Hazard Tree Inspection

Inspection procedures

Hazard tree inspection is an art. Rating systems, procedures and guidelines have been developed for inspection and decision-making, but knowledge, judgment, common sense and experience are an important part of the process. As an inspector grows in knowledge and experience, decision-making and interpretation of defects become more nuanced and complex.

Preliminary Decisions

If only one or a few designated trees are to be inspected (typical for an arborist with a homeowner as a client), there really are no decisions to be made before inspection.  But if the project is large, with hundreds of trees, several things must be considered before beginning.

1.  Target Distance

How far out from the targets of concern will trees be inspected?  If a tree or one of its parts could not strike the target, there is no point in inspecting it.  But how far one goes depends on a variety of factors.  With high target values and relatively few, large trees, it may make sense to go 1.5 to 2 tree heights from the target to account for broken branches and tops sailing in the wind or the domino effect – a falling tree knocking down a closer tree.  When there is a great number of trees to be inspected in dense stands and targets are of lower value or less consistently occupied, one tree height is usually considered sufficient.

2.  Tagging

Temporary (heavy foil) or permanent aluminum tags can be used with aluminum nails. Wildlife (often the 2-legged kind) sometimes tear off tags. If tags are used without mapping, there can be quite a job finding the tree with the right tag. Tags work best as a supplement to mapping.

3.  Marking

Ideally, decisions about treatment, such as what hazard rating levels to treat, are made after the data are in. In some situations, particularly where time is pressing, it may be desirable to decide before hand that all trees above a certain hazard rating will be treated. In that case, such trees can be marked for treatment (e.g., removal) at the time of inspection.

4.  Minimum Tree Size

A minimum DBH (diameter at breast height, 4.5 feet) for inspection must be decided. Usually minima are in the range of 6-8″.

5.  Mapping

In a large population of trees, a means of finding the trees again is needed in order to implement treatment and perhaps to track changes between inspections. In some cases it may be desirable to map and tag all trees that are inspected; in others a threshold hazard rating can be determined, above which trees will be so located. There are various possibilities:

  1. Sketch mapping. A map of the site is obtained or prepared. During inspection, trees are located on the sketch map by number. This is somewhat less accurate but faster than the following.
  2. Reference point mapping. Along with other tree data, the distance and azimuth from a fixed reference point are recorded. On large sites, multiple reference points will be needed. This is a more precise method, but takes a bit longer. If a map is desired, it can be generated from these data with some geometry.
  3. A note on GPS. Although GPS would seem to be a good way to locate trees, it is still (even after the military removed selective availability) not precise enough unless: a) trees are widely spaced, or b) a radio or satellite correction is applied to the GPS signal. Without real-time correction you can generally get within a 10-feet radius, which is often not precise enough to distinguish nearby trees.

Assessing internal decay

Without advanced scientific knowledge and experience with wood decay, most observers do not recognize early stages of decay as decay. And yet, the most rapid strength loss can take place in those early stages, when decay is barely perceptible by appearance. Don’t make the common mistake of only considering advanced decay as important.

In order from low- to high-tech, here are some tools for detecting and measuring internal decay in stems and large roots.


Strike the tree sharply with the hammer side of a hatchet or a mallet. A very hollow tree will be easy to detect by the bass drum sound. Trees with decay but no large void may be much more difficult. Sound varies among species independent of decay. Practice sounding lots of trees to tune your ear to the sound of decay. Sounding is an art, and some people are better at it than others, but it can be a useful part of initial examination, especially when there is a great many trees to inspect.

Increment boring

An increment borer is a long, hollow, steel tube with threads on one end. After screwing the tube into the tree, an extractor is used to remove the wood core from inside the tube. Advanced decay with a void is detected by the fact that the tube spins freely when the threads enter the void. Also, the core can be examined for symptoms of decay. The thickness of sound wood can be measured fairly accurately on a core. Increment borers provide good information but: a) create a somewhat significant hole (up to a cm or so) that could lead to problems, and b) borers require much maintenance to avoid problems and are difficult to use on some hardwoods. Trees should not be cored unless there is reasonably strong suspicion of decay.

Increment Borers – Should We Stop Using Them?

Forest pathologist John Pronos prepared an excellent review and presentation on the use of increment borers in hazard tree inspection, the potential damage they cause, and recommendations on their use. His review is summarized in the powerpoint at right, which you can open in a new window for a larger view. Thanks John!

Cordless drills

One of many cordless drills that can be used to feel the hardness of wood inside the tree.

The easiest and most efficient way to routinely detect decay in living trees is with a cordless drill and long bit. DeWalt drills have many of the features needed. Look for the following features:

  • 1/2 inch drill/driver
  • 18-volt system or higher
  • XR+ heavy-duty battery packs (2)
  • circuits that prevent memory development in the battery

There are a number of constantly changing DeWalt drills that meet these criteria. A single battery pack should last a long day under heavy use, but most sets come with a second battery pack, which is nice to have. For questionable cases, it is useful to also carry an increment borer for more precise measurement and examination of the wood.

