[1] https://wiki.openstreetmap.org/wiki/Mapping_techniques
https://www.mobiddiction.com.au/drone-mapping-in-4-steps/
Depends a lot on what you mean by "site survey" though.
The photogrammetry used there also created a depth map from the stereography where images overlap:
https://www.mobiddiction.com.au/wp-content/uploads/2019/11/T...
You can see in the bottom right corner of that the elevation changes for the roundabout and the ~120mm high curbs at the edges of the roads - and the gradual slope of the whole site towards the top right corner. (There's a lot more detail and calibration data than that image shows in the full data set there.)
For an "on the cheap, lots of DIY" approach - I've got an inexpensive (~$40 from memory) laser rangefinder with sub cm accuracy and a range of a bit over 100m (it'll get ~120m for things with "good" reflective properties, sometimes only 50m or so for "bad" surfaces). If the park is a suitable size, you could probably start with a Google Satellite view, and measure enough distances to triangulate everything to calibrate the satellite imagery. I've considered (but never gotten around to) attaching a 3 axis digital compass module so I can record angles and elevations for each distance measurement. That'd help a lot of there are lots of objects you want to plot positions for - you wouldn't need 2-3 measurements from different known locations for each object that way.
See also my other comment about using a drone and photogrammetry.
It is a small park, 600ft square, but conservation means we would like to map tree locations precisely. Half a trunk diameter would do: around about 6”.
In metric that’s around 200m with accuracy of 200mm.
Budget is up to $500. We have time and tech ability on our side, so don’t need a Total Station (something I’ve learned about recently) that can do everything, but it would be nice.