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Tree Ordinance Guidelines
Simplified guide to measuring DBH
In the US, tree diameter is usually measured at 4.5 ft (137 cm) above ground
level. Measurement at this height is referred to as diameter at breast height
or DBH. DBH can be measured with a specially calibrated tape measure called
a diameter tape (d-tape) available from arborist or forestry supply dealers.
In a tree with a clear gradually tapering trunk, measuring DBH is straightforward,
but there are a number of circumstances in which questions arise about how to
measure DBH. The following guide can be used to solve some of the more common
complications. In the guide below we have generally used the simplest methods
we found recommended in other sources. Other guides with illustrations can be
found at:
The Tree Register of the British Isles - http://www.treeregister.org
Canada's National Forest Health Monitoring Plot Network Manual on Plot Establishment
and Monitoring (Revised) from the Environment Canada Ecological Monitoring and
Assessment Network (EMAN) site - http://www.eman-rese.ca/eman/reports/publications/arnews/arnews.html
USDA Forest Service's Forest Inventory and Analysis (FIA) program manual -
http://fia.fs.fed.us/library.htm/
(rules for determining DBH heights for forked trees become very complicated
in this manual)
| 1. The tree tapers in such a way that the diameter at a
point below 4.5 ft is actually smaller than the diameter at 4.5 ft.
Measure diameter at the smallest point and record the height at which diameter
was measured on the data sheet. |
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| 2. Tree has branches or bumps which interfere with DBH
measurement. Measure DBH below the branch or bump. Some references say
to measure a foot below the branching point, which assumes this point is
the smallest diameter of the trunk below 4.5 ft. Record the height DBH is
measured at. |
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| 3. Vertically growing tree is on a slope. There are
several commonly accepted ways to find the DBH height. Probably the easiest
method is to measure diameter 4.5 ft from the ground on the upper side of
the slope. This method is used by the US Forest Service. Some references
(e.g., International Society of Arboriculture's Tree Appraisal Manual) say
to measure 4.5 ft from the midpoint of the trunk along the slope. However,
finding the location of the trunk midpoint is probably subject to more error
than finding the upper side of the trunk, so the USFS method is likely to
be more repeatable than the ISA method. |
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| 4. Tree leans. There are several commonly accepted
ways to find the DBH height. The US Forest Service measures 4.5 ft up the
stem in the direction of the lean. Some references (e.g., ISA) say to measure
4.5 ft from the midpoint of the lean. As noted under 3 above, the USFS method
is probably less prone to error and more readily repeatable by different
observers. |
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| 5. Tree forks below DBH or near DBH. The measurement
is recorded at the narrowest part of the main stem below the fork. The height
of the DBH measurement and the fork should be noted (e.g., 3 ft diameter
@ 2 ft [Forks @ 4 ft]). |
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| 6. Tree splits into several trunks close to ground level.
Measure DBH of each trunk separately, using the principals shown in categories
1-5 above. The DBH for the tree is found by taking the square root of the
sum of all squared stem DBHs. |
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