Global Mapper Pro

Can't maintain file dimensions

anticus
anticus Global Mapper User
edited April 2008 in Technical Support
I have a WGS 84 datum / UTM projected raster file that I need to export to geotiff.
I need to accurately maintain the original proportions of the file (22.5" x 29").
When I export to UTM using the defaults the length and width expand to 113" x 144" .
Why is this happening and how can I control the proportions ?
Both exported files are 300 dpi geotiffs.

Thanks for any help !

Comments

  • global_mapper
    global_mapper Administrator
    edited March 2008
    By default Global Mapper does not write out any DPI information to exported maps as there is no way to automatically determine a value for that based on spatial information alone. However, when you export a GeoTIFF file, there is an option on the export dialog to manually specify a DPI, so try specifying a DPI of 300 there and see if that fixes your problem.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • anticus
    anticus Global Mapper User
    edited April 2008
    I tried exporting to 300 dpi but the output files are still five inches too wide. The original map sheet is 22.5" H x 29" W.
    The files were originally georeferenced in GM at geo lat lon, which streched the pixels horizontally. Dimensions 27.18" W x 29" H
    I reprojected the maps to UTM ( the original projection of the map) and exported at 300 dpi.
    The file dimensions are 27.183" W x 34.683. The proportions are now correct, but the chart is oversize by 5" in both dimensions. I have a large number of charts like this, so a fix is preferable to starting over from scratch. Can you suggest a workaround ?

    Thanks
  • global_mapper
    global_mapper Administrator
    edited April 2008
    The multiple changes of projections will likely have increased the spatial resolution in the X direction, which indirectly affects the DPI and size in inches used in other programs. When you do your export your file, multiply the X sample spacing by 1.208 (this is the ratio of 27.18" to 22.5"). This should reduce the X size of the image back down to the number of pixels that would result in the desired size. Also make sure to un-check the box to use square pixels when exporting so that you can get the desired ratio with this change.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • anticus
    anticus Global Mapper User
    edited April 2008
    This calc gives me the correct overall page size for the georeferenced file, unfortunately the chart area inside the neat line is now .161-.170" too small (W) and .193" to short (L) compared to the original map sheet.
  • global_mapper
    global_mapper Administrator
    edited April 2008
    It sounds like perhaps your map was expanded a bit during the multiple reprojections that you performed, like due to the rotation of the bounding boxes causing the export bounds to be enlarged. Use the Export Bounds tab of the Configuration dialog to clamp your export to just what you actually want in the output file to fix this (you'll need to scale your sample spacing back down to get the same printed size in inches since you will be exporting less pixels in this case).

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • anticus
    anticus Global Mapper User
    edited April 2008
    Here's an interesting wrinkle-I georeferenced again ,this time referencing the dumb tif to the original projection (no reprojection, no stretch). Resolution held constant at 300dpi.The amount of difference in dimensions (of the area inside the neat line) between the original map sheet and the new georeferenced tiff file was almost exactly the same as before :
    .162 " less than the original map sheet !?!
  • global_mapper
    global_mapper Administrator
    edited April 2008
    If you load both the original TIF file and the new one in Global Mapper, then open the Overlay Control Center, select each and press the Metadata button, what is reported as the NUM_COLUMNS and NUM_ROWS values for each file? Assuming the DPI value stored in each file is the same (i.e. 300), then I would expect those values to be different if you are getting different sizes for the printed image in another non-spatial application.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • anticus
    anticus Global Mapper User
    edited April 2008
    The files compare this way:

    original tif NUM COLUMNS= 6750 NUM ROWS = 8700 (not georeferenced)

    UTM geotif NUM COLUMNS= 6830 NUM ROWS = 8700
  • global_mapper
    global_mapper Administrator
    edited April 2008
    It looks like the NUM_COLUMNS has changed. What options are you choosing when doing your GeoTIFF export. Make sure to uncheck the box to force square pixels, otherwise it is possible that either your X or Y sample spacing will be reduced to match the other and potentially cause a different size that the input if your georeferencing resulted in non-square pixels (which is extremely common).

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • anticus
    anticus Global Mapper User
    edited April 2008
    Would a higher order polynomial produce a more accurate result ?
    For example, does GM allow a third order polynomial ?
    Is there a general rule for sample spacing ? What affect does doubling or halving the sample have on file dimensions ?
  • global_mapper
    global_mapper Administrator
    edited April 2008
    A higher order polynomial would not make any difference for this situation. Once you get beyond the 2nd order polynomial, Global Mapper automatically switches to the triangulated rectification method, which is a piece-wise affine transformation that exactly preserves all control points regardless of how many there are.

    Sample spacing has a linear relationship with how many pixels are in the output image, and thus on the printed file dimensions assuming that the DPI value is not changed. For example, if you have an image that is originally 6000 pixels wide with each pixel being 1 meter across (this means the image represents 6km of horizontal distance) and you double the sample spacing to 2 meters per pixel, the output result would be 3000 pixels wide. If you maintained a DPI of 300 throughout this price, the size in inches of the image would go from 20" across (6000 pixels / 300 dpi) to 10" across (3000 pixels / 300 dpi). The same relationship applies in the Y direction.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • anticus
    anticus Global Mapper User
    edited April 2008
    Mike,

    On the reprojection itself, what kind of model does Global Mapper use to make the transforamtion from Geo lat/lon to UTM ? Does it make a calculation similar to GEOTRANS . or some other math model ?
  • global_mapper
    global_mapper Administrator
    edited April 2008
    For UTM projections Global Mapper uses the math for the Transverse Mercator projection that defines the particular UTM zone. The actual math is done using the USGS GCTPC library. The formulas should match what every other piece of software uses as well, including GeoTrans.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
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