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Using Matplotlib and Cartopy, plotting a map from Swisstopo is shifted by about 170m. The official coordinate system of Switzerland is EPSG 2056 ("LV95"). I create an axes in matplotlib with this projection and use wms_add to plot the map. I know the coordinate of some objects and they do not match in the plot. Also, when I overlay objects downloaded from Swisstopo (in LV95) they do not match the wms map. The same offset is seen in: https://gis.stackexchange.com/questions/4035/problem-with-swiss-coordinate-system-overlay-in-wms?newreg=db3988a2d7e4441993f3fb1795f76bbf

import matplotlib.pyplot as plt
import cartopy.crs as ccrs

LV95 = ccrs.epsg(2056)
# in LV95: 
x0 = 2682956; y0 = 1246234; extent = 500; 
xmin=x0-extent/2; xmax=x0+1.5*extent; ymin=y0-extent; ymax=y0+extent

fig = plt.figure(figsize=(8,8), dpi=96)
ax = fig.subplots(subplot_kw={'projection':LV95})

ax.add_wms(wms='https://wms.geo.admin.ch/',
           layers='ch.swisstopo.swisssurface3d-reliefschattierung-multidirektional')

ax.set_xticks([xmin,0,xmax]); ax.set_yticks([ymin,0,ymax]) 
ax.set_xlim(xmin,xmax); ax.set_ylim(ymin,ymax)
ax.ticklabel_format(axis='x', useOffset = x0)
ax.ticklabel_format(axis='y', useOffset = y0)
ax.set_xlabel("(LV95) X [m]"); ax.set_ylabel("(LV95) Y [m]");

I tried to specify explicitly the coordinate system for the wms query with

wms_kwargs = {'srs':'EPSG:2056'}

but get an error (double specification). Maybe it has to do with the default order for SRS in the query:
https://scitools.org.uk/cartopy/docs/latest/_modules/cartopy/io/ogc_clients.html I don't know if this is a bug, or I do something wrong. Any help please!

Jörg Rychen
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  • same offset here: https://gis.stackexchange.com/questions/411758/transforming-coordinates-from-swiss-reference-system-lv95-to-wgs84-using-r – Jörg Rychen Oct 14 '22 at 16:04
  • Did you try to adapt your code to WGS84 EPSG:4326 or webmeractor EPSG:3857 ? since wms.geo.admin.ch supports it https://www.geo.admin.ch/en/geo-services/geo-services/portrayal-services-web-mapping/web-map-services-wms.html – scriptDaddy Oct 15 '22 at 06:43
  • @scriptDaddy I need the EPSG:2056 since I have other features in this coordinate system to plot onto the map. – Jörg Rychen Oct 15 '22 at 07:22

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