[GRASS-user] Re-projected Data Position Mismatch

[Anna Petrášová]

On Thu, Aug 31, 2017 at 3:47 PM, Markus Metz
<markus.metz.giswork at gmail.com> wrote:
>
>
> On Thu, Aug 31, 2017 at 5:59 AM, Anna Petrášová <kratochanna at gmail.com>
> wrote:
>>
>> On Wed, Aug 30, 2017 at 4:46 PM, Markus Metz
>> <markus.metz.giswork at gmail.com> wrote:
>> >
>> >
>> > On Mon, Aug 28, 2017 at 9:11 PM, Jeshua Lacock <jeshua at 3dtopo.com>
>> > wrote:
>> >>
>> >>
>> >> > On Aug 27, 2017, at 8:54 PM, Anna Petrášová <kratochanna at gmail.com>
>> >> > wrote:
>> >> >
>> >> > On Tue, Aug 22, 2017 at 7:46 PM, Jeshua Lacock <jeshua at 3dtopo.com>
>> >> > wrote:
>> >> >>
>> >> >> To hopefully help troubleshoot; I just reprojected one of the raster
>> >> >> tiles (from epsg: 3857), into the source location of one of the
>> >> >> lonlat
>> >> >> vectors (reverse projections from my OP), and the datasets are
>> >> >> offset by the
>> >> >> same distances. Since the dimensions of the raster are being changed
>> >> >> (by
>> >> >> r.proj), it leads me to think it must be a datum or coordinate
>> >> >> system
>> >> >> misalignment and not a projection issue.
>> >> >
>> >> > I have the same problem, working with NAIP imagery. It is related to:
>> >> > https://trac.osgeo.org/grass/ticket/2229
>> >> >
>> >> > I have to remove nadgrids: @null from the PROJ_INFO file to be able
>> >> > to
>> >> > reproject into that location, but then it is shifted. gdalwarp helps.
>> >
>> > EPSG:3857 is problematic because it
>> > "Uses spherical development of ellipsoidal coordinates. Relative to WGS
>> > 84 /
>> > World Mercator (CRS code 3395) errors of 0.7 percent in scale and
>> > differences in northing of up to 43km in the map (equivalent to 21km on
>> > the
>> > ground) may arise."
>> >
>> > Therefore you would need to use the WKT EXTENSION PROJ4:
>> >
>> > +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0
>> > +k=1.0 +units=m +wktext  +no_defs
>> >
>> > without +nadgrids=@null
>> >
>> > In GRASS, you could use this proj4 definition to create a new location,
>> > then
>> > import the data (the -o flag might be needed), then reproject to the
>> > desired
>> > CRS.
>> >
>> > Considering this particular CRS (EPSG:3857), it is strange than gdalwarp
>> > works while GRASS reprojection does not work because GRASS uses GDAL to
>> > convert WKT to proj4, then to GRASS terminology. Some debugging is
>> > needed to
>> > figure out why within GRASS, the conversion from WKT to proj4 (using
>> > GDAL)
>> > produces wrong results.
>>
>> Thank you, yes that works. I looked at lib/proj/convert.c where I
>> assume the problem is. If I interpret it correctly it basically
>> discards +a and +b from the EXTENSION and instead picks WGS84
>> ellipsoid because of wkt
>>
>> DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]]
>>
>> But I don't really know what that extension actually means or how
>> GRASS should treat it.
>>
>>
>> This is the WKT which is processed:
>>
>>
>>
>> PROJCS["WGS_1984_Web_Mercator_Auxiliary_Sphere",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.017453292519943295]],PROJECTION["Mercator_Auxiliary_Sphere"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",0.0],PARAMETER["Standard_Parallel_1",0.0],PARAMETER["Auxiliary_Sphere_Type",0.0],UNIT["Meter",1.0],EXTENSION["PROJ4","+proj=merc
>> +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0
>> +units=m +nadgrids=@null +wktext +no_defs"]]
>
> This WKT is processed correctly, GDAL converts this with OSRExportToProj4()
> to
>
> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0
> +k=1.0 +units=m +nadgrids=@null +wktext +no_defs
>
> the only problem is +nadgrids=@null.
>
> Unfortunately, GPJ_osr_to_grass() discards any datum and/or ellipsoid info
> in the proj4 term at
> https://trac.osgeo.org/grass/browser/grass/trunk/lib/proj/convert.c#L477
>
> Datum and/or ellipsoid definitions are determined later on from the original
> OGR spatial reference, which causes problems for EPSG:3857 and possibly
> other CRS's.

 Are there actually more cases like this 3857 we need to fear?
>
> The reason for this special handling is that GRASS has its own datum and
> ellipsoid definitions in lib/gis, i.e. in datum.table, datumtransform.table,
> ellipse.table, ellipse.table.solar.system.
>
> It seems like a review of GRASS handling of GDAL CRS definitions is
> needed...
>

That sounds rather complicated. Would some workaround be an option?
Maybe checking if the datum from GDAL matches the datum from the proj4
string?

Anna

> Markus M



>>
>>
>> Anna
>>
>> >
>> > Markus M
>> >
>> >>
>> >> Hi Anna!
>> >>
>> >> Thank you very much for confirming that! I am working with the NAIP
>> >> imagery as well.  :)
>> >>
>> >> I have found that their original Geotiff assets work perfectly.
>> >>
>> >> In fact, I was very happy to stumble on to the fact that the complete
>> >> NAIP
>> >> archive (~250 terabytes) is available as a bucket drive on Amazon Web
>> >> Services (AWS). So I setup GRASS instances to process the tiles, then
>> >> download the processed, reprojected images compressed as JP2s. I am
>> >> paying
>> >> for the bandwidth and compute time, but I think its worth it for my
>> >> purposes. I’ll be able to process and download the imagery I need in
>> >> about
>> >> 60 days compared to over 300 days without AWS!
>> >>
>> >>
>> >> Cheers,
>> >>
>> >> Jeshua Lacock
>> >> Founder/Engineer
>> >> <3DTOPO.com>
>> >> GlassPrinted.com
>> >>
>> >> _______________________________________________
>> >> grass-user mailing list
>> >> grass-user at lists.osgeo.org
>> >> https://lists.osgeo.org/mailman/listinfo/grass-user
>