Quick tip. I’m using data from air pollution sensors provided here. The Koordinaten are of the form 2’710’500 / 1’259’810. These use the Swiss coordinate system. pyproj is the defacto standard for doing coordinate processing, this page also helped.

But I could make it work, those coordinate values were too big.

Eventually I realised one needs to remove the leading 1 and 2 from the two numbers. This is mentioned in the wikipedia article:

In order to nonetheless achieve a clear distinction between the two systems, an additional digit was added to the coordinates of LV95: any East coordinate (E) now starts with a 2, and any North coordinate (N) with a 1. Consequently, LV95 coordinates are given by pairs of 7-digit numbers, whereas LV03 used pairs of 6-digit numbers – for instance the coordinates (2 600 000m E / 1 200 000m N) in LV95 would be expressed as (600 000m E / 200 000m N) in LV03.

https://en.wikipedia.org/wiki/Swiss_coordinate_system

In summary, my code now looks like:

```
from pyproj import Proj, transform
sites = []
sites.append({'name':'Zurich, Schimmelstrasse (ZH)','E':681942,'N':247245,'height':415})
sites.append({'name':'Zurich, Heubeeribüel (ZH)','E':685126,'N':248460,'height':610})
pWorld = Proj(init='epsg:4326')
pCH = Proj(init='epsg:21781')
for site in sites:
print(transform(pCH,pWorld, site['E'], site['N']))
```