usually laser ranging is used to establish a baseline, rather than using a theodolite and chain like OG surveyors.

the rest is trigonometry, withe the GPSS defining points relative to the baseline.

https://www.gps.gov/trilateration

https://apps.dtic.mil/sti/html/tr/ADA165932/index.html

https://app.studyraid.com/en/read/15241/528230/determining-d...