You can sometimes find very high resolution orthomaps from your county GIS department. My county has maps with a GSD that sometimes gets down to 1 foot (Made from an airplane using Eagle view Pictometry). You could use this data for some decent 2D GCPs. You basically find items in the images and get the latitude/longitude of the corners of the items or centers of objects and use these as GCPs. Keep in mind though that this will not get you "survey grade" control points. You are using data with a GSD of 1 foot and up to begin with, adding in the loss of accuracy of then marking them.
You could also look for free state LIDAR data and do the same and this would allow for 3D GCPs. This workflow would have similar accuracy/precision as above and all depends on the accuracy/precision of the LIDAR data being used as well as how dense the LIDAR point cloud is as a thin cloud would make it very hard to find the objects an thus losing more accuracy.
Both options above represent getting ballpark absolute accuracy and would be a decent way to get maps to overlay each other in the same area. But these methods are not acceptable if you want to try to acheive "survey grade" data.
From your other thread I see you have a M3E. Take advantage of the RTK by receiving corrections to the drone from either a personal base (DJI DRTK or
Emlid Reach) setup on a known point or by receiving corrections from a paid service or a free state DOT service. But you will still need a handheld rover to measure checkpoints to "check" the accuracy.
Emlid RS2 or 3 work great for this purpose.
My Sample workflow
Receive corrections from NTRIP RTK service to my drone and my handheld rover (
Emlid RS2)
Measure control points in the area to be mapped. I use checker board targets easily seen from even 300' AGL.
FLy mission with drone receiving corrections.
Use the measured control points as only check points, but I usually measure extra control points that can be used as Ground Control Points, just in case I do not achieve my desired absolute accuracy with my check points. But in almost all cases so far, I have not needed the ground control points.
Long baselines (< 20km) from a services Base/CORS will entail a change in workflow by switching to PPK or setting up my
Emlid RS2 as a local base on either an NGS monument or my own known point that has been logged for 6 to 12 hours and run using OPUS or PPK.