# [Proj] Map projection used for CADRG polar areas?

Mikael Rittri Mikael.Rittri at carmenta.com
Thu Oct 23 08:01:09 EST 2014

```Thanks, Even.

> Did you check if the maths given in 30.5.1 "North polar region"
> are consistant with an aeqd method ?

Good idea. The math in 30.5.1 is definitely a spherical aeqd method.

> my experience with data showed that the 4
> corner coordinates reprojected from long/lat to aeqd
> form a rectangle. But that might also be true of polar
> stereographic...

I found some polar sample data, and the corner coordinates
do not form a rectangle if I try to use polar stereographic.
Almost, but not quite - I found differences of about 1 km.

So I am now certain that the mention of "stereographic" is
just a typo in the document.

Mikael

-----Original Message-----
From: Even Rouault [mailto:even.rouault at spatialys.com]
Sent: Wednesday, October 22, 2014 11:54 AM
To: proj at lists.maptools.org
Cc: Mikael Rittri
Subject: Re: [Proj] Map projection used for CADRG polar areas?

Mikael,

Based on my code in GDAL, the answer would be Azimuthal Equidistant :
https://github.com/OSGeo/gdal/blob/trunk/gdal/frmts/nitf/nitfdataset.cpp#L1005

But it is perhaps wrong. However, my experience with data showed that the 4
corner coordinates reprojected from long/lat to aeqd form a rectangle. But
that might also be true of polar stereographic...

Did you check if the maths given in 30.5.1 "North polar region" are consistant
with an aeqd method ?

Another clue is that the ASRP specification
(https://www.dgiwg.org/digest/documents/asrp_1_2.pdf), quite close to CADRG
regarding the ARC system, mentions : "3.2.3 Polar zones are square arrays of
tiles overlaid mathematically on a polar azimuthal equidistant projection".
And its maths at "B.2.2 North Polar Case" look very much the same as the one

Even

Le mercredi 22 octobre 2014 11:34:52, Mikael Rittri a écrit :
> Hello,
> I am reading the specification for CADRG,
> . But I am confused about the polar areas.
>
> Section 3.5.5 says: "CADRG shall use a polar stereographic projection".
>
> And Section 3.5.6 says "Polar frames are defined in polar stereographic
> projection and non-polar frames are in the ARC projection."
>
> But Section 3.6.2 says: "Pixel coordinates in the polar zones are
> proportional to rectangular coordinates of the Azimuthal
> Equidistant projection, polar aspect, spherical form (as
> defined in Map Projections-A Working Manual, page 191)."
>
> And section 30.1 says: "Polar zone equations are based on
> the Azimuthal Equidistant projection, polar aspect,
> spherical form."
>
> Since Polar Stereographic is not the same thing as Azimuthal Equidistant
> in polar aspect, I assume that one of the two alternatives is just a typo
> (unless I am missing something). But which one is the typo of
> "Stereographic" and "Azimuthal Equidistant"? Does anyone know?
>
> Best regards,
>
> Mikael Rittri
> Carmenta
> Sweden
> http://www.carmenta.com

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