No.26  Safe Anchor Choice

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Key Words - 

1.    Strength

2.    Risk Assessment

3.    Visual Tree Inspection

4.    Physics

5.    Considerations and


6.    Mapping the tree

7.    Access

8.    Work Positioning

THE FIRST QUESTION IS: Why climb anyway?


I see working climbers loosely set into two categories. 


The first are those that climb pragmatically, perhaps they would choose first to cut from the ground or mechanical platform rather than risk themselves to tie into organic anchor points.  One can not argue the safety, though, in the long term this approach may limit the arborist to that of technician, albeit a highly skilled one, but kept at a distance from the tree system.  The second category is one that I work hard to place myself in.  A climber that uses arboreal skill to develop understanding of the tree system through the act of climbing.  So rather than climbing being only a means to an end, to cut a branch, to set a rope etc,  the climber develops physical and intellectual sensitivity in order to more deeply understand the organic and ever changing nature of the tree system.


This article has been complicated to write because the subject is hard to systematise; trees are complicated.  

Forces of tension and compression within the wood, the unchanging presence of gravity, the huge variety of wood characteristic and stages of development and decay.  That as climbers we need to reach high into the structure to set anchors and often from a distance with throw-lines… …we have to keep on our toes and as hard as it may seem find and use systematic approaches within our work, we must try.


I am interested in developing safe, pragmatic and transparent technique to allow climbers a freedom to find efficiency and enjoyment in their work, of which ever category they see themselves in.  The purpose of this article is to propose something, ideas to think about, generalisations toward making safe anchor choices, I hope that it may help you a little with your own journey of discovery while working with trees.


Strength is inherent within design and material.  Karabiners have specific loading points that can be tested, approved, certified and clearly marked as such but the moment mis-configuration occurs it stands for nothing.  The same for rope. Loading in one direction to another with a specific differences in interface can have a dramatic change in strength.  Tests at ODSK have shown the enormous difference between 16 Strand, Double Braid and Semi-Static Kernmantle when shock loaded with the same toothed cam interface.  

Strength alone doesn't create a safe system, it is the careful design and configuration of components that make it so.

And so it is true of trees.  Strategic and carefully configured loading patterns at the anchor are vital to retain the inherent strength within the design, the morphology, of the tree structure.  Certainly there are BAD ways to choose and install anchors, RISKY ways and most of the time this stems from lack of training that come be overcome by pragmatic risk assessment.




There is nothing more fundamental than the ever perpetuating skill of risk assessment.  It is foundational to every technique, from beginner to expert.  It is a skill that we carry like a shield, to protect us from harm.  

We must be open to see the potential risk in our activity and capable of finding solutions in order to abate them. Risk Assessment is half the solution.  Creative and logical risk abatement is the other.

Study and learn how to use this risk square for practical risk assessment while working:  RISK SQUARE

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Continue to the trees crown.  Check the health of the leaves and for signs of heavy insect infestation.  Use binoculars and search for decayed sections and dead wood in the upper crown.


Look for previous activity,  storm damage and pruning.  Trees always show their history.


The assessment will continue into the access part of the climb as often it is difficult to make a full assessment from the ground.  Remember, trees are 3 dimensional, look from all sides and never take some ones word that a tree is safe to climb.  Always check, check and check again and hold the confidence to say a structure is unknown, unsafe or unclimbable, your life can rest upon these decisions.





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Begin the risk assessment at a distance from the tree, get an overview of the surrounding environment as everything close to the tree can have a pronounced effect upon it.  Look for dying branches at the top of the tree as this can be a strong indicator of root damage.


As you get closer the root system and lower trunk will take your attention, look for unusual structure in the soil which may indicate root movement.  There may be signs of fungal attack with fruiting bodies on the lower and mid height trunk.  Fungus identification is crucial because fungus has different attack strategies on each tree.  This means that it eats different parts of the tree so sometimes the tree becomes soft and sometimes brittle.

