[Tagging] Peak-based mountain range proof of concept

Wed Dec 30 00:50:33 UTC 2020

This is very cool.  While you're at it, the step just before you apply the
2% buffer ALSO looks like it gives you a whole bunch of very reasonable
boundaries that you could use for "valleys" if any of those little inlets
are named..

On Tue, Dec 29, 2020 at 7:41 PM Kevin Kenny <kevin.b.kenny at gmail.com> wrote:

> On Tue, Dec 29, 2020 at 10:01 AM Brian M. Sperlongano <
> zelonewolf at gmail.com> wrote:
>
>> I went with what I could objectively verify with the help of local
>> knowledge.  The Green Mountains are not the only mountain range in Vermont,
>> there is also the Taconic range which runs through Vermont and over the
>> line into New York.  It's west of US7 in southern Vermont, and there are a
>> few mountains in the "Green Mountain National Forest" that are actually
>> part of the Taconic range.  Part of the discussion was "should foothills be
>> included?" - and the answer was that "Vermonters wouldn't consider the
>> foothills to be part of the mountains".  So, I specifically attempted to
>> exclude them.  I excluded anything with "Hill" in the name which eliminated
>> both the foothills and many (but not all) of the sub-peaks.
>>
>> The initial data set was formed by pulling down a rectangle around VT
>> with natural=peak objects, then filtering out for elevation so I could pick
>> out the parallel spines that make the shape of the mountain range that
>> matched the rough shape of where I understood the range to be.  (I've done
>> some hiking in the GM, but certainly not enough to declare any sort of
>> local knowledge).  From that starting point, I consulted actual local
>> knowledge to figure out what to add or subtract from that set.
>>
>
>  I decided to run with Brian's idea to see how far I could take it, taking
> into account some reasonably reilable, public-domain, but non-OSM
> information (plus some local knowledge).  Here's a detailed analysis of how
> an indefinite 'Catskill Mountains' polygon might be produced - and some
> comments on the challenge of identifying a 'spine' as a linear feature.
>
> Warning: This is quite technical. You're welcome to skim and look at the
> pictures. I suggest having the Flickr set open in a separate window, so you
> can hit the 'forward' control to go through them.
>
> I began the analysis by asking the wrong question.  I picked Slide
> Mountain, the highest peak in the range, and asked, 'what's the lowest
> contour that encircles Slide but no higher peak?'  That' I thought, would
> give the ultimate outer border of the range - anything outside that contour
> would be either in the flatlands/valleys, or would "look to" a different
> range. It was surely an outer limit: it wound up spanning a comma-shaped
> area from the eastern Finger Lakes to the Capital Region of New York, down
> almost to Harrisburg, Pennsylvania, encompassing not only the Catskills,
> but also the Poconos, a lot of the rolling hills of central New York, and a
> sizable chunk of the Appalachian ridge.
> https://www.flickr.com/photos/ke9tv/50777004162/in/album-72157717614241653/
> Clearly, I needed a tighter boundary than that!  (Although this shows just
> how prominent Slide Mountain is: the nearest higher peak along its
> prominence line is in West Virginia, and the nearest more prominent peak
> along the line is all the way in North Carolina.  The deep valley of the
> Mohawk River cuts it off from the nearer higher summits to the north and
> east.)
>
> Instead, I asked the question, 'what is the highest-elevation contour that
> still encloses the 35 summits of the Catskill high peaks.
> https://en.wikipedia.org/wiki/Catskill_High_Peaks   The resulting polygon
> was encouraging:
> https://www.flickr.com/photos/ke9tv/50777003997/in/album-72157717614241653/.
> The lowest point needed to make a single connected polygon with the 35
> peaks under consideration was a pass on New York State Route 42 separating
> the drainages of West Kill - Schoharie Creek - Mohawk River - Hudson River
> and Bushnellsville Brook - Esopus Creek - Hudson River.
> https://www.flickr.com/photos/ke9tv/50777003802/in/album-72157717614241653/
> Raising the contour by another ten metres divides the polygon there. (I
> actually took a shortcut: I asked "what's the HIGHEST enclosing contour for
> Hunter Mountain (the second most prominent peak in the range) that excludes
> Slide Mountain or any higher peak?" and found that thresholding at that
> elevation produced a polygon that enclosed all 35 high peaks.)
>
> Along most of its border, it appeared to be nearly coterminuous with the
> Catskill Park, and a quick glance (informed by intimate local knowledge)
> made me say, "yeah, that's all mountainous"... and prompted me to look for
> further gaffes.  Here's where subjectivity starts to come in a little bit -
> I'm applying local knowledge about what the inhabitants would say "are the
> Catskills" or "are not the Catskills". Fortunately, none of my calls would
> be at all controversial locally.
>
> https://www.flickr.com/photos/ke9tv/50777003787/in/album-72157717614241653/
> - This area is accounted by the locals, including the local geologists, as
> belonging to the Helderbergs rather than the Catskills.  The demarcation is
> pretty clear - the narrow neck at the pass on County Route 3.  Given this
> local knowledge, I cut this area off manually - without changing the
> elevation threshold elsewhere.
>
> https://www.flickr.com/photos/ke9tv/50776134543/in/album-72157717614241653/
> - The elevation threshold isolated Bald Mountain, Tower Mountain and some
> unnamed heights of land in the Town of Jewett.  Retaining the same
> elevation threshold, I added the missing polygons.
>
> https://www.flickr.com/photos/ke9tv/50776892081/in/album-72157717614241653/
> - The elevation theshold isolated Mount Tremper, Carl Mountain, Mount
