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"]] 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 _______________________________________________ grass-user mailing list [email protected] https://lists.osgeo.org/mailman/listinfo/grass-user
