On Thu, Aug 31, 2017 at 5:59 AM, Anna Petrášová <[email protected]> wrote: > > On Wed, Aug 30, 2017 at 4:46 PM, Markus Metz > <[email protected]> wrote: > > > > > > On Mon, Aug 28, 2017 at 9:11 PM, Jeshua Lacock <[email protected]> wrote: > >> > >> > >> > On Aug 27, 2017, at 8:54 PM, Anna Petrášová <[email protected]> > >> > wrote: > >> > > >> > On Tue, Aug 22, 2017 at 7:46 PM, Jeshua Lacock <[email protected]> > >> > 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. 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... 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 > >> [email protected] > >> https://lists.osgeo.org/mailman/listinfo/grass-user
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