LibreNMS just uses the coordinates replied by the system.
Just set the “syslocation” like the example below:
syslocation RANDOMTEXT [48.057446, 6.453289]
It’s important to have the brackets as LibreNMS indicates the coordinates by them.
Also just have a look at this:
https://community.librenms.org/t/librenms-location-on-map-gps-data/1569/5