Rajsekar,
One of the reason that the -full image is larger than the -small one is
that the former includes > 500 MB of geodetic grids, whereas the later
only includes very basic ones. (I've just amended
https://github.com/OSGeo/gdal/blob/master/docker/README.md to make it
clearer)
As you do a WGS 84 to NAD83 transformation, the ubuntu-full image can
use the HPGN grids, and in particular the Pennsylviana grid
$ echo -77.75 40.16666666666666 | PROJ_DEBUG=2 bin/cs2cs
'+init=epsg:4326' '+to' '+init=epsg:2271'
[ ... snip ... ]
Using coordinate operation axis order change (2D) + Inverse of NAD83 to
WGS 84 (57) + SPCS83 Pennsylvania North zone (US Survey feet)
pj_open_lib(us_noaa_pahpgn.tif): call
fopen(/home/even/proj/PROJ-data/us_noaa/us_noaa_pahpgn.tif) - succeeded
1968499.39 -0.15 0.00
Whereas the -small image has no such grid and will assume that NAD83 ==
WGS84.
In theory the result with the -full image should be more accurate, but
it depends if somewhere in your workflow someone has not assumed that
NAD83 == WGS84 ...
Anyway WGS 84 is literaly a moving and somewhat fuzzy target, so you
shouldn't expect in general results to have an accuracy better than ~ 2m
in any transformation involving it.
Even
Le 10/01/2022 à 15:21, Rajsekar Manokaran a écrit :
Hi gdal devs and users,
I came across a discrepancy in proj. values between ubuntu-small and
ubuntu-full docker images even for the same gdal version tag. I tried
to convert a few coords from epsg:4326 to epsg:2271. The coords are:
Input:
-77.75 40.16666666666666 # this is the center meridian, lat of 2271
-75.18094433657971 40.25904959199275 # another sample point
The outputs of all ubuntu-small-* images seem to be correct, but the
ubuntu-full-* images have about 1 feet error (2271 uses US_SURVEY_FEET
units).
Here's how I run it:
$ docker run --rm -i osgeo/gdal:"$tag" cs2cs '+init=epsg:4326' '+to'
'+init=epsg:2271' < input
where tag is the docker tag: "ubuntu-full-3.2.2", or
"ubuntu-small-3.4.1", etc.
Here are the outputs for the ubuntu-full images:
output-ubuntu-full-3.2.2
1968499.39 -0.15 0.00
2685535.97 44295.90 0.00
output-ubuntu-full-3.3.3
1968499.39 -0.15 0.00
2685535.97 44295.90 0.00
output-ubuntu-full-3.4.1
1968499.39 -0.15 0.00
2685535.97 44295.90 0.00
Here are the outputs for the small images:
output-ubuntu-small-3.2.2
1968500.00 -0.00 0.00
2685536.18 44296.27 0.00
output-ubuntu-small-3.3.3
1968500.00 -0.00 0.00
2685536.18 44296.27 0.00
output-ubuntu-small-3.4.1
1968500.00 -0.00 0.00
2685536.18 44296.27 0.00
The first input is the center and so should translate to "(false east,
false north)". Going by that, the small versions seem to be more
correct. Any idea why this difference in the values?
-
Rajsekar
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