Hi folks, Technical problem. Perhaps something to do with PROJ. Perhaps I have my declinations backwards? survey testdirection -title "Test direction" centreline date 2021.08.13 cs OSGB:ST #change this to test other grid squares fix startst 0.0000000000 0.0000000000 0.0000000000 station endst "end" entrance data normal from to length compass clino #for SU # startst endst 10 0.22666712328 0 #for ST startst endst 10 0.7235958904109588 0 #for SH # startst endst 10 1.51343150685 0 endcentreline endsurvey thconfig: source "testdirection.th" export cave-list -surveys on -location on -o testpositions.html Those compass bearings match the declination at those locations, on those dates. I can see this in the Therion output pane: geomag declinations (deg): 2021.1.1 -0.8475 2022.1.1 -0.6465 I have double checked them against https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination and I get the same numbers there (rounded to 2 dp). Therion seems to be doing the right thing so far. I have converted the startst location into lat,long (using both Helmert and OSTN5+drift just in case it made a difference). Then applied a northerly 10 metre geodesic to see where it lands (using two separate geodesics formulae just in case it made a difference), when given that bearing and declination. When doing this at the SU0000000000 line (where OSGB grid and true north line up), Therion gets the same answer as me for the endst coordinates: SU 0.000 10.000 -0.000 When doing this at the ST0000000000 line (where OSGB grid and true north do not line up), the geodesic calculation gives this (1 mm rounding errors possible due to imperfections in geodesic formulae): ST 0.276501 9.994217 0.000 (Converted using Helmert, to be compatible with Therion, but the results are within 1 mm when converting more accurately.) Therion gives an answer that is shifted to one side by 8 cm: ST 0.192 9.998 0.000 This seems to cause a rotation to our entire survey. The amount of rotation depends on the location of the cave. Where we are working, it is a 12 cm shift within a length of just 10 metres. By the time you reach SH0000000000 it is a 23 cm shift. Am I wrong somewhere? Or is Therion (and perhaps Survex?) applying the declinations in the wrong place? If someone wants to help debug this, you can do conversion from grid to lat/long using Helmert here (note, use SH 00000.000000 00000.000000 format with several decimal places to avoid it using the middle of a grid square): http://www.howtocreate.co.uk/php/gridref.php And you can perform geodesics here: https://geographiclib.sourceforge.io/cgi-bin/GeodSolve?type=D&input=50.79091... Thanks for any help, Tarquin