To relocate an old bole diameter, we looked at 1993 data sheets and measured at one half of the 1993 stem length. Measurements are taken with the paint spot aligned between the forks of the calipers, showing on the open side where possible. Old spots were repainted. Recorded in cm. Some of these are becoming quite hard to reach as the trees grow.
Diameter tape is sometimes easier to use.
Any unusual basal swelling or scars are also noted in the comments. Some basal diameter markings are really buried and should be relocated and marked above ground. If a new basal diameter is marked, this is noted in the comments. 0.1mm Recorded in cm. If the last measurements for a stem were basal and bole diameters, but the stem is now long enough to get a DBH, we will take all three measurements.