Hi Steve,
We’ve tested that Reach RS2 can transmit all necessary corrections to the DJI Phantom 4 RTK. It should allow the drone to calculate precise image center coordinates. However, I can hardly comment on the particular accuracy DJI Phantom 4 RTK can achieve, as we haven’t done any thorough testing of that.
Let me elaborate a bit on how GCPs work to make it clearer.
GCPs are points with known coordinates on the ground in the area of your interests. You can use them to improve and check the accuracy and get the map’s correct scale and orientation. Also, they are needed for absolute positioning. To find some tips on placing GCPs, you can explore the Breaking Down the Drone Mapping: A Step by Step Guide.
If you work with a drone that can save image center coordinates in RTK, you will need only a few GCPs to check the accuracy. With a lot of GCPs placed, you can get centimeter-accurate results without RTK/PPK mapping drone. Please note that in this case, enough GCPs quantity highly depends on the site area.
As Gabriel said, GCPs should be linked to the ground and be independent of the flight data. That’s why a subset of photos can hardly help to check the result.