Treetec Arborists Melbourne Victoria

TPZ / SRZ calculator | Incursion calculations

(calculators align with Australian Standard AS 4970-2009)

Proof Safe ProofSafe digital forms for data collection workplace safety OHS WHS

Tree calculator incursion or encroachment into Tree Protection Zone (TPZ)

DBH calculator

Use the calculator below to determine the Diameter at Breast Height (DBH) of a multi-stemmed tree.

Use the red boxes to input the measurements, always in metres eg. 65 cm calliper - input: ' 0.65 '

The result is displayed in the green box. If there are more than 5 stems enter the total from the first 5 back into a red box and continue.


Tree Protection Zones - TPZ / SRZ calculator

Enter the relevant figures (in metres) in the red boxes below to calculate the dimensions of a

Tree Protection Zone (TPZ) or Structural Root Zone ( SRZ).

The result is a measurement of the radius (R) of a circle taken from the centre of the trunk.


Incursion into the TPZ

Enter the TPZ radius and the applicable distances in the red boxes to calculate the degree of incursion into a TPZ. All figures should be entered in metres.

The first image is for situations where an incursion does not cut entirely across a TPZ.

These calculators are based on the Australian Standard AS4970-2009 Protection of Trees on Development Sites, commonly used by arborists. They can be used by anybody for working out the calculated Tree Protection Zone (TPZ) or Structural Root Zone (SRZ) of a given tree, or degree of TPZ incursion for a given set of plans. This information is normally provided by a consulting arborist in an arboricultural report (tree report) and helps guide planning departments and architects etc on how far from a given tree works should be kept. Within AS 4970-2009 it states that an incursion of any more than 10% of the calculated TPZ is considered a 'major incursion' and certain conditions should apply; it may trigger the requirement for root mapping, design revisions or low impact construction methods to help protect the tree, often there will be a requirement for all planned works within a TPZ to be supervised by an arborist.

It is recommended that a consulting arborist be engaged very early in the planning process to help determine which trees, on or near a site, are of higher retention value. Once high value trees are identified subsequent planning can incorporate tree protection, this will save delays and re-working of designs. If a tree is identified by the arborist as being of high retention value, however it needs to be removed, then that removal application and permit process can commence early, saving on delays and further unexpected costs.

Designing around trees should be done in consultation with an arborist. Where it is not possible to restrict all works from within a calculated TPZ there are tree sensitive construction methods and materials that can facilitate some TPZ encroachment whilst still protecting the longer term viability of the tree.

As 4970-2009 considers both above and below ground tree components, whilst it is imperative that roots are protected it is also important that any pruning of the canopy is appropriate and undertaken by a suitably qualified arborist in line with the Australian Standard on the Pruning of Amenity Trees (AS 4373-2007). Tree lopping causes unnecessary and permanent damage.

Standards Australia. 2009, AS 4970-2009 Protection of Trees on Development Sites.

Standards Australia. 2007, AS 4373-2007 Pruning of Amenity Trees.

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