You can use external orientation data from UAV. But is is not too accurate. You allways need some control points to scale a model in photogrammetry. For small areas, you can use total station to measure coordinates of control points.

Hi,

It depends on what you want to do with your DEM/DSM, but even a good onboard GNSS system, working in RTK with a ground station isn't enough, you will always need ground control points. Read the work by Brasington or Javernick, they have addressed some of those issues.

As Ondrej pointed out, you can use relative control points with a total station, but you do need geometries to control your final DEM/DSM.

Chris

I agree Ondrej and Christopher, the usefulness of control points is not only providing an absolute reference system but also they are a tool to structure and analyze how good is the geometry of your resulting model. For example, you run a first survey using SfM and you just scale your model with 2 or 3 points and you use points in this model as reference for later surveys, the inconsistencies in the geometry of the initial model will be inherited by the subsquent models. In my opinion, the use of GCP in SfM is, up to now, necessary.

Álvaro.