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:

[hidden email]]

Sent: Wednesday, October 22, 2014 11:54 AM

To:

[hidden email]
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#L1005But 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

of CADRG.

Even

Le mercredi 22 octobre 2014 11:34:52, Mikael Rittri a écrit :

> Hello,

> I am reading the specification for CADRG,

>

http://earth-info.nga.mil/publications/specs/printed/89038/89038_CADRG.pdf> . 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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