I have a map in the form of .bil & .hdr using WGS84. I'd like to convert this into another map using a 2D projection while keeping the lat/long information. What I'd really like with this new map is to have a consistent distance/pixel. With the WGS84, the distance/pixel changed from top/bottom and left/right. The map size is about 100 miles. Here are my question.

  1. Is this possible to do this to a level of accuracy of +/- 100 ft.
  2. What would be a good 2D projection? I have seen people recommending not using mercator but I dont know what my other options are.
  3. What is the command to transform this using gdal. I think this is it but the # of options were a bit overwhelming and I wasnt sure if I was doing this right.

    gdalwarp -t_srs '+proj=merc +datum=WGS84' input_wgs84.bil out_merc.bil
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  • You can't have both. Either it's geographic (lat/lon) or it's projected (feet, metres, inches..). When you view geographic raster in projected coordinates the width (and height) of the pixels changes in accordance with your projection. Why do you need both? depending on your software you can get graticules overlaid if that's what you're after, you can determine your geographic coordinate from your projected... what are you trying to achieve here? – Michael Stimson May 13 '15 at 0:34
  • i'm using this for flight software. so the inputs will be in lat/long. the trajectory planner likes square grids in x, y coordinates. when i try to window a block that is 10 miles x 10 miles, the window isnt rectangular due to the wgs84. – jti107 May 13 '15 at 5:04
  • Mercator might be your best bet but don't count on distances being very accurate. Are you writing the software or using it? When I did a similar exercise (in reverse) I used SRTM in WGS84 and clipped to the extent of the flight path in DD then projected it to the appropriate UTM Zone to calculate distances.. I would suggest to go bigger than you want (11 by 11 miles) clip, project and reclip to keep it neat if you like... but then again I was writing the software so I was in charge of what it did and had freedom to decide, it's a bit different if you're just using someone else's program. – Michael Stimson May 13 '15 at 5:33

If you're most interested in maintaining distances, you could use a customized azimuthal equidistant with the center/origin at the center of your area of interest. Distances from the center point only would be correct.

Depending on the shape of your 100 (square?) mile area, you might be able to use another projection that maintains a different trait like areas or shapes and still not have distances that are too distorted. An area of interest that it more east-west could be mapped with a conic projection like Albers (equal area) or Lambert (conformal/shape), while a north-south extent could use transverse Mercator (conformal) or Lambert azimuthal equal area.

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