Different results when exporting Global Mapper Grids.

OkawaOkawa Global Mapper UserPosts: 10Trusted User
edited December 2013 in Bug Report
Hi there, I have imported a bathymetric csv file into Global Mapper 14 (64 bit), and then exported it as a Global Mapper Grid, and have consistently been getting some odd results. I have been exporting as various different box averages to try to get the best compromise between smoothing out the effects of the swell etc, and keeping as much detail as possible.
Every time I change the re-sampling settings in the export settings box, the resulting .GMG file is very small (about 600kb from a 425mb csv file, and the result is a very un-detailed map. But if I keep all the settings the same and export it again, the result is 25mb file, and a much better looking map. All I have to do is simply export the data twice, and I can get the results I want, but it just seems a bit odd.
I hopefully have attached some pictures to help explain.
The top level is a small section of the imported csv, using bicubic re-sampling.
The second level is from the first export, at 9x9 box average re-sampling.
The third level is exported again with the same settings (9x9 box average re-sampling and lossless compression).
Any ideas as to what is going on?
And just by the way, I have found that the results have been better looking when exporting the data as a GMG file than when I use the equivalent box averaging directly on the imported csv.
Keep up the good work,

.GMG exports.jpg


  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited December 2013
    Is the 'use lossless compression' setting the same between both runs? Also which elevation units are you selecting when exporting? The GMG compression stores the values as whole numbers in the units that you select, so if your data has detail down to 0.01m, use centimeter. If it's not that resolution you can get much better compression by using decimeters or meters for the stored units. It could be that your bump is a very small height difference such that the 'lossy' size is enough to smooth much of it away to gain better compression.

    One other thought, when you choose a Box Averager of some size the default X and Y sample spacing automatically multiplies from full resolution to whatever the multiplier is, so if it was 10m resolution data, a 9x9 box averager would kick it to 90m resolution. If you want to use the box averager to smooth but keep the 10m resolution you will need to change the X and Y sample spacing back. Now that I think about it this is almost certainly what is happening, so just restoring the original sample spacing should do the trick.

    Let me know if I can be of further assistance.


    Global Mapper Guru
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • OkawaOkawa Global Mapper User Posts: 10Trusted User
    edited December 2013
    Thanks for the reply Mike,
    That is starting to make sense now. When I export as 9x9 box average it comes out as 55x108m, and 5x5 box average comes out at 31x60 metres, etc. Then it seems that if I keep the settings the same and export again, the spacing reverts back to what must be it's original, 6x12 metres.
    The original csv file is made up from track lines, so areas close to home have very dense data, and further away the data becomes sparse, and was in Geographic lat/lon format. Don't know if any of that has any bearing or not.
    On the second export, the resulting files all end up about the same size, whether it is 3x3 or 9x9 box average,and the results get progressively smoother as the box average size goes up.
    export box.jpg
    Thanks, Michael.
Sign In or Register to comment.