Resistograph demonstrated by Julian Dunster.
Arbotom sonic tomography unit produced by RinnTech, demonstrated by Frank Rinn.
Picus sonic tomograph machine, demonstrated on an aspen by Tom Smiley.

The bit to use is a 1/8″ x 12″ brad bit, which can be ordered from Grizzly Industries, (570) 546-9663, catalog #G2704.

A cordless drill is very worthwhile to carry (carrying is easier with a drill holster). Inspection crews learn to appreciate them quickly. You can quickly and easily drill trees that have indicators or suspicious sounding results. Because it is easy and the wound is slight, it reasonable to drill trees that you might be reluctant to core, and you may quickly drill several radii to get a better “picture” of the decay column. Cordless drills have come a long way. The 18-volt systems are powerful and modern battery packs (XR+) hold a good charge all day and have circuits that prevent “memory” development. See sidebar for a recommendation.


This machine is a sophisticated cordless drill with a device that measures and records changes in torque as the bit goes in. It produces a paper and/or digital graph (depending on the model) of the strength of wood along the bit path. It is accurate and allows easy measurement. It is so sensitive that you may have difficulty distinguishing between changes in wood density (often due to changes in growth rate) and pockets of early decay. The only drawbacks are the expense (around $4000 and up) and bulk/weight. For rapid inspection of large numbers of trees it may not be worthwhile, but the resistograph would likely provide useful additional information for intensive inspections of few, high-value trees.

Sonic tomography

Other machines are available for internal inspection of trees, but their usefulness and power to distinguish decay from other internal variables in a tree (such as moisture content, wood density, resin, branch traces, etc.) is not yet clear. Among them are the Arbotom, a complex system of impulse tomography (almost like a CAT scan but it uses stress waves from striking the tree instead of x-rays). Another instrument is the Arborsonic 3D Acoustic Tomograph, sold by Fakopp. It is an ultrasonic instrument, which measures the strength and speed of propagation of high-frequency sound waves through the wood. Knowledge, experience and common sense are much more important than expensive equipment.

Target Distance

How do you determine which trees around a target need inspection?  In many cases it is obvious.  However, as the distance from the target increases and approaches your threshold, it becomes difficult to determine. Rather than measuring tree heights and distances, Jason Worrall worked out a simple method.

Stand at the edge of the target facing the tree.  Measure the angle from horizontal to the base of the tree; call it A.  Measure the angle from horizontal to the top; it is A′.  Assuming you are inspecting out one tree height, calculate 2A′ − A.  If > 90°, the tree would reach you if it snapped at the base and fell toward you.  The diagrams at right show how this works for trees based above you, level with you, and below you.

This can be modified for 1.5 or 2 tree heights.

How to measure angles?  Classically we would use a clinometer, a forestry instrument costing $190.  However, the free app SpyGlass on an iPhone provides a great and amazingly precise way to measure such angles.  In the example of the heads-up display shown below, the angle to the base is -2° and that to the top is 41°.  Would the tree reach you if it fell toward you?

Precise measurement of the angle to a tree base using the free iPhone app SpyGlass. A = -2°.
Precise measurement of the angle to a tree top using the free iPhone app SpyGlass. A’ = 41°.

Inspection hints and tips

Here are some tools often used in hazard tree inspection:

  • Forms (inspection and failure report forms)
  • Site maps
  • Compass (for mapping)
  • Diameter tape (to measure DBH)
  • Measuring tape (100′ or so)
  • Clinometer (to measure tree height)
  • Tree tags and nails (aluminum)
  • Pulaski (see sidebar)
  • Hatchet
  • Camera (for important trees)
  • Binoculars
  • Cordless drill (see sidebar)
  • Increment borer
  1. Work the site in a logical, consistent sequence so that tree numbers are in a logical sequence that can be followed later.
  2. Assess and rate the target for each tree. If the tree would not hit a target, ignore it.
  3. Look at the tree from 2-3 perspectives, close and far and all around. When you are learning, it will help to go through each defect and decide if it applies.
  4. Also based on indicators (wound, bird cavity, sounding, etc.), you may need to increment core or drill the tree to assess internal stem decay. At a minimum you should have 1/3 of radius left sound (sound shell). Use DBH/2 to get radius, not the core.  See Hazard Tree Defects for details.
  5. Calculate the rating. In some cases, you may feel the rating does not accurately reflect the hazard of the tree. Tree inspection and hazard rating are a combination of science and art: do what experience and sound judgment tell you is right, but make notes about what you do.