Map projection used for CADRG polar areas?

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Map projection used for CADRG polar areas?

Mikael Rittri

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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Re: Map projection used for CADRG polar areas?

Even Rouault-2
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
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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Re: Map projection used for CADRG polar areas?

Mikael Rittri
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#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
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

--
Spatialys - Geospatial professional services
http://www.spatialys.com
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