[Proj] Equal Arc Projection, GeoTIFF, and GDAL/Proj4

Charles Karney charles.karney at sri.com
Sat Mar 1 20:08:53 EST 2014

Equal arc just means equal arc increments in latitude and longitude.  In
other words the projection is just ordinary longitude and latitude.
Here is gdalinfo on a sample file.  This is for a CADRG (compressed arc
digitized raster graphics) map.  To comply with ARC specifications, it
may be necessary to pick the size, resolution, and tiling of the imagery
specifically.  However, the projection is simple.


     Driver: NITF/National Imagery Transmission Format
     Files: 00rwk043.ja1
     Size is 1536, 1536
     Coordinate System is:
     GEOGCS["WGS 84",
	    SPHEROID["WGS 84",6378137,298.257223563,
     Origin = (48.283562710984143,24.333652924256953)
     Pixel Size = (0.000365164892236,-0.000337068552253)
       NITF_SERIES_NAME=Joint Operation Graphic - Air
     Image Structure Metadata:
     Corner Coordinates:
     Upper Left  (  48.2835627,  24.3336529) ( 48d17' 0.83"E, 24d20' 1.15"N)
     Lower Left  (  48.2835627,  23.8159156) ( 48d17' 0.83"E, 23d48'57.30"N)
     Upper Right (  48.8444560,  24.3336529) ( 48d50'40.04"E, 24d20' 1.15"N)
     Lower Right (  48.8444560,  23.8159156) ( 48d50'40.04"E, 23d48'57.30"N)
     Center      (  48.5640093,  24.0747843) ( 48d33'50.43"E, 24d 4'29.22"N)
     Band 1 Block=256x256 Type=Byte, ColorInterp=Palette
       NoData Value=216
       Color Table (RGB with 217 entries)
	0: 247,242,105,255
       216: 0,0,0,0

On 03/01/2014 05:05 PM, Frederik Ramm wrote:
> Hi,
>     I've been pointed to this list after posting my question to the
> German FOSSGIS-talk list (FOSSGIS is the German OSGeo chapter).
> Among the things I do for a living is making raster images from
> OpenStreetMap data, usually through the "Mapnik" renderer which has
> Proj4 support built-in. Someone recently asked whether I could make them
> an image in the "Equal Arc" projection in GeoTIFF format, and I naively
> said "sure, give me the EPSG code and I'll work it out". I had never
> heard the term "Equal Arc" and I somewhat assumed that the inquirer
> probably meant one of the common "Equal Area" projections.
> But I was wrong; what the client-in-spe is looking for is indeed the
> "Equal Arc" projection that is defined here
> https://portal.dgiwg.org/files/?artifact_id=3923
> and which, as far as I can see, has neither an EPSG code nor anything
> remotely machine readable like a WKT or Proj4 projection description.
> Even so, it seems to be widely used in aviation, and it seems to be an
> integral part of so-called ADRG files. These ADRG files can even be
> "experimentally" written by GDAL:
> http://www.gdal.org/frmt_various.html
> but of course these aren't GeoTIFFs then. I wonder:
> 1. Is it at all technically possible to have a GeoTIFF in Equal Arc
> projection, given that I could not find any represenation of that
> projection suitable for including in a GeoTIFF header?
> 2. If the answer to (1) is "yes", then can I create such a GeoTIFF with
> GDAL, or possibly even by passing a clever Proj init string to the
> Mapnik renderer?
> 3. If the answer to (2) is "no", then is this just an issue where
> someone would have to spend some time adding support to Proj (with any
> luck I could get the client-in-spe to fund this work), or are there
> fundamental reasons barring us from doing that?
> My internet research regarding "Equal Arc" was unsatisfying; the name
> "FalconView" appeared a number of times in conjunction with this
> projection.
> Thank you for any insights you might have on this.
> Bye
> Frederik

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