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 Top Anchor

A =        Top Anchor/Anchor

B =        Re-direct/Load Sharing Anchor

A to Climber =    Working End (rope beneath the climber is Standing End)

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 Bottom Anchor

A =        Apex

B1 =        Re-direct/Compressive Re-direct

B2 =        Re-direct/Load Sharing Anchor

C =        Bottom Anchor

A - C =        Anchor Leg

C - Climber =    Working End (rope beneath the climber is Standing End)


The following points should always be considered when choosing an Anchor.

  • Tree species 

  • Health 

  • Age (older wood is less elastic but usually stronger due to it building reaction growth) 

  • Length of Moment/Lever

  • Branch angle (compressive/tensile force)

  • Input force (kn)

Arborists use a specific vocabulary that describes technique and parts of the system.  


Because of the longer lever the small boy can generate more force than the heavy man.  Tree branches are like levers, some of them stand vertically, some horizontally and many more at every angle between.  They are connected to the structure through an amazing bio-mechanical process that squeezes fibre together in 

layered growth.  The tree continuously builds wood where it is needed.  Fungus, bacteria, insects and mammals use the wood and by doing so change its strength properties.  Trees are complex structures.  They fill and drain water, they process sunlight into sugar with zero loss of energy, they are designed to withstand strong wind and rain and build their structure according to the local environment. 

Trees are sensitive to the world around them and have specific parameters of strength due to this.  They can withstand huge tropical storms because they are accustomed to them, they have designed there structure to be flexible and static.  In February 2016, Tobiraonsen in Nagano saw a freak heavy and wet snowfall which destroyed 100’s of Red Pine and Larch trees.  The trees were unaccustomed to such intense tensile force and could do nothing but collapse and break.  It was akin to the small boy in the illustration, such loading puts intense tension into the wood fibre and breaking point is quickly reached.  

Tree Climbers have two choices, they can be like the heavy wet snow and jolt and break the structure or they can be sensitive to the trees inherent strength and load rope systems accordingly.  The tree is a living history, a map, that shows lines of strength (compressive loading) and lines of weakness (tensile loading) and so rope systems can be set accordingly.


An anchor refers to a place in the tree where a climbing system is attached.  A Top Anchor usually has one point of attachment and a Bottom Anchor more than one point.  We can say that the Top Anchor system is Isolated (one attachment point) and the Bottom Anchor system is Non-isolated (more than one attachment point).  Either way the uppermost point on the system must be considered carefully as branch angles and lever lengths affect the strength and integrity. 





Anchor Location = Species + Species Condition + Time of Year + Branch Angle + Lever Arm.

Force = Branch Angle + Rope Angle + Lever Arm + Mass. 

Category ‘A’ Anchor

The climber is tied into a long yet upright lever (ground to anchor) with no deviations from the main trunk so the force exerted will compress the wood.  This is the strongest type of Top Anchor

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Category ‘C’ Anchor

Although the anchor appears to fall into the first category it does not.  The anchor point has deviated from the main trunk so that the branch attachment point is horizontal and will load tensile force. 

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A = 0 - 20 degrees.  Top Anchor

Top Anchor point for stationary (SRT) or dynamic (DdRT) system.  Trunk anchored or crown anchored.  As the branch moves toward 20 degrees shortening of the Moment Arm (Lever) must be considered.


B = 20 - 45 degrees.  Apex + compressive anchor leg.  

Floating anchor for stationary (SRT) or dynamic system (DdRT).  Trunk anchored.  Ensure that mass is vectored to compress the structure.


C = 45 - 90 degrees.  Re-direct.  After choosing a higher placed Apex or Top Anchor, stationary (SRT) or dynamic (DdRT) systems may be re-directed here.  


Anchor Location = Species + Species Condition + Time of Year + Branch Angle + Lever Arm.

Force = Branch Angle + Rope Angle + Lever Arm + Mass. 

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When the tree has been OK’d to climb by both manager and climber the climber has a number of technical possibilities for Access.  If the tree is being removed the climber may use Spur’s and Lanyard to access toward the Working Anchor.  If the tree is to remain then Spur’s will damage the tree so a climber may choose to Access the lower portion of the tree by ladder and proceed with A.L.T. from there.  If the lower branches are higher than the ladder a Throwing Knot or Throw-line can be used.  More than Spur, Ladder and ALT the Throw-line is a highly skilled means of Access and must be used responsibly.  


