ok so this has been bugging me for a while and i finally have time to post about it. doing a mapping job for a landscaping client, roughly 12 acres, not huge. i laid out 6 GCPs with my Emlid Reach RS2 and the checkpoints are all over the place in Pix4D ? like 8-10cm horizontal error on some of them which is way worse than what i was getting on smaller jobs without any GCPs at all using just the RTK tag from the drone.

the targets are the standard 50cm checkerboard ones printed on foam board and i marked the centers as carefully as i could. im wondering if its a coordinate system thing, i set the project to NAD83 / UTM Zone 17N because thats what the Emlid was outputting but maybe something got mixed up somewhere. or maybe my point marking in the rayCloud is off. honestly not sure where to start debugging this.

coordinate system mismatch is definitely the first thing id check. specifically look at whether your geoid model is set right ? if the Emlid is giving you ellipsoidal heights and Pix4D is expecting orthometric or vice versa you'll get weird vertical errors that can kind of bleed into your horizontal too once the bundle adjustment tries to make sense of everything. also double check what datum your base station was tied to. if you just hit survey-in instead of using a known point the absolute accuracy will be rough even if the relative is fine.

also 6 GCPs on 12 acres isnt bad but placement matters a lot ? if they're all clustered toward the center or all on flat ground the adjustment can get weird at the edges. what does the distribution look like?

had something simlar happen to me last year and it turned out i was clicking the wrong corner of the checkerboard pattern consistently lol. sounds dumb but when you're marking like 40 images per GCP it's easy to drift. pix4d has that zoom in view when you're marking ? i started using it religiously after that. probably not your issue but worth sanity checking