Hi Jean-Daniel

Technically I wouldn't expect the numbers to be exactly the same, as the elevation in OSM needs to be metres above above mean sea level as defined by the EGM96 geoid model. This is unlikely to really differ a lot, but should be taken in to account, if you see differences that are in the 30-50 m range then they are likely values above the GPS ellipsoid (and should be corrected).  See https://wiki.openstreetmap.org/wiki/Key:ele https://www.swisstopo.admin.ch/en/maps-data-online/calculation-services/navref.html might help with checking this.

Are you actually using the 1:25'000 map? If yes I would use Carte Nationale SwissTopo and add the scale. On the other hand I'm fairly sure their https://www.swisstopo.admin.ch/de/geodata/landscape/names3d.html dataset includes elevations too (general warning about the swissnames dataset, the names need to be taken with a lot of salt, don't use them without checking locally first).

Simon

Je constate souvent que certaines altitudes (montagnes, cols...) dans OpenStreetMap sont très imprécises, c'est-à-dire présentent plusieurs mètres de différence par rapport à la carte nationale SwissTopo.

Donc question concrète : si je corrige ces altitudes erronées dans OpenStreetMap, dois-je ajouter au sommet/col l'attribut "source=SwissTopo" ? Ou est-ce suffisant que, lorsque j'upload le changement, je mette juste dans le champ "Specify the data source for the changes" (j'utilise JOSM), le commentaire "Carte Nationale SwissTopo" ?