> Tobias and Mount Guardian, on a ridge between Mount Tremper village and
> Woodstock.  Again, I kept the same elevation threshold and added the
> missing polygons.
>
> https://www.flickr.com/photos/ke9tv/50776892026/in/album-72157717614241653/
> - Ticetonyk Mountain is below the Catskill Escarpment, but is counted among
> the Catskills.  It would have been isolated even using the criteria of the
> failed attempt with Slide Mountain. I retained the elevation theshold and
> added the missing area.
>
> https://www.flickr.com/photos/ke9tv/50777003512/in/album-72157717614241653/
> - Ashokan High Point, and the rest of the peaks on its ridge, are cut off
> by the pass between the Rondout and Ashokan valleys. Again, they're very
> close to the boundary of the original region. I retained the elevation
> theshold and added the missing areas.
>
> https://www.flickr.com/photos/ke9tv/50776891851/in/album-72157717614241653/
> - Plattekiill Mountain and Mount Utsayantha are accounted among the
> 'Catskill Hundred Highest', and are again separated by only a short
> distance (and the East Branch of the Delaware). The same drill - retain the
> elevation threshold and add the missing areas.
>
> https://www.flickr.com/photos/ke9tv/50776134113/in/album-72157717614241653/
> - The good burghers of Delhi and Andes consider the hills to their
> southwest to be among the Catskills, and who am I to argue?  Again, the
> elevation threshold can be kept and gives a clean polygon.
>
> https://www.flickr.com/photos/ke9tv/50776133938/in/album-72157717614241653/
> - The mountains are really petering out into rolling hills here, and hardly
> anyone would argue with fixing the East Branch of the Delaware as the
> ultimate western limit of the range. I excluded this region.
>
> That pretty much got me around the whole border of the region, and left me
> with a reasonably tidy, and objectively mountainous, area shown in
> https://www.flickr.com/photos/ke9tv/50777002877/in/album-72157717614241653/
> .
>
> The chief purpose of having such an area would be shaped label placement.
> Rendering its boundaries would never be appropriate. But having the shape
> would be important, particularly in rendering paper maps whose area of
> interest partially overlaps with the region.  For instance, a map showing
> only the extreme eastern portion might well want to have a curved label,
> 'CATSKILL MOUNTAINS', following the curve of the ridge above the eastern
> escarpment.    I know that this sort of sophisticated label placement is
> beyond our current capabilities, but we have a chicken-and-egg problem that
> it's impossible to do better cartography without the data to support it.
>
> In many cases, we'd want to simplify (more accurately, generalize) the
> area for rendering at a smaller scale.  For stand-alone areas like this
> one, one of my favourite methods for generalization is morphological
> closure.  I include a few images indicating how this can work.  For the
> closure, you have to pick a length scale.  A workable scale is often about
> 1-2% of the largest dimension of the area feature; that suffices to retain
> the shape but to fill in gaps and smooth out rough edges. We get this
> dimension by calculating the smallest enclosing circle.
> https://www.flickr.com/photos/ke9tv/50777002732/in/album-72157717614241653/
> - here I found that the radius of the circle was 51.7 km.
>
> I therefore buffered the area by 2.07 km - 2% of the diameter, to arrive
> at
> https://www.flickr.com/photos/ke9tv/50776133413/in/album-72157717614241653/
> - which is overly large, but fills in all the gaps.  The 'morphological
> closure' operation is then accomplished by buffering by _negative_ 2.07 km,
> removing the excess around the perimeter but leaving gaps and indentations
> filled.  This should be a very nice simplified enclosing polygon for
> applications that need one.
> https://www.flickr.com/photos/ke9tv/50776890916/in/album-72157717614241653/
>
> I also did a rough ridge line analysis at
> https://www.flickr.com/photos/ke9tv/50776308698/in/album-72157717614241653/
> .  The most significant ridge - the only one separating the basins of
> first-order rivers - is the Catskill Divide, with the basin of the Delaware
> to the west and that of the Hudson to the east.  It's hard for me to see a
> good way to choose a 'spine' for the range with the ridges running as they
> do in every direction.  If I were asked to choose the next most significant
> ridges, they would be the one separating the East Branch and West Branch of
> the Delaware, running southwest from the Catskill Divide at top center and
> ending at the border of the mountains near the Cannonsville Reservoir; and
> the one separating the Mohawk River basin from those of other Hudson River
> tributaries. It starts from the Catskill Divide near the west border of the
> Hunter-West Kill Wilderness, runs generally east almost to the eastern
> escarpment, then turns abruptly north and follows the ridge of the
> escarpment, continuing north out of the region into the Helderbergs
> beyond.  But then I'd feel to add the ridge of the Pepacton range to the
> west (south of the reservoir near the center of the image), so as not to
> lose all linear labeling in that area. I think that the 'label the ridges'
> idea has been defeated by the complex topography (and equally complex
> topology).
>
> Fortunately, I think that maintaining a fixed elevation threshold for a
> given range gives us some hope of having manageable (multi)polygons with a
> modicum of objectivity.  I'd even be willing to make the sacrifice of,
> 'that lobe to the north is really part of the Helderbergs, but OSM rules
> insist it has to be grouped with the Catskills' as a less-than-ideal
> solution if it will get the verifiability police off my back.
>
>
>
>
>
>
>
> --
> 73 de ke9tv/2, Kevin
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