I have been sent the following coordinates, they belong to Switzerland:
I need to convert those to EPGS:4326, but I don't know what the source SRID is.
Does someone recognise the coordinate system?
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There could be two possibilities. If the coordinates are in km and is the top of a ridgeline/mountain (approximately lat:46.853795 lon:7.985557), the CRS could be CH1903 / LV03, EPSG::21781.
If the coordinates are in meters and using CH1903 / LV03C-G (EPSG::21782), they're near Greyerzstrasse in Berne as Joseph suggested. However. EPSG states that 21782 is only used in Liechtenstein.
I've found a couple of useful tools to find possible CRSes if you know the general area.
epsg.io : this is a web site which is a search engine for CRSes. Type in something like 'Switzerland' and it'll list possible matching CRSes. You can also see the CRS extent on a map and see metadata in various formats (proj4, WKT etc) so you can check that units and accuracy are suitable.
A few months back they also added a feature to project coordinates so you can check to see if the results look sensible (it helps if you know some reference points).
Projestions (QGIS plugin) . If you use QGIS, this plugin lets you search using the current canvas extent for CRSes whose bounds intersect. You can zoom in (perhaps using a web basemap) to an area of interest and list coordinate systems which intersect this area.