ArcGIS Pro 2.9 - How Collapse Hydro Polygon behave?

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12-06-2021 07:54 AM
VincentLaunstorfer
Regular Contributor

Hi,

I discover the Collapse Hydro Polygon and I still have a question on how does the tool behave at bends. The screenshot below show the unique polygon converted to centerline and I wonder why there is a dangling line at the bend? Is it a normal behavior of the tool? And what is the rational?

Thanks

 

Collapse_Hydro_Polygon.png

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DanPatterson
MVP Esteemed Contributor

Collapse Hydro Polygon (Cartography)—ArcGIS Pro | Documentation

it looks like it is partially collapsed, according to the help, you may have to play with the parameters to get what you want as an output


... sort of retired...
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VincentLaunstorfer
Regular Contributor

Thanks Dan.

I double-checked again the parameters but nothing really to deal with dangles in sharp bends. To me, it looks like the Collapse Hydro Polygon tool is confused in sharp bends as you can see here below. Small bends are ok Vs dangles in sharp bends! I guess the algorythm tries to follow the shape of the polygon to the further possible way but ends up thw wrong way, and then retries the centerline in a better direction... If this makes sense. This may explain why centerline always ends the wrong way at polygons ends, as below. In the tool notes, there are some warnings/limitations about possible confusion with certain complex polygon (which to me is not the case here). For info, polygons are not multipart.

Sharp BendsSharp BendsPolygon EndsPolygon Ends

 

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DanPatterson
MVP Esteemed Contributor

as explained in the help, there are limitations  and it is for cartographic issues and not designed for analysis.  You would have to collapse to a skeleton, split the line(s) at vertices and remove those that are dangles.  doable, but hardly worth the effort for most cartographic representations.  you seem to have hit the "issues" case with all the sharp bends


... sort of retired...
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VincentLaunstorfer
Regular Contributor

True, it is in Cartography Tools, meant for visual representation.

I wonder how the similar Polygon to Centerline in the Topographic Production Tools would behave... Similar algorithm or smarter? I cannot try it as it requires an extension!

Also, the Collapse Hydro Polygon is new in Pro 2.9 and perhaps not perfect (yet!)

https://pro.arcgis.com/en/pro-app/latest/tool-reference/topographic-production/polygon-to-centerline...

 

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DanPatterson
MVP Esteemed Contributor

I haven't tried polygon to centerline either.  I have used skeleton/medial axis and simply trimmed segments with a terminal node with no adjoining segments (aka, dangles).  A smart algorithm ignores the end segments for the long axis and only removes offshoots meeting a certain length criteria.  Lots of papers on the subject with only few practical applications other than to keep people interested in computational geometry off the streets 😉 

PS, I notice that the width of the bend at the inflection point is greater than the width of the remaining polygon normal to the main line.  So this is likely to produce the dangles that you are seeing since collapsing the main "channel" will still leave an uncollapsed area at the inflection point (as compared to a "buffered" line which should in theory have a constant width along its length


... sort of retired...
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