[GRASS-user] Incorrect re-projection from EPSG:4326 to 3857

[Markus Metz]

On Thu, Jan 4, 2018 at 9:08 AM, Helmut Kudrnovsky <hellik at web.de> wrote:
>
> Zhiyu (Drew) Li wrote
> > Hi there,
> >
> >
> >
> > I was trying to re-project a EPSG:4326 coordinate into EPSG:3857 using
> > GRASS, but I found GDAL gave me a different result.
> >
> > GRASS (7.2.2):
> >
> > Re-project this " -111.648251 40.246882" lon-lat pair into a existing
> > location with EPSG:3857 projection
> >
> > m.proj -i input=in4326.txt
> >
> > -12428626.45 4874271.67
> >
> >
> >
> > GDAL (2.2):
> >
> > gdaltransform -s_srs EPSG:4326 -t_srs EPSG:3857
> >
> > -111.648251 40.246882
> >
> > -12428626.4492796 4901883.62422139 0
> >
> >
> >
> > Results on X axis are almost the same, but the offset along Y is pretty
> > big.
> >
> > Any suggestion would be appreciated.
>
> have a look into a related ticket:
>
> https://trac.osgeo.org/grass/ticket/2229
>
> in my experience EPSG:3857 isn't the best projection for analysis.

Regarding EPSG:3857:
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.

This has been fixed in recent GRASS 7.4 and GRASS 7.5, providing same
results like GDAL.

Markus M
>
>
>
>
> -----
> best regards
> Helmut
> --
> Sent from: http://osgeo-org.1560.x6.nabble.com/Grass-Users-f3884509.html
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