How to measure DSH properly
Everything downstream of this number depends on it. The zone, the encroachment percentage, the classification, and whether a tree gets retained. Slopes, forks and buttress flare are where it goes wrong, and the error is rarely where people look for it.
What is covered
DSH is Diameter at Standard Height, which AS 4970:2025 renamed from DBH. Same measurement, 1.4 m above ground, but a defined name instead of an inherited convention. If you want the background on what else changed in the revision, that is covered separately.
This piece is about the tape.
1. Why small errors matter more than they look
The Notional Root Zone radius is twelve times DSH, so a diameter error scales the radius by the same proportion. But encroachment is assessed against the area of the NRZ, and area goes up with the square of the radius. That means measurement error does not carry through, it compounds.
| Error in DSH | Error in NRZ radius | Error in NRZ area |
|---|---|---|
| 2% | 2% | 4.0% |
| 5% | 5% | 10.3% |
| 10% | 10% | 21.0% |
On a 600 mm tree, being 30 mm out is a 5% error. That is a tape sitting a bit low on a flaring trunk, or a stem measured on the downhill side. It moves the zone area by more than ten percent, which is enough to push a borderline encroachment from minor into moderate and change what the report has to recommend.
Nobody argues about the millimetres on site. They argue about the classification three months later, in writing, with a planner.
2. Where the tape actually goes
1.4 m above natural ground level, measured up the trunk, with a diameter tape held level and square to the stem axis.
- Natural ground level means the existing soil surface at the base of the trunk, not the top of imported fill, not mulch, and not a garden bed that has been built up around the tree.
- Use a diameter tape if you have one. A standard tape measuring circumference works too, divided by pi, but doing that arithmetic on your phone in the wind is exactly where transcription errors creep in.
- Level and square. A tape that slips down at the back reads high. On a big trunk that error is easy to make and invisible in your notes afterwards.
3. Trees on a slope
Measure 1.4 m up from the high side of the slope.
This is the single most common site error, and it is always in the same direction. Ground level on the uphill side is higher, so 1.4 m from there sits lower on the trunk, where the stem is thicker. Measure from the downhill side instead and you are further up a tapering trunk, reading smaller, and understating the zone.
On a steep bank the difference between the two reference points can be several hundred millimetres of trunk height. On a tapering eucalypt that is a real difference in diameter, not a rounding matter.
4. Trees that fork below 1.4 m
This is the one that produces genuinely wrong numbers, because the intuitive approach is wrong and the wrong answer looks reasonable.
Where a tree forks below standard height, you measure each stem separately at 1.4 m and then combine them. The combination is not addition. It is the square root of the sum of the squares:
The reason is that you are combining cross sectional areas rather than widths. Two stems do not make a trunk twice as wide, they make one with the equivalent area of both.
What the difference actually looks like
Take a tree forking into stems of 45 cm and 30 cm.
| Method | Combined DSH | NRZ radius |
|---|---|---|
| Correct | 54.1 cm | 6.49 m |
| Adding the stems | 75 cm | 9.00 m |
That mistake overstates the protection zone by about 122 square metres. On a tight residential block it is the difference between a workable design and a refusal, and it is your report the objection will be aimed at.
Our free calculator has a multi stem mode that does this for you and shows what the naive sum would have given, so you can see the gap.
5. Leaning trunks
Measure 1.4 m along the stem, following the lean, with the tape square to the stem axis rather than to the horizon.
Being straight with you: this is accepted practice rather than something the standard spells out in detail. The logic is that standard height refers to a position on the trunk, and on a leaning tree the vertical distance from the ground and the distance up the stem stop being the same thing. Measuring vertically on a heavy leaner puts you further up the taper and reads low.
On a significant lean, write down what you did. A sentence in the report explaining where the measurement was taken costs nothing and answers the question before it is asked.
6. The other measurement: above the root buttress
The SRZ uses a different measurement entirely, and mixing the two is a common slip. It is the trunk diameter immediately above the root buttress, at ground level, where the flare finishes and the trunk becomes cylindrical.
Judging where the buttress ends is a matter of eye, and on old figs and heavily buttressed eucalypts reasonable people will pick slightly different points. Measure where the flare has clearly stopped rather than where it is still tapering, and note it if the tree is unusual.
Unlike DSH, the SRZ formula is nonlinear, so it is more forgiving of small errors at the top end and less forgiving on small trees. Either way there is a 1.5 m minimum.
7. Five ways people get it wrong
- Measuring from the low side on a slope. Always understates. Always in the same direction.
- Adding multi stem diameters together. Substantially overstates the zone and is easy to spot in a report.
- Measuring from mulch or fill. Natural ground level, not whatever the landscaper added.
- Using the DSH measurement for the SRZ. Different measurement, different height, different formula.
- Writing the number down later. The most avoidable one. Between the tree and the ute, digits swap, and a schedule that does not match the plan is a fast route to a request for further information.
On this article. The measurement heights, the multi stem method and the slope rule come from AS 4970 and published guidance on it. The leaning trunk section reflects accepted practice rather than explicit text in the standard, and I have flagged it as such above. If you are producing reports, work from your own copy of the standard, and tell us at hello@timbrfield.com.au if anything here does not match it.
Record it once, at the tree
TIMBR Field captures DSH and buttress diameter in the field, works out the NRZ and SRZ as you type, and keeps them attached to the tree with your photos so nothing gets written down twice or transcribed wrong. In private beta with a small group of Australian consultancies, five places left.