "Rectify (Georeference) imagery"? This would assume I could get a base image or base map and manually enter control points...


Method 2: If I can get some description of the coordinates the creators used, I should be able to define a coordinate system - but which projection should I use? I have to assume that the Y axis of these coordinates are not aligned with Longitude (e.g. "up" is not north).

Thanks
Cody Benkelman

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2008
    Cody,

    Once you have your CAD files loaded (using whatever projection you want to eventually use), you can graphically rectify that data by opening the Control Center, right-clicking on the layer, then selecting the option to Reposition the data.

    Typically local coordinate systems are based on a Transverse Mercator projection with the appropriate central lat/lon and a false easting/northing if applicable.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • benkelmanbenkelman Global Mapper User Posts: 12
    edited October 2008
    Mike

    Thanks! Very cool! This is exactly what I was seeking for what I called "Option 1".

    Sorry to bother you with help on a standard feature, but I simply hadn't found it.

    So far I don't know if I can get adequate information for defining a custom projection ("Option 2"), but as noted I do not expect these CAD coordinates to have any correlation to standard projections. I'll let you know more assuming I get more info.

    Thanks again.
    Cody B.
  • Trail BanditTrail Bandit Global Mapper User Posts: 3
    edited November 2008
    I have been using a combination of gps tracks, rectified aerial photos, and other data to produce a map. The gps tracks agree very well with the aerial photo data. I export vector the data I have traced off the aerial photo as a DXF file and open it in Adobe Illustrator which seems to be the format that my printer likes. All is fine so far. I am trying to bring a layer from Adobe Illustrator, back into Global Mapper to check that I haven't screwed things up.
    I export the layer as a DXF file to Global Mapper and georeference it as instructed. The 4 points I used to georeference the file show up perfectly located on the image, but as you move away from the reference points, there is a larger and larger error. Any ideas?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2008
    This likely indicates that the projection that you are rectifying in is not similar to the true native projection of the layer, so the warping induced by 4 control points is less true the further that you get from those control points. Try changing your rectification projection after entering your control points to something like UTM and see if that works better.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • Trail BanditTrail Bandit Global Mapper User Posts: 3
    edited November 2008
    Thanks for the input. When I georeferenced the vectors, I set the projection to "Geographic latitude and longitude" Before hitting OK, I entered State Plane coordinates into the projection and hit OK. The aerials were provided to me in SPC. I will try your UTM ans see what happens. The worst errors are a few hundred feet, which is way too much for my project.
    Thanks
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2008
    When you changed your projection to State Plane and were prompted to reproject or reinterpret, which did you select? Reproject is the one that you needed to select.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • Trail BanditTrail Bandit Global Mapper User Posts: 3
    edited November 2008
    Thanks Mike,
    I hit "Reproject". I will try again tomorrow.
    Thanks,
    Bob
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