UKTC Archive

RE: Plotting groups

Subject: RE: Plotting groups
From: Mike Ellison
Date: Dec 17 2008 13:39:20
 -----Original Message-----
From: [] 
Sent: 15 December 2008 11:36
To: UK Tree Care
Subject: RE: Plotting groups

Thanks Ron and Luke,
I suppose my question is more along the lines of...
"Is there any case law out there which has dealt with the failure of a
tree from within a group of trees, where the tree was previously
surveyed as "one-of-a-group," as opposed to an individual?"
eg. Lets say, a mixed woodland group is plotted within a GIS database as
such, and a large ash failed from within this group.
In this instance, both the local authority and surveyor might struggle
to provide proof that a detailed survey had been undertaken in respect
of that particular tree.

I ask on behalf of a local authority, who wish to survey trees as 
groups and only plot individual trees as exceptions to this rule.   I 
wonder if Mike Ellison could help?

Brendan contacted me off forum and we have agreed to post our emails
here to continue the thread.



I have responded to your email below, but in essence, if what we are
assessing is the risk of significant harm occurring - generally we are
not concerned with minor harm - then there must be something of
significance to be harmed by tree failure. Otherwise, I will advise my
clients that expending resources on tree safety assessment is most
probably disproportionate to the risk that they represent.

In responding to your email, I have gone through the approach that I
generally take and hope that this is helpful. It would be good to
discuss the issues further.


BT. <My question revolves around a methodology that a local authority
have devised.  They wish to plot trees as large groups, and to plot
individual trees "only in exception.">  

MJE. As you might expect, I can't comment on a particular methodology
without seeing the detail, but I will comment on the more specific
points that you have raised.

BT. <This is driven by cost savings and a wish for a limited number of
data entries within their database.>

MJE. This is an interesting point and might be viewed from a different
perspective.  Under the Health and Safety at Work Act 1974 S3 a council
has the following duty "(1)It shall be the duty of every employer to
conduct his undertaking in such a way as to ensure, so far as is
reasonably practicable, that persons not in his employment who may be
affected thereby are not thereby exposed to risks to their health or
safety." Of particular relevance to your statement is the matter of what
is 'reasonably practicable'.  As the 'enforcing authority' the Health
and Safety Executive (HSE) provides guidance particularly to assist the
compliance of duty holders and to assist HSE's enforcement officers in
their consideration of breaches of Regulations that apply under the Act.
With particular regard to the 'reasonably practicable' issue, HSE
guidance advises that "a proposed control measure must be implemented if
the 'sacrifice' (or costs) are not grossly disproportionate to the
benefits achieved by the measure". In other words, controls should be
implemented (only) so long as the cost of reducing the risk can be
justified by the benefit of the projected risk reduction.  So what might
be referred to as 'cost saving' might be better described as eliminating
needless costs and thereby acting in a reasonably practicable manner.
Councils are charged with managing the taxpayers' services and
resources.  Allocating those resources to disproportionate risk
assessment will either reduce resources available for other services or
result in increased taxes. It doesn't end there because the closer we
look the more we see and with the more detailed tree safety assessment
usually comes more remedial work with its associated costs.

BT. <I am aware that QTRA plots trees as groups and group composition is
determined on a number of factors, including target occupancy.  But my
question does not relate to QTRA methodology directly.

I am trying to ascertain whether I could be found negligent, as a tree
surveyor, if a tree that I surveyed (which had defects) failed and the
quality of the information recorded for the individual specimen was
poor, because the methodology required trees to be plotted as groups?  
I should state that we all plot trees as groups where circumstance best
suits this purpose, but I usually plot large street trees, for instance,
as individuals.>

MJE. I understand how, given recent court judgments, one might be a
little confused as to how the duty of the council might reasonably be
discharged. On issuing a fee proposal, I set out what it is that I am
supplying to my client and in doing this make it clear that I will
record the generality of the tree population in appropriate groupings
and that I will assess the trees at a level that is appropriate to the
circumstances.  This may in some situations involve plotting individual
trees but not necessarily making a separate database record for each

When I visit a group, a woodland or some wider collection of trees to
carry out a safety assessment I always assess the context first.  Often
I have assessed the distribution of trees and their relationship with
land-use using aerial photographs before I leave the office. A
preliminary assessment will inform my approach and I will know that the
large declining tree over the busy highway will require more of my
attention than the smaller tree of good vitality that overhangs a
private driveway. I will also know that I don't need to assess the copse
that is located in agricultural land and has no particular access. Where
the circumstances dictate a close assessment of each tree then each tree
will be plotted and closely assessed, but they may be described as a
group and only those trees that require some particular reference - be
that for safety or some other management reason - will be logged
individually.  Even then, the reference may be limited to a note in the
more general record for the group. QTRA is my risk assessment method of
choice and I use this to assess the most significant risk in the group.
If that risk falls well within the client's acceptable threshold then
all of the other trees must represent an equally low or lower risk. If
the risk associated with the worst tree falls above the client's
acceptable threshold then the next worst case is assessed and so on.

I go through a structured process of assessment and recording based on
my client's policy (or adopted approach) and my professional judgment.
If I have carried out a proper risk assessment with the outcome that
there were no unacceptable risks and a tree from the group subsequently
fails and causes significant harm, this is not, as some would have you
believe, that the risk was higher than I had assessed, it is simply that
the risk has been realised.  The approach requires that I assess and
consider only what is significant otherwise I would be focussing on
every defect almost irrespective of significance, simply because it
could fail and could cause harm. Worse still, I would be producing long
schedules of remedial work because I can, because it shifts
responsibility back to the owner or manager of the tree and because
there is no cost to me.  When arboriculturists provide value for money
tree safety management then they might reasonably expect more land
owners to buy into their services.  At the moment it seems to me that
many in our industry are seriously risk averse. Barrister Richard Stead
seems to have picked up on this in a recent article in the
Arboricultural Journal when he pointed out that "The standard of
inspection required is influenced by the higher standards of the
arboricultural profession which may impose upon individual landowners
unrealistic demands. This may lead to a defensive approach to tree

In an earlier email you said "We know that all trees are slightly
different and a single database entry for a group of trees will only
ever record the 'average tree' within the group."  What I am advocating
is not that we record the average tree for the group but that we record
everything that we consider significant to the task in hand. If the task
is a tree safety assessment then we should record the significant risks.
What you and I and our clients consider significant may be different
things and it is by setting such matters out in a clear contract -
usually based on either the clients policy or their adopted approach -
that we can share the responsibility for the risk assessment process
where we advise how risky the trees are and the client makes informed
management decisions based on our judgment and advice.  The alternative
seems to be that those clients who can find the cash try to push the
liability over to us and we in turn specify unrealistic risk controls
and push it back to them.  Of course this process has potential to
generate work for the industry.

BT. <I appreciate that the methodology would need to be looked at in
detail. Do you know of any case law, where a tree has failed from within
a "group record" within a database?  If so, did the prosecution question
the quality of the survey data?>

MJE. No, but the Atkins judgment is useful in respect of how little you
might do and still defend your approach. 

The UK Tree Care mailing list
To unsubscribe send

The UKTC is supported by The Arbor Centre