3.4 Field Technique Tips for Measuring Tree Diameter
Measuring DBH is relatively easy, but it is important to keep the following in mind while measuring and recording.
1. To speed up your work, determine where DBH is on your body and then use that point as a reference for locating DBH on each tree. This is much faster and easier than measuring up 4.5 feet on every tree. It is best to check this periodically the first several days you measure to make sure you are consistent. Over time it is natural to slip a little, and measure where it is most comfortable rather than at true DBH.
2. Make sure the d-tape is level, at right angles to the tree trunk. Slack in the tape on the back side of the tree will inflate its true diameter.
3. When using a d-tape, “hug the tree” to wrap the tape around it. This is much faster and less tiring than hooking the tape and walking around the tree, and you will soon find the diameter limit of your arms. This can come in handy. On large trees, try swinging the tape behind the tree and catching it in your other hand to avoid having to walk around the tree. This takes practice, but is worth mastering. Walking around large trees in heavy brush on steep slopes, while trying to hold a d-tape at the correct height can get really old really fast.
4. When determining DBH, always record measurements to the precision of the instrument being used. In other words, if you can measure to the nearest 0.1 inch, record to the nearest 0.1 inch. This maintains the highest flexibility in using the data later for whatever analyses are needed. So, don’t round off in the field.
5. Try to guess the DBH before measuring it. You will be amazed at how quickly your eyes calibrate. This not only makes a game out of the work, but can come in handy in situations in which you need to check for errors.
6. Work safely with the d-tape. The hook on the end can injure your hand or eyes, and the metal edges can cut your fingers.
7. Once the data are brought back to the office, diameters may be placed into their appropriate diameter classes. This is a way of grouping diameters for easier data analysis. Regardless of whether one-inch or two- inch diameter classes are used, the diameter class “numeral” is always the midpoint of the diameter class. This is the easy way to remember how to assign the correct class. They are grouped as follows:
One-inch classes: Two-inch classes:
8” class = 7.6” – 8.5” 8” class = 7.0” – 8.9”
9” class = 8.6” – 9.5” 10” class = 9.0”- 10.9”
10” class = 9.6” – 10.5” 12” class = 11.0” – 12.9”
11” class = 10.6” – 11.5” and so on….
12” class = 11.6” – 12.5”
and so on…
Note that if you record “11” in the field, it is not clear if the tree should be placed in the 10” two-inch class or the 12” class. Was the actual measurement 11.2” (12-inch class), or was the measurement actually 10.7” and rounded up to 11, placing the tree in the 12” two-inch class instead of the correct 10” class. So leave the rounding until the data are actually being analyzed. If the field measurement is 11.0”, record “11.0.”
8. As with all field data collection, when working with a partner, echo back your measurements to make sure the correct number is written down or entered into the data collector.