UKTC Archive

Re: Tree canopy cover as a sustainability metric: Planning for the future - consultation

Subject: Re: Tree canopy cover as a sustainability metric: Planning for the future - consultation
From: Bill Anderson
Date: Sep 28 2020 08:44:32
Perhaps your average urban-based arb understands that his day-to-day
employment is likely to mean the figures obtained for canopy cover can very
rapidly become inaccurate. And I suspect that counting trees is often
confused with measuring canopy cover as well. Not to mention the conundrum
that a spreading weeping Willow might be only 10 metres tall but have a
crown projection greater than a 30 metre tall Wellingtonia. It might be
better, in terms of particle interception at least, to attempt to work out
total crown volume per hectare. I mean, a recently planted parcel of Sitka
Spruce would bear no resemblance to a similar sized parcel immediately
prior to harvesting, but still provide more or less the same "canopy cover."
Wouldn't it?

On Mon, 28 Sep 2020 at 09:27, Brynley Andrews <brynley.andrews@xxxxxx.com>
wrote:

Hi Jon

Thanks for the questions. I wish I had the time to answer your questions
thoroughly, & besides this thread is really looking for info to backup the
principle of using TCC as a metric of sustainability - because of the
arboricultural implications and ease of use/transparency - so as to raise
the issue of tree cover within the planning for the future consultation
process.

1. The <2% I meant is the threshold set by forestry research (FR) for
standard deviation within their UK ward tree canopy mapping project. As a
baseline for further consideration within the planning process. FR give
info linked here - that indicates the UK has an average of 15.8% TCC, which
based on the hundreds of wards done seems like it is fairly accurate.
https://www.forestresearch.gov.uk/research/i-tree-eco/urbancanopycover/

2. The above FR TCC process is repeatable/reproducible and transparent/open
access, and so for the sake of a baseline scoping metric of
sustainability measurement I suggest it is good. And simply reflects
common-sense; also, it is readily understandable by all and Google Earth /
site visits enables verification.

3. As you know, CAD shapes can be derived from drawn/produced polygons of
any shape, from multiple survey methods thereby giving good enough accuracy
again. I did not envisage circles.

What concerns me more than the technical issues you raise, is that there is
seemingly a lack of engagement with urban TCC measurement by the arbs. I
hope I am wrong? But that is a whole other topic for another day, perhaps.

Thanks

Brynley


On Fri, 25 Sep 2020 at 17:58, Jon Heuch <jh@xxxxxxxx.co.uk> wrote:

Brynley



i)                    You can’t measure accuracy! (that is - from
sampling).

ii)                   Please provide me with a reference that suggests
they
are estimating something to “2%”.  If they do have a measure it will be
of
precision. Something very different to accuracy. If you apply more sample
points your precision increases & that appears to give you a good
measure.
But it doesn’t because of the problems I describe.

iii)                 CAD will very accurately measure what you put into
CAD.
That is not the problem. If you draw a circle around a tree, CAD will do
a
wonderful job of telling you the area of the circle. The first big
problem
is that images have shadows, varying from image to image; if you are
dealing
with only one image, there will at least be some uniformity across the
image. The second is that trees are not uniform cylinders, spheres or
other
shapes so whatever you measure, however you measure.



You might want to read the musings of Francisco J. Escobedo (used to work
with I Tree; Google Scholar gives you an entrée to his work) who gave a
talk
at the AA conference in 2016. He at least started to discuss some of the
statistical issues related to inventory of the urban forest. I can tell
you
simply you won’t pick up modest but real changes in canopy cover!
Undoubtedly you will generate numbers. Undoubtedly they will be
different.
Will they be meaningful? Most unlikely.



Jon






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Brynley M M Andrews MSc., C.Env., M.Arbor.A.
www.brynleyandrewsassociates.com
01935 XXXXXX
07970 XXXXXX



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To unsubscribe send mailto:uktc-unsubscribe@xxxxxx.tree-care.info

The UKTC forum is supported by Bosky Trees arboricultural consultancy and
Stockholm Tree Pits
https://www.stockholmtreepits.co.uk