In bundle adjustment tasks, how are the weights for reprojection errors and GCPs set?

In bundle adjustment tasks, how are the weights for reprojection errors and GCPs set? (GCP = Ground Control Points in photogrammetry) What are they generally set to? Is there any physical basis for these settings? Why are these parameters typically hardcoded in implementations like Ceres or g2o, instead of varying with the number of error terms?

computervision control errors gcp parameters photogrammetry set tasks