On 4/13/2024 1:26 PM, Javier Jimenez Shaw wrote:
On Sat, 13 Apr 2024 at 17:35, Stephen Woodbridge via gdal-dev
<gdal-dev@lists.osgeo.org> wrote:
Thanks, this is NOT the standard Web Mercator projection. I am
aware of EPSG:90013 and EPSG:3857. This projection is used with
HYCOM data that I have extracted into geotif files so that I can
accurately project that onto EPSG:3857. It took some fiddling with
the values to get to overlay visually correctly. HYCOM data is
weird in that it uses two different projections based on if the
data is above or below some latitude.
I found something like that in the Internet. But I was not sure it was
the right one https://polar.ncep.noaa.gov/global/about/ It didn't
specify the projections, just a short description as Arctic bi-polar
patch north of 47N, and Mercator south of it.
Yes this what is happening with the HYCOM data. The projection
definition in PROJ4 worked quite well for my case of web mapping in
mapserver.
I do not know if you can specify a projection "in parts" respect to
parallel 47. Maybe in WKT2.
Where did you found those parameters for the datum and projection?
They are quite strange.
For the ellipsoid there are a few already with a similar radius:
SELECT * from ellipsoid where semi_major_axis BETWEEN 6371000 and 6371010
EPSG 7035 Sphere
PROJ EARTH 6371000.0 EPSG 9001
6371000.0 1
EPSG 7048 GRS 1980 Authalic Sphere
PROJ EARTH 6371007.0 EPSG 9001
6371007.0 0
ESRI 107047 Sphere_GRS_1980_Mean_Radius Sphere with mean radius
based on GRS80 PROJ EARTH 6371008.7714 EPSG 9001 0.0
0
And datums
SELECT * from geodetic_datum where ellipsoid_code in (7035, 7048, 107047)
EPSG 6035 Not specified (based on Authalic Sphere)
EPSG 7035 EPSG 8901
1
EPSG 6047 Not specified (based on GRS 1980 Authalic Sphere)
EPSG 7048 EPSG 8901
1
ESRI 106047 D_Sphere_GRS_1980_Mean_Radius GRS 1980 Mean Radius
Sphere ESRI 107047 EPSG 8901
0
and geographic crs
SELECT * from geodetic_crs where datum_code in (6035, 6047, 106047)
EPSG 4035 Unknown datum based upon the Authalic Sphere
geographic 2D EPSG 6402 EPSG 6035
1
EPSG 4047 Unspecified datum based upon the GRS 1980 Authalic Sphere
geographic 2D EPSG 6422 EPSG 6047
1
ESRI 104047 GCS_Sphere_GRS_1980_Mean_Radius
geographic 2D EPSG 6422 ESRI 106047
0
See that EPSG ones are deprecated. (surprisingly the ellipsoid
EPSG:7048 is not deprecated, but the datum that uses it is deprecated).
On 4/13/2024 4:19 AM, Javier Jimenez Shaw via gdal-dev wrote:
If what you need is really EPSG:3857, yes, use it.
However I have seen strange parameters on your projection. The
radius of the sphere is the "average" 3671 km, and you set a
false easting and northing of just 4.4 km. Is that trying to
correct the radius of the sphere? I do not know why you need that.
Bas, are they really equivalent?
In proj you can convert to WKT1 (see that I added +type=crs):
projinfo "+proj=merc +a=6371001 +b=6371001 +lat_ts=0.0 +lon_0=0.0
+x_0=-4448 +y_0=-4448 +k=1.0 +units=m +over +nadgrids=@null
+no_defs +type=crs" -o wkt1_gdal
Ok I get this adding by +type=crs but how do I add it to the proj
database so I can access it referencing it by something like
EPSG:90014?
Is a WKT string enough?
PROJCS["unknown",
GEOGCS["unknown",
DATUM["unknown using nadgrids=@null",
SPHEROID["unknown",6371001,0]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]]],
PROJECTION["Mercator_1SP"],
PARAMETER["central_meridian",0],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",-4448],
PARAMETER["false_northing",-4448],
UNIT["metre",1,
AUTHORITY["EPSG","9001"]],
AXIS["Easting",EAST],
AXIS["Northing",NORTH],
EXTENSION["PROJ4","+proj=merc +a=6371001 +b=6371001 +lat_ts=0.0
+lon_0=0.0 +x_0=-4448 +y_0=-4448 +k=1.0 +units=m +over +nadgrids=@null
+no_defs"]]
(generated with the projinfo line from prev email) Using a geographic
CRS as described above will make it nicer, and probably more compatible.
You can use it in QGIS for instance. I am not sure how does it behave
in a GeoTIFF, as it has some special tags. You can try to generate the
geotiff with gdal with that WKT, and see that happens.
About adding to proj.db as EPSG:900914, I am not sure of all the
steps. But I would say that you have to create a datum and a
geographic crs (or use one of the above), in addition to the
projection (conversion, parameters, etc) and the projected crs. And
then "compile" the database.
Is it worth it?
In case you modify proj.db, instead of using EPSG, you can create your
own authority, or use "PROJ" authority. It will be cleaner.
Ok, thanks, I think I can work with this information. I appreciate the
help.
I'm also considering rewriting the extraction utility to totally avoid
using this projection. This might be the better choice long term.
Thanks,
-Steve
Thanks,
-Steve
On Sat, 13 Apr 2024 at 06:17, Sebastiaan Couwenberg via gdal-dev
<gdal-dev@lists.osgeo.org> wrote:
On 4/12/24 11:24 PM, Stephen Woodbridge via gdal-dev wrote:
> and was able to access it in gdal, mapserver, postgis, etc
with
> "EPSG:900914"
I used to do that too, but switched to EPSG:3857 its
non-deprecated
equivalent. I would recommend that instead of trying to keep
using a
non-standard projection.
Kind Regards,
Bas
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