GCP issues/lack of understanding

Hi,
I am having issues using GCPs that were surveyed in using EPSG:27700 (OSGB 1936).
When I create the orthomosaic without GCPs it comes out pretty well, with less than a metre error (X,Y) on known points. However, when I try it with GCP’s the orthomosaic shift to the northwest by about 50 miles. I know the GCP co-ordinates are correct (surveyed in to UK National Grid - EPSG:27700 (OSGB 1936) . I have tried processing this but WebODM doesn’t seem to recognise EPSG:27700. So I converted these co-ordinates to WSG84 (EPSG:4326) - again I have checked that the lat long results are correct), but I get this 50 mile shift. Clearly I am doing something wrong.
My GCP list is provided below, any suggestions. Thanks in advance.

EPSG:4326
53.528607 -1.131578 9.912 2031.9076012657201 1920.6133528300745 DJI_0625.JPG GJCP09
53.528607 -1.131578 9.912 1673.4590876868854 2592.5070133576664 DJI_0632.JPG GJCP09
53.528607 -1.131578 9.912 4379.689061566473 1453.0975868610733 DJI_0676.JPG GJCP09
53.529707 -1.130709 9.219 897.8194176762089 696.5385279223024 DJI_0557.JPG GJCP14
53.529707 -1.130709 9.219 2896.7483385885153 2451.556311360217 DJI_0583.JPG GJCP14
53.529707 -1.130709 9.219 2903.158565836227 1048.7501813035697 DJI_0613.JPG GJCP14
53.529766 -1.13259 10.595 3134.569771294938 1659.5284082177948 DJI_0418.JPG GJCP15
53.529766 -1.13259 10.595 2692.730770997473 2583.57298054429 DJI_0451.JPG GJCP15
53.529766 -1.13259 10.595 890.9507409208708 1616.2199589332663 DJI_0460.JPG GJCP15
53.530035 -1.130168 9.202 3756.5165895265704 1095.0121365471489 DJI_0585.JPG GJCP16
53.530035 -1.130168 9.202 2224.4752577601844 1823.3960132082016 DJI_0610.JPG GJCP16
53.530035 -1.130168 9.202 1406.5551029789972 1962.3656994359687 DJI_0646.JPG GJCP16
53.530179 -1.131079 10.05 4395.968830436962 3257.6162170923712 DJI_0469.JPG GJCP17
53.530179 -1.131079 10.05 1172.0676085749576 2660.10505125909 DJI_0496.JPG GJCP17
53.530179 -1.131079 10.05 2413.105652343651 1343.979745110781 DJI_0521.JPG GJCP17
53.528777 -1.129141 9.952 1165.5026449354636 1137.6163632063615 DJI_0769.JPG GJCP19
53.528777 -1.129141 9.952 2578.95562712309 2008.632938927086 DJI_0791.JPG GJCP19
53.528777 -1.129141 9.952 3241.906666824283 2687.0460157509683 DJI_0814.JPG GJCP19
53.530934 -1.130859 10.742 4656.790813767222 2508.7072168136 DJI_0433.JPG GJCP20
53.530934 -1.130859 10.742 2754.9310820552887 1278.9947716026213 DJI_0473.JPG GJCP20
53.530934 -1.130859 10.742 3061.8810898022703 3205.1043239744977 DJI_0490.JPG GJCP20

1 Like

Welcome!

Sorry for the trouble.

Our EPSG database doesn’t contain every definition, so using the PROJ4 string is usually a good/safe bet:
epsg.io/27700.proj4

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs 

Can you try with the PROJ4 defintion?