A Comparison of Douglas-Fir Tarif and form Class Timber Measurement Systems for Woodland Owners in the Pacific Northwest

We evaluated the accuracy of OSU Extension Forestry's Varplot timber measurement system compared to two professional-level, commercially available cruising programs. One hundred forty-five Douglas-fir (Pseudotsuga menziesii) trees, 45 years of age, located on the eastern slopes of the coast range near the central Willamette Valley in western Oregon were felled and measured which provided the volume comparison baseline and the data utilized by the systems in the study. Varplot is a tarif system which uses a taper equation and tarif tables for estimating log volumes, while the other two programs in the study utilize form class. Results showed the form class programs to be 8.33% and 7.59% less than the measured volume and Varplot to be 10.57% less than the measured volume. For small woodland owners with limited experience in timber measurements, the added difficulty in obtaining accurate tree measurements utilizing form class based programs versus the more simplified tarif system will likely result in the two systems having similar accuracy.