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<div class="moz-cite-prefix">On 2013-08-29 16:57, Colin Smale wrote
:<br>
</div>
<blockquote cite="mid:46bdd1416e7285faf3901c6fa7003f5c@xs4all.nl"
type="cite">
<p>Won't the calculated radius depend on the number of points on
the way, and the width of the road? If you look closely at the
geometry of a curved road in OSM it is of course made up of
straight segments with a certain angle between them. A right
angle junction might be a 45 steps of 2 degrees each, giving a
smooth curve with a certain radius, but it can also be
represented by a single node with an angle of 90 degrees. How do
you work out the turning radius from that?</p>
</blockquote>
I said on the Osmand forum that for the same total angle, the radius
can be long or short. It does not really depend on the number of
points (1). The most significant parameters are the straight ways
with their points at which the road starts turning. If the bend is
as simple as that, with a constant curvature, then it's simply the
radius of the circumference that's tangent to the straight ways at
the said points. But, of course, the road may not be an arc of
circumference, which makes the radius smaller (1).<br>
But, having seen what I saw, I doubt that the mapping of many bends
is precise enough, especially those I moved 20 m or much more back
to their real place, which some sometimes called micro-mapping.<br>
Regarding 90° junctions, their radius is theoretically infinite so
that rotating a car around its vertical axis is unsafe at any speed
;-) In reality, it depends on the minimum turning radius which
you'll find in car catalogs. But as the cars stop or almost at such
junctions, you divide a very small speed by that radius and that's
why the cars very rarely skid there ;-)<br>
<br>
<div class="moz-cite-prefix">On 2013-08-29 17:12, Pieren wrote :</div>
<blockquote
cite="mid:CAPT3zJrq6B6ME8DNmxB_DbrnJ9NeROwMh5EcVkPoz2AM+yoW9Q@mail.gmail.com"
type="cite">Then, instead of going on site to measure it with a
decameter and add a new tag on a splitted segment, I would simply
add more nodes....</blockquote>
With a new tag precisely_computable_radius=yes and a trig lib in the
GPS to compute it.<br>
With an average of 1 per continent, the turning_radius tag is mapped
on a node, no split, they forget to specify which but logically at
the smallest radius.<br>
As to measuring it, you're right about the decameter, the first
problem is finding the center and the second one spinning round with
trees in the way and cars passing on the road ;-)<br>
And well, amusing methods for funny tags, I would use a varying
radius circumference to overlay the road on an ortho, the largest
radius so that no part of the road extends outside the
circumference. Even a single circumference in a zoomable transparent
layer does it. No math even needed with the two zoom factors. Slide
the circumference under the scale of the road layer to measure it,
or use a measuring tool in the road layer.<br>
<br>
Did I say fun?<br>
Drive carefully,<br>
<br>
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<td>André.</td>
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<br>
(1) 2+3 points exactly on a circumference make a good bet that it is
one, but more points are needed to make sure there is no very
improbable zig-zag in between. Take some time to read about the <a
href="https://en.wikipedia.org/wiki/B%C3%A9zier_curve">Bézier
curves</a> for more complicated ones if you like.<br>
<br>
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