[PROJ] [EXTERNAL] Re: package 'geosphere' -- transforming spherical-based geographic coordinates to WGS84

[Douglas, David C]

Even,

Thank you for your prompt and detailed reply.  You've given us a lot to digest, but 2 things seem to be apparent: 1) use EPSG:XXXX notation to specify proj parameters, and 2) be glad we are not attempting any survey-precision applications with those 2 map projections.  In fact, the data sets we analyze (which are distributed in those projections) are Arctic-wide, coarse pixel-resolution rasters. Notwithstanding a desire to be as spatially accurate as possible, perhaps the imprecision of transforming coordinates between these 2 projections is inconsequential with respect to the broad scales across which we are quantifying spatial information.

Thanks again.
Be well.

David


________________________________
From: Even Rouault <even.rouault at spatialys.com>
Sent: Wednesday, April 22, 2020 11:24
To: Douglas, David C <ddouglas at usgs.gov>
Cc: proj at lists.osgeo.org <proj at lists.osgeo.org>; Fischbach, Anthony S <afischbach at usgs.gov>; r.hijmans at gmail.com <r.hijmans at gmail.com>; Durner, George M <gdurner at usgs.gov>
Subject: Re: [EXTERNAL] Re: [PROJ] package 'geosphere' -- transforming spherical-based geographic coordinates to WGS84


David,



That could be the matter of days of discussions. I'll try to make it short :-)



- the +proj= syntax you mention for EPSG definitions has been deprecated since PROJ 6, since it was a rather imperfect way of capturing datum expressions. With GDAL 3 & PROJ >= 6, we encourage strongly to use directly EPSG:XXXX syntax instead of deprecated PROJ.4 strings



- ... but for the code you mention, the datum behind EPSG:3408 is datum EPSG:6053 "Not specified (based on International 1924 Authalic Sphere)" and the one for EPSG:3411 is EPSG:6054 "Not specified (based on Hughes 1980 ellipsoid)". So even with PROJ 6, there's no hope to get any significant datum transformation



- EPSG:3408 has been recently deprecated by EPSG in favor of EPSG:6931, and EPSG:3411 in favor of EPSG:3413. Both use the WGS84 datum, and are thus not equivalent to their non-deprecated code.



- With PROJ.4 syntax, there's no toXYZ qualifier, only towgs84. WGS84 in PROJ.4 era was used as the universal pivot when transforming between 2 datums (sometimes it was abused a bit to transform directly between a CRS A and B when A or B was sufficiently close to GRS80/WGS84 for the approximation to be valid). Starting with PROJ 6, one can do direct transformations, but with your rather unspecified datums, I've no idea what would be the right way of transforming them...



Even



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