[Proj] Map projection used for CADRG polar areas?
Mikael.Rittri at carmenta.com
Thu Oct 23 08:01:09 EST 2014
> 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
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.
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?
Based on my code in GDAL, the answer would be Azimuthal Equidistant :
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
Le mercredi 22 octobre 2014 11:34:52, Mikael Rittri a écrit :
> 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
Spatialys - Geospatial professional services
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