[GRASS-user] Projection errors with Aster GDEM and SHP Files

[Markus Metz]

On Wed, Jun 28, 2017 at 4:02 PM, preiaen <nils.hempel at gmx.de> wrote:
>
> Dear Markus,
>
> thank you for your answer. I did as you suggested and below are the
results.
> As far as I can understand it, it looks fine, however the error remains.
>
> *For the shapefile: *
>
> *g.proj -g:*
>
> -PROJ_INFO-------------------------------------------------
> name       : WGS 84 / Pseudo-Mercator
> datum      : wgs84
> ellps      : wgs84
> proj       : merc
> lat_ts     : 0.0
> lon_0      : 0.0
> x_0        : 0.0
> y_0        : 0
> k          : 1.0
> wktext     : defined
> no_defs    : defined
> -PROJ_EPSG-------------------------------------------------
> epsg       : 3857
> -PROJ_UNITS------------------------------------------------
> unit       : meter
> units      : meters
> meters     : 1
>
> *ogrinfo:*
>
> PROJCS["WGS 84 / Pseudo-Mercator",
>     GEOGCS["WGS 84",
>         DATUM["WGS_1984",
>             SPHEROID["WGS 84",6378137,298.257223563,
>                 AUTHORITY["EPSG","7030"]],
>             AUTHORITY["EPSG","6326"]],
>         PRIMEM["Greenwich",0,
>             AUTHORITY["EPSG","8901"]],
>         UNIT["degree",0.0174532925199433,
>             AUTHORITY["EPSG","9122"]],
>         AUTHORITY["EPSG","4326"]],
>     PROJECTION["Mercator_1SP"],
>     PARAMETER["central_meridian",0],
>     PARAMETER["scale_factor",1],
>     PARAMETER["false_easting",0],
>     PARAMETER["false_northing",0],
>     UNIT["metre",1,
>         AUTHORITY["EPSG","9001"]],
>     AXIS["X",EAST],
>     AXIS["Y",NORTH],

So far, all looks fine, but

>     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"],

a (equatorial radius) and b (polar radius) are the same, i.e. this is a
sphere, not an ellipsoid like wgs84. This can make a difference. You can
try to use the proj4 term
+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

to create a new location, then force import the shapefile with v.in.ogr -o,
then reproject and check if it is the geolocation is correct.

>     AUTHORITY["EPSG","3857"]]

See
http://epsg.io/3857

"Remarks: 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."

Markus M

>
> *For the DEM:*
>
> g.proj -p
> -PROJ_INFO-------------------------------------------------
> name       : WGS 84
> datum      : wgs84
> ellps      : wgs84
> proj       : ll
> no_defs    : defined
> -PROJ_UNITS------------------------------------------------
> unit       : degree
> units      : degrees
> meters     : 1.0
>
> *gdalinfo:*
>
> GEOGCS["WGS 84",
>     DATUM["WGS_1984",
>         SPHEROID["WGS 84",6378137,298.257223563,
>             AUTHORITY["EPSG","7030"]],
>         AUTHORITY["EPSG","6326"]],
>     PRIMEM["Greenwich",0],
>     UNIT["degree",0.0174532925199433],
>     AUTHORITY["EPSG","4326"]]
>
>
> Nils
>
>
>
> --
> View this message in context:
http://osgeo-org.1560.x6.nabble.com/Projection-errors-with-Aster-GDEM-and-SHP-Files-tp5324655p5325865.html
> Sent from the Grass - Users mailing list archive at Nabble.com.
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