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Base Sensor

The Base sensor is located at ground level, on the base of the trunk. The base sensor records the tilt values of the Structural Root Zone as it tilts under dynamic wind loading. These tilt values provide the critical data used to establish the stability of the tree.

Comprehensive Assessments

A comprehensive evaluation of the dynamic response of a structural root plate, monitoring periods should include:

  • Wind events above 40km/h
  • A wind event lasting at least 3 hours
  • Periods where wind comes from at least two directions
  • Directional changes during the same wind event

Control Sensor

The Control sensor is placed higher up the trunk at approximatley 2.5 to 3.0 metres high on the main trunk.  Data from the Control sensor records trunk movement and is used to confirm if a wind event has occurred. 


Dynamics involves change and movement of objects and forces. In structural analysis, dynamic methods consider forces that change, often very rapidly. Wind is dynamic and the wind forces on trees are always changing, both in size and direction. Trees sway dynamically in the wind and the forces are dynamic because they change with each gust of wind.

Dynamics is more complex than statics because it includes forces that do not occur in statics such as;

  • Inertial forces created by moving objects
  • Damping forces that absorb or dissipate energy

The tilt sensor uses dynamic methods to accurately record the tilt of the tree’s structural root plate under natural wind conditions. This is the only instrument available which accurately records dynamic tree response in winds.

Monitoring Period

The period the sensor records data on a tree

Overturning moment

An overturning moment is the tendency of the wind force on a tree canopy to twist or rotate the tree at the base.

This is an important, basic concept in engineering and physics. The overturning moment is calculated mathematically by multiplying the wind force on the canopy by the distance (or height) that the wind force acts above the base of the tree.

A wind blowing on a tree creates an overturning moment at the base of the trunk and causes the structural root zone to tilt by a small amount.

The rotational angle (Φ) of the structural root zone is therefore a function of the turning moment (M). Measurement of the rotational angle Φ of the structural root zone can be used as a method to determine the overturning moment (M) on a tree

Resultant Tilt

The resultant tilt is calculated to find the maximum tilt. The sensor measures tilt in two horizontal directions that are a right angle to each other. The resultant tilt is the combination of these two values and always gives the maximum value of tilt for the tree, no matter from which direction the wind blows.


A tri-axial accelerometer used as a tilt sensor. The PiCUS Tree Motion Sensor (TMS) that has the following features:

  • Records tilt (X,Y & Z) of a tree’s structural root zone to an accuracy of 0.01 degree
  • Records dynamic tree movement by taking 20 readings per second
  • Internal rechargeable battery (approximatley 20 days of continuous operation)
  • Internal data recorder and expandable memory
  • GPS to locate the approximate tree position
  • GPS-linked clock for accurate timing
  • An electronic compass to give directional information
  • A removable micro SD card


Constant or steady conditions. In structural analysis, static methods consider forces that do not change rapidly. Since wind is a dynamic and changing force, static methods for wind loading on trees can only approximate real conditions. Examples of static loads on trees include self weight of large, over extended branches, ice loads or snow loads on branches.

Stability Range

The tilt data from this tree plotted on a graph that uses an international dataset of dynamic tilt values taken from trees under natural wind loading.

The tilt data from this tree is plotted as the circular icon, showing the maximum tilt value recorded (in degrees) using the scale on the left side of the graph. 

The tilt value for this tree can then be evaluated according to stability ranges which consist of;

  • Low Range of tilt indicates that the tree is within the limits of stability.
  • Medium Range of tilt indicates that the structural root plate has suspect movement.  It is recommended that a second monitoring period be conducted during a wind event above 60km/h and include a comprehensive evaluation.
  • High Range indicates a tree that requires extended monitoring that should include wind events above 70km/h and a comprehensive evaluation.

To obtain a comprehensive evaluation of the dynamic response of the tree additional tilt monitoring should include periods where wind comes from at least two directions.  Using this information assists in evaluating the stability of a tree but does not predict failure.

Structural Root Zone

The structural root zone is the area around the base of a tree required for a tree’s stability in the ground (Protection of trees on development sites, AS 4970-2009, Australian Standards). This is the part of the tree base and root system that supports the weight of the aerial parts of the tree and anchors it into the soil (Biddle 1998).

Treesensor Report

The report will compile your tree data to produce a report in PDF format that: 

  • Includes your summary details of the tree
  • Generates a Google Map, which spatially identifies the location of the tree using the GPS information from the sensor
  • Identifies the maximum tilt values and the date and time they occurred
  • Shows a comparison graph between the Base sensor and the Control sensor giving a visualisation of comparative movement between the monitoring points
  • Presents comprehensive graphs from the tilt data that clearly show the dynamic tilt movement of the tree’s structural root plate, when the tilt occurred and the direction of the tilt
  • Generates graphs of the entire monitoring period providing a visualisation of the wind events
  • Generates a graph showing the stability range of the tree’s structural root plate
  • Provides a summary list of the top ten files that contain the maximum tilt values
  • Provides a complete list of all the files that were uploaded to

The report gives a complete summary of the dynamic movement of the tree’s structural root plate that occurred during the monitoring period.  It is an accurate and factual record of the tree’s response to the winds that occurred during the monitoring period.  The data and graphs clearly show the dynamic movement of the tree’s structural root plate and the direction in which this movement occurred.

The generated report can then be used as a summary report for the client or an appendix to a larger, more detailed report.

Wind event

A period where the wind is at or exceeds a 'strong breeze' as described in the Beaufort Scale for a period greater than 3 hours.

Wind loading

Wind pushing on a tree’s canopy creates a turning moment (M) at the base that is resisted by the root system, and primarily by the root plate or the structural root zone