Hello List As mentioned in my post before, sometimes I need to do a resurvey. As the cave is an underwater cave and therefore the waterlevel can change I need to "correct" the depth readings. E.g.: The initial survey was done and the water depth at the start station was -5m at this time. The resurvey was done at a different time and the water level at the same start station was -9m. (to use positive numbers inside the data I use: "calibrate depth 0 -1") initial survey: centerline data diving from to length compass backcompass fromdepth todepth left right up down calibrate depth 0 -1 10 11 2.04 160 - 5.2 3.9 [ - -] [ - -] [ - -] [ - -] ... endcenterline resurvey: centerline data diving from to length compass backcompass fromdepth todepth left right up down calibrate depth 4 -1 0 1 3.09 284 - 9.2 11.4 [ 0.2 2.6] [ 0.2 0.9] [ - -] [ 2.2 0.2] 1 2 2.92 298 - 11.4 12.5 [ 2.6 1.8] [ 0.9 1.3] [ - 1.1] [ 0.2 1.1] 2 3 1.96 259 - 12.5 13.7 [ 1.8 1.2] [ 1.3 1.2] [ 1.1 1.2] [ 1.1 0.0] ... endcenterline Station 10 from the initial survey is identical to station 0 from resurvey and I do connect these using equate command. Due to the different waterlevel there is a difference of 4.0m in depth readings. To correct this I used "calibrate depth 4 -1" within the resurvey data. My assumption for e.g. station 0 was: (9.2 - 4) * -1 = -5.2 (identical to station 10 from initial survey: (5.2 - 0) * -1 = -5.2) (as discribed in the therion book on page 19: measured value = (read value - zero error) * scale) But doing so I get an error from the compiler: therion 6.0.0 (2021-07-13) - using Proj 5.1.0, compiled against 5.1.0 initialization file: C:\Program Files (x86)\Therion/therion.ini reading ... done configuration file: thconfig-survey.thc reading ... done reading source files ... done preprocessing database ... done C:\Program Files (x86)\Therion\therion.exe: error -- test.th [64] -- length reading is less than change in depth writing xtherion file ... done Line 64 is station 2 from resurvey. Depth difference is 13,7-12,5=1,2 and Distance is 1,96 (1,96 > 1,2). I don't understand this error. When changing "calibrate depth 4 -1" to "calibrate depth 0 -1" in the resurvey there is no error compiling the data. But the depth readings are not correct then. What do I get wrong with calibrate command? Torsten