[Tagging] Estimated values for height

Kevin Kenny kevin.b.kenny at gmail.com
Tue Nov 13 16:36:06 UTC 2018


On Tue, Nov 13, 2018 at 10:09 AM Sergio Manzi <smz at smz.it> wrote:

> Yeah, agreed. And I think in our context "*estimate*" should be more
> taken as "*quesstimate*", i.e. "*a first rough approximation pending a
> more accurate estimate, or it may be an educated guess at something for
> which no better information will become available*" [1].
>
> Now... how do we tag this... "*thing*"?  :-)
>
> My personal idea is that it should be:
>
> *either*
>
> *measure*:accuracy=estimate (e.g.: height=10 + "height:accuracy=estimate")
>
> *or*
>
> accuracy:*measure*=estimate (e.g.: height=10 + "accuracy:height=estimate")
>
> *and*
>
> get rid of all the est_* tags (e.g.: est_height=10)
>
>
 I'd mostly agree. When this gets wikified, let's make it clear, though,
that the ruleis "map what you know" rather than "don't map until you have
all the detail." (Too many discussions here come up with schemes where
mappers have to do additional research beyond what they can see in the
field before they can map in a conformant way. We don't want that.)

Now, as to the height of a tree. I've been tempted to map a few locally
spectacular examples, particularly of _Tsuga canadensis_ (and decided to
refrain: https://8thlnt.wordpress.com/). I had been thinking in terms of
just putting in the number of metres - but if I were to map such a thing,
ought I to distinguish:

- step back and simply visually estimate how many copies of my six-foot
hiking partner it would take to make the height of the tree.
- pace back a known distance (with the usual inaccuracy of pacing off a
distance) and use a clinometer to measure the angle subtended by the tree.
- frame the tree with stadia marks in a transit and tape off the distance
to the tree
- (I've never done this!) Climb the tree and drop a weighted tape.

The accuracy of these techniques varies by 4-5 orders of magnitude. The
simple visual method is probably going to have a standard error of a few
metres on a big tree (because I'm reasonably skilled at such things), which
can be reduced to tens of cm with the clinometer, a few cm with a transit
and tape, and sub-cm using direct measurement with a tape.  With the
specific example of a tree, nobody cares about a few cm, because trees
flex, and grow, and measurements aren't going to be repeatable to that
level (With a tower, it might become significant.)

At that point, for a tree, the only significant difference becomes "are
measurement errors of the same order of magnitude as the natural variation
in the measured quantity?" With a visual estimate, the answer is most
likely "no", with a clinometer and pace count, it becomes "yes," and
everything more sophisticated is mostly "wasted precision". So for a tree,
"measured" and "estimated" really are the only two categories that matter.
(And perhaps the date of the measurement. Trees grow, and the big ones all
eventually take storm damage and die back.)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstreetmap.org/pipermail/tagging/attachments/20181113/f879a37e/attachment.html>


More information about the Tagging mailing list