[PROJ] GPS to MSL

[Even Rouault]

Paul,

Great post.

I've noticed you point to the https://www.agisoft.com/downloads/geoids/ 
page which mentions they use the PROJ GTG format. A lot of their files 
have the same names as the ones provided by https://cdn.proj.org/ , but 
they are not exactly the same... Looking at au_ga_AUSGeoid98.tif, it 
appears they have reprocessed them to modify the TIFFTAG_COPYRIGHT 
(removing the license information) and TIFFTAG_IMAGEDESCRIPTION. Also 
adding the nodata value tag (PROJ grids only contain it when they are 
actual grid values at the nodata value). And more annoying they have 
also modified the GDAL_METADATA tag in a way it is not currently 
understood by current GDAL versions (adding the <?xml version="1.0" 
encoding="UTF-8"?> marker before the root <GDALMetadata> element, 
whereas GDAL expects the string to start directly with <GDALMetadata>), 
so the GTG specific metadata is hidden. I'm going to fix the GDAL 
GeoTIFF reader to be more robust to that.

I've also noticed that the dk_sdfe_dvr90.tif file they provide is quite 
different from the PROJ CDN: it has not the same extent and spatial 
resolution.

A bit annoying to reuse the same file names as the ones we provide but 
with different content.

Anyway this is rather for any Agisoft employee reading this email....

Even

Le 28/09/2022 à 10:06, Paul Harwood a écrit :
> This is not a question but a request.
>
> I had to work out an API to convert from GPS coordinates to Height 
> above MSL. Having made it work, I wrote a blog post about the 
> solution, since I like to document things. I have posted it here for 
> three reasons :
>
>  1 As a thank you to the team. The simplicity of the actual solution 
> shows just how good PROJ is!
>
> 2 In case you want a good laugh, and
>
> 3 more seriously, given the impressive competence on this list, I 
> would like to know if I have made any glaring (or even subtle) mistakes.
>
> Thank you
>
> https://medium.com/p/70b723767787
>
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