(32) Permanent tree-girth tape
Manual measurements with permanently installed measuring tapes provide high-resolution data on changes in tree stem circumference (girth) with an accuracy of 1/10 mm. The permanent girth tape is tensioned by a spring and has a scale with vernier (nonius) reading. It is permanently attached to the tree to avoid deviations due to different measuring positions. The scale is in pi centimeters, i.e., the girth at breast height (GBH - or circumference at breast height) is directly converted into the diameter D (cm), with D=GBH/pi.
Normally permanent diameter measurements are aiming at the assessment of changes with an accuracy 1/10 of mm. Usually a permanent girth band tightened by a spring, a scale (with nonius
reading) enables this precision when permanently fixed on the tree to avoid variation due to differing measurement positions.
Aims of the measurements
We use girth tapes to assess annual changes in diameter (i.e., radial growth), as these changes are an important indicator of the vitality of selected trees and of forest condition.