Creating GeoTIFF causes file to appear warped/crooked

I'll outline the steps I took:
1. Grabbed a 50K MIM, cropped out a specific part via photoshop.
2. Uploaded it to Global Mapper, performed geo-rectification with 4 GCPs at four corners along the existing MGRS grid lines (WGS-84, UTM 11N)
3. Success.

The problem is that whenever I open the file in ArcMap, it shows up all distorted and contorted, as if someone took the flat map and turned it 45 degrees. Other times, when redone, it appears upside down, or somehow contorted in other ways.

For what pvrpose does this happen?

Best Answer

  • MykleMykle Global Mapper User Posts: 447
    edited January 2017 Answer ✓
    It's complicated.  Rather, it's hard to comment without having the file to play with, or knowing the gory details about your workflow.  Like: what version of GM are you using (the build number at the top of the window)?

    When you rectify, are you selecting MGRS projection and entering MGRS coordinates?
    When done, is your display set to MGRS, so you can turn on grid lines that match the map?
    When you export, your display should still be set to MGRS, and ensure that you select ancillary files that are used by ArcMap, and that all of these exported files are visible when importing into ArcMap.

    These steps do not always need to be this rigorous, but since you are having issues, keep it (less) complicated.

    If I had a copy of the map, I would simply open it in GM and turn on grid lines, then look for anything that seems odd.  (I don't have ArcMap, but others do)

Answers

  • When I do the rectification, I select UTM projection and I enter in the grid-line numbers. Normally on an MGRS map, the long series 8-10 digit grid numbers) are displayed at the end. For this map, however, it wasn't displayed, so I was not entering the full numbers. Bad on my part. 

    Given what you highlighted, I think the screw up is somewhere on my part. Even when I followed the youtube tutorial, I noticed that I wasn't entering the full grid number because it wasn't showing on the map. However... if I did have the full grid number (for the project I am working on), then I think it would work. 

    Thanks for your help. I'd provide a copy of the map but it is FOUO. Still, your advice was really helpful. 
  • MykleMykle Global Mapper User Posts: 447Trusted User
    I see that it doesn't look like GM provides a way to rectify a map using MGRS coordinates.  So you need to figure out the full UTM values somehow, like using map info as you describe. 

    Another method would be to open a blank instance of GM (for simplicity), then use the digitizer tool to add points (use the MGRS entry box), for points near each corner of the map at grid line intersections (or at least two diagonal corners).  When these points are on the display, then you can import a map and rectify it (remember to set the appropriate projection). 

    The middle pane of the rectification window will show your map, and the right pane will show your reference points.  Click on a grid intersection on your map (red dot), then click on the corresponding point in the right pane (another red dot).  Note the coordinates that appear and tweak them (probably by rounding off) to suit your map point. 

    You could probably get away with only placing one point before loading a map and rectifying it.  Use the point to help define the coordinates for one point on the map and use it for reference to devise the complete UTM coordinates for other grid line intersections. 

    Or, your map boundaries may be lines of latitude and longitude.  You can rectify your map using the corner coordinates.  Then switch your display to UTM.  GM provides for loading maps of a variety of datum/projections and will adjust each to suit your choice of display. 

    Thanks for providing an interesting problem!
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