[PROJ] Big grid files

[Even Rouault]

> During the summit I asked about the interpolation method. The metadata 
> says that it should be biquadratic. The answer was: "for the 
> undulation there is almost no difference. But for other magnitudes, 
> like vertical deflection, it is more noticeable. However, to match our 
> values you have to do biquadratic interpolation"
> Do we have biquadratic intepolation for the geoid models?
If you use the generalized +proj=gridshift method, yes, with the 
+interpolation=biquadratic flag: 
https://proj.org/en/stable/operations/transformations/gridshift.html#cmdoption-arg-interpolation
>
> I would be very surprised that they release something at a different 
> resolution. Some people asked for a GeoTIFF, and they were reluctant 
> (but not completely oposite). They want to minimize the different 
> formats and file to deal with (one ggxf for everything)
>
> But there are other problems:
>  - velocities: the full model, considering time evolution, has a 
> velocity component. N_t = N_2020 + (t-t0) * v. The resolution of that 
> velocity is different if I checked correctly. And we do not have such 
> a model in PROJ. (It will appear in EPSG at some point)

For difference resolution per file, we already have such case with 
us_noaa_nadcon5_nad83_2007_nad83_2011_alaska.tif:

$ gdalinfo 
GTIFF_DIR:1:/home/even/proj/proj-data-1.21/us_noaa_nadcon5_nad83_2007_nad83_2011_alaska.tif
[...]
Origin = (171.958333333333343,73.041666666666671)
Pixel Size = (0.083333333333333,-0.083333333333333)
[...]
Band 1 Block=256x256 Type=Float32, ColorInterp=Gray
   Description = latitude_offset
   Unit Type: arc-second
Band 2 Block=256x256 Type=Float32, ColorInterp=Undefined
   Description = longitude_offset
   Unit Type: arc-second
   Metadata:
     positive_value=east

$ gdalinfo 
GTIFF_DIR:2:/home/even/proj/proj-data-1.21/us_noaa_nadcon5_nad83_2007_nad83_2011_alaska.tif
[...]
Origin = (171.875000000000000,73.125000000000000)
Pixel Size = (0.250000000000000,-0.250000000000000)
[...]
Band 1 Block=241x93 Type=Float32, ColorInterp=Gray
   Description = ellipsoidal_height_offset
   Unit Type: metre

> One more thing: I do not expect them publishing the intra frame 
> deformation model (IFDM2022). Every time they mentioned it they added 
> "Danger!". That leave a feeling of "incompleteness" among the 
> audience, as that model is needed for some time dependant 
> transformations (like bringing measurements to the frame epoch, 
> 2020.0). They will offer it in their webpage somehow.

What about NAD83(2011) to NATRF2022: did they mention something ? 
Currently PROJ uses NAD83(2011)->ITRF2020 (EPSG:10334) and 
ITRF2020->NATRF2022 (using Euler Pole Rotation).

-- 
http://www.spatialys.com
My software is free, but my time generally not.
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