It depends... were there any markers already present? In a past life I worked as a surveyor, and if a marker was present(it looks like a silver-dollar on the ground, with a spike holding it in place, these can last 100+ years), we would use that to measure from. If, however, they set up what looked like a tripod with a oval-shaped dish on top, that's a GPS receiver, and could be affected by this outage.
No - they placed new markers (orange disks on long spikes) driven flush into the ground, but they were walking around using GPS to do this (with supposedly centimetre precision, which I understand is what GNSS is for). This is in Eastern EU.
Link: here are some survey markers for sale, linking it so you can see a picture of what they commonly look like: https://www.berntsen.com/Surveying/Survey-Markers/Aluminum-S...