[gdal-dev] How do I add a projection to proj 8?
Javier Jimenez Shaw
j1 at jimenezshaw.com
Sat Apr 13 10:26:52 PDT 2024
On Sat, 13 Apr 2024 at 17:35, Stephen Woodbridge via gdal-dev <
gdal-dev at 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.
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.
> Thanks,
> -Steve
>
>
> On Sat, 13 Apr 2024 at 06:17, Sebastiaan Couwenberg via gdal-dev <
> gdal-dev at 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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