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: Brynley Andrews
Date: Sep 28 2020 08:27:06
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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The UKTC forum is supported by Bosky Trees arboricultural consultancy and
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