The objective is to Access up into the tree via a number of techniques to obtain the Working Anchor.  The Working Anchor is the final and highest placement of the climbing system so must be able to withstand:

  • Compressive and tensile forces.

  • Shock loads from the climber slipping off a pendulum while branch walking.

  • 2 Man loads while rescuing.


The systems that I teach encourage climbers to 'get into the tree’ while accessing.  Often SRT climbers will use high powered launchers to install the line to the top of the trees crown and away from the trunk and while this has an efficiency of time it has safety drawbacks and I encourage all to get in close to the structure while accessing in order to properly risk access the structure.   


Throw-line Access Flow Chart

If you install throw-line into parts of the tree that you cannot see this means that you are unable to Risk Assess the process and open yourself and colleagues to an accident.  Placing 2 people on a remotely installed line and Bounce Checking offers little security as wood fibre is complex and so is the repetitive low input forces generated while accessing.  



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 Onigiri Anchor/Ring and Ring Anchor

Retrievability and low friction in the anchor system are important for tree workers for a number of reasons.  There are many designs available and the common factor is that a Large Ring (or Pulley) is combined with a Small Ring (or Thimble).  A Retrieving Ball is attached to the Working End of the rope, which passes through the Large Ring, catches on the Small Ring and is able to pull the anchor system out of the tree.  The Onigiri Anchor is useful because the Large Rings are small enough to Knot Block and change between the DdRT and SRT systems, which is convenient for both climbing and anchoring technique and allows the climber a number of anchoring possibilities.

DdRT (Doubled Rope Technique)

 Nakahara kun shows a basic set-up for DdRT.  

His anchor position is a category A which gives a high

Safety Factor for compressive and tensile forces.  

The low friction at the aluminium rings allow him to move easily 

in all directions and he can retrieve the system 


 The MULTIANCHOR can be used in a number of ways, one of which is to create a load sharing top anchor.  Care must be taken when setting the angle of the anchor.

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 Load Sharing with Multianchor

SRT (Stationary Rope Technique)

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Sinnet Chain (Daisy Chain) 

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Locked Alpine Butterly 

Here Nakahara kun has created a knot block and converted his DdRT system into an SRT system.  As mentioned earlier the rings must be small enough to block the knot safely (note.  some ring and ring anchors are incompatible as the large ring is too big)  Using a midline knot such as an Alpine Butterfly is very convenient.  

ODSK ran several tests to find the breaking strength of knot blocks with various ropes.  What is apparent is that a ropes safety factor effectively halves when climbing on SRT compared to DdRT.  This is not to say that SRT creates a weak rope although it is important to understand inherent strength within system design.  The knot block does appear to have capacity for shock absorption because the knot will move and tighten under load, this is an aspect that cannot be found in the DdRT set-up.

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Because SRT is stationary it can be rigged in a number of ways.  Here the working end beneath the knot block has been lengthened and tied on to a lower and stronger branch.  In the event of excessive branch movement due to side loading from strong wind the lower branch will begin to load share and brace the top anchor.


The knot block may be removed to form a load sharing anchor.  The working end may be passed over Category B and C anchors if care is used to maintain compressive loading to the the structure.  It is often complicated and unnecessary to do so but that it is possible makes it a useful technique.  As the rope shares the climbers mass on both legs either side of the anchor force will increase at the top and care must be taken to ensure that this doesn’t become a dangerous levering tool like the small boy on the see saw.


All SRT anchors need forward planning to make them retrievable.

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Bracing Anchor

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Load Sharing


Organic anchor points will always leave a grey area because one can never truly understand their strength.  Climbers can arm themselves through knowledge, training and systematic and logical thinking and if they do so the benefits of being a climber will deepen the experience of working with trees and hopefully to will seep into wider society as we create arboriculture.

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