[Proj] Azimuthal Equidistant question

Joseph White JWhite at terragotech.com
Fri Sep 28 08:33:59 EST 2012

Hi, all,

I'm trying to track down an issue I'm seeing with Azimuthal Equidistant
values when compared to ESRI.  The ESRI coordinate system I'm using is
World Azimuthal Equidistant.  I'm using some sample data that is in the
Tucson AZ area.


The coordinate strings we're passing into PROJ to set up the transform

Src: +proj=aeqd +lat_0=0 +lon_0=0  +x_0=0 +y_0=0 +datum=WGS84 +units=m

Dest: +proj=longlat +datum=WGS84 +no_defs


I pass in the following coordinates (Easting,Northing):

1.        -9939304, 6673447

2.       -9930938, 6684888


I get this result:

1.        -69d 0' 0.12", 32d 15' 14.159"

2.       -68d 59' 24.215", 32d 19' 5.934"

Needless to say, this isn't anywhere near Tucson, and is about 31-32
degrees off longitudinally.


When I try using a sphere (I've tried both semi major axis and
equal-area), the values are much closer, but still off.  

Src: +proj=aeqd +lat_0=0 +lon_0=0  +x_0=0 +y_0=0 +a=6378137.0
+b=6378137.0 +units=m +no_defs

Dest: +proj=longlat +a=6378137.0 +b=6378137.0 +no_defs


1.        -110d 50' 50.416", 32d 6' 24.361"

2.       -110d 51' 24.032", 32d 10' 15.985"


Src: +proj=aeqd +lat_0=0 +lon_0=0  +x_0=0 +y_0=0 +a=6371007.1809
+b=6371007.1809 +units=m +no_defs

Dest: +proj=longlat +a=6371007.1809 +b=6371007.1809 +no_defs


1.        -110d 59' 11.622", 32d 4' 58.135"

2.       -110d 59' 45.523", 32d' 8' 49.575"


Does anyone out there have any idea why this is happening?  I'm at my
wits end to explain it.




Joe White



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