[Proj] Equal Arc Projection, GeoTIFF, and GDAL/Proj4
frederik at remote.org
Sat Mar 1 17:05:58 EST 2014
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
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:
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
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
Thank you for any insights you might have on this.
Frederik Ramm ## eMail frederik at remote.org ## N49°00'09" E008°23'33"
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