interpret datum info?

JSLJSL Global Mapper UserPosts: 434Trusted User
Hi, I'm trying to import a DWG from the Bureau of Ocean Energy Management:

This is the datum information it gives:

"Datum Information: These files have the GCS_North_American_1927 coordinate system, datum D_North_American_1927, spheroid Clarke_1866. Linear units are Foot_US"

How do I translate that into the projection settings in GM?

Thanks for any help...


  • MykleMykle Global Mapper User Posts: 451Trusted User
    Download the seep-anomaly-positives-zip file containing shape file data. 
    Download the file. 

    Load the seep-anomaly-positives-zip into Global Mapper.  You don't need to unzip the files, just drag the zip file into the display area of GM.  These files include a PRJ file containing the datum and projection information that GM will use. 

    Load the file into Global Mapper.  You don't need to unzip the files.  Accept the datum and projection for this file and it should overlay the seep files.  Then you can hide the seep files, leaving only the DWG.

    For reference, you can add a background map from Online Data Source (the globe icon on the toolbar).  I used a layer under WORLDWIDE DATA called World Topo Map, just to verify that I was in the right area.  In the Overlay Control Center, you can move the World Topo Map layer to the top, so the others display on top of it. 

  • JSLJSL Global Mapper User Posts: 434Trusted User
    Thanks, got it!

  • JSLJSL Global Mapper User Posts: 434Trusted User
    Now I've got another problem--I downloaded a bathymetry tiff from the same website:

    and I'd like to overlay it over an ETOPO DEM, but there's a collar around the detailed bathymetry that blocks the underlying DEM. I tried to get rid of it using "Clamp out of bound values to Valid Range" in the Alter Elevation Values tab of the Elevation options, but that didn't work, and neither did "Crop a Specified Number of Pixels around the Edges of the Layer" in the Cropping tab.

    Thanks for any help.

  • MykleMykle Global Mapper User Posts: 451Trusted User
    TIFF files are normally images and cover rectangular areas.  They MAY be formatted as elevation data, where each pixel provides an elevation rather than color information. 

    I downloaded and the East file, then loaded them into Global Mapper.  When asked, I selected Elevation data rather than image data.  As I move my cursor around, the bottom line of the window shows elevation while over the defined area, and no value when the cursor is outside of the defined area. 

    I am presently exporting the data to a Global Mapper elevation file, but the data set is very large, so it will take several hours.  An advantage of this export is changing the data to a normal elevation file, perhaps avoiding undesirable TIFF behavior like undefined areas within the rectangular area of each file. 

    However, it is looking like you may have imported these files as images rather than as elevation data, and are seeing undesired behavior.  I'm not seeing any issues with rectangular areas with files loaded as elevation data. 

    But I'm still playing (!)

  • JSLJSL Global Mapper User Posts: 434Trusted User
    Thanks for the help!  When I load the tiff into GM, it doesn't give me an option for elevation vs image--it just brings it straight in.
  • MykleMykle Global Mapper User Posts: 451Trusted User
    We are diving into the picky details, so you could add your version info (from the top line of the GM window).

    Here's what I'm using:
    Global Mapper v18.2.2 (b080217) [64-bit] - REGISTERED

    When I simply drag the file into an empty Global Mapper window, it sez:
    "Extracting (0%)".

    Eventually (these are large files) a message is displayed:

    My response has been to select YES.  
    (note that this is a common type of message when a raster file may contain elevation OR image data)
    Eventually (I did note that these are large files) the data appear. 
    I was able to load both the East and West (an even larger, large file) and display them without problems, other than doing other stuff while I waited.  I'm still exporting both files to a GM file in another instance, so it is taking even longer to reload one of the files into a second instance of GM.  Otherwise there have not been any problems. 

  • JSLJSL Global Mapper User Posts: 434Trusted User
    I have GM v. 18.2(b070617)(64-bit).

    I always get that window when I import the ETOPO DEM, which is a large geotiff as well, but for some reason, GM opens up the BOEM DEM without asking me for any options.  Should I try to re-download the BOEM DEM from the site?
  • MykleMykle Global Mapper User Posts: 451Trusted User
    There may be differences with newer (daily) versions, so you might start by updating yours (as I am):

    If you are using the same file that I am, there should not be any differences.  I would not expect re-downloading the same file to be of any help.  If you are using a different file, tell me the name so we can be looking at the same file. 

    When the dust has settled here (the save-to-file completed that is only at 34% now, and updating my version), I will look for any options to reinterpret a loaded raster file as elevation rather than as image data. 

  • MykleMykle Global Mapper User Posts: 451Trusted User
    I have updated my copy of Global Mapper (Global Mapper v18.2.2 (b081617) [64-bit] - REGISTERED)
    I have downloaded wide-area bathymetry from British Oceanographic Data Centre covering the Gulf of Mexico (requires establishing a free account).
    I have loaded the BODC and BOEM data sets (the file created from the BOEM files by exporting to GM GMG format is much faster to work with).
    The two data sets overlay nicely.  The BOEM is of higher resolution, but the BODC is not much more coarse. 

  • JSLJSL Global Mapper User Posts: 434Trusted User
    I did the same thing, but this is what I get when I bring n the GEBCO GMG first, and then the BOEM GMG for the eastern Gulf:

    aa.jpg 73.6K
  • MykleMykle Global Mapper User Posts: 451Trusted User
    Draw a line across the BOEM data, extending outside into the GEBCO data, and show us the profile for that line. 
    What we are interested in is the elevation values of both data sets and the boundary (white) area. 
    It is looking like the BOEM is still a raster image and not elevations.  We also note the difference in the shading of both data sets, like they aren't using the same units (you can check the metadata for the elevation specs). 
  • JSLJSL Global Mapper User Posts: 434Trusted User
    I figured out what the problem was--even though I thought I had downloaded the meters version of the BOEM DEM, it was using feet.  It got fixed when I changed Feet to Meters in the Elevation Units Interpretation box under the "Alter Elevations Tab."

    Thanks for all the help!
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