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

GRASS GRASS user
,
1

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.

Thanks & Happy New Year!

Drew

Brigham Young University

Provo, UT USA

2

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.

-----
best regards
Helmut
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3

On Thu, Jan 4, 2018 at 9:08 AM, Helmut Kudrnovsky <hellik@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

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4

On Thu, Jan 4, 2018 at 6:25 PM, Markus Metz
<markus.metz.giswork@gmail.com> wrote:

On Thu, Jan 4, 2018 at 9:08 AM, Helmut Kudrnovsky <hellik@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):

...

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.

I have added a related note here for the upcoming 7.4.0 release:

https://trac.osgeo.org/grass/wiki/Release/7.4.0-News#Projectionsupport

markusN

5

Hi there,

I just tried the same 4326 to 3857 re-projection with dockerized 7.5 trunk. The results are correct.

Thanks so much!
Drew

···

On Thu, Jan 4, 2018 at 10:38 AM, Markus Neteler <neteler@osgeo.org> wrote:

On Thu, Jan 4, 2018 at 6:25 PM, Markus Metz
<markus.metz.giswork@gmail.com> wrote:

On Thu, Jan 4, 2018 at 9:08 AM, Helmut Kudrnovsky <hellik@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):
…
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.

I have added a related note here for the upcoming 7.4.0 release:

https://trac.osgeo.org/grass/wiki/Release/7.4.0-News#Projectionsupport

markusN

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grass-user@lists.osgeo.org
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6

This has been fixed in recent GRASS 7.4 and GRASS 7.5, providing same

results like GDAL.

can https://trac.osgeo.org/grass/ticket/2229 be closed?

-----
best regards
Helmut
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