[GRASS-user] Criteria for v.clean tool=rmdangle

[Dwight Needels]

On Jun 17, 2009, at 11:13 AM, Markus GRASS wrote:

> Maybe start with a lower threshold first, e.g. 5 ft to remove D and E
> first, then increase the threshold for any other, longer dangles you
> would like to remove

This is exactly what I ended up doing, and it works great. The only  
question is how fine a distinction I want to make (do I really care if  
it removes a 25 ft dangle while leaving a 30 ft dangle or removes a 5  
ft while leaving a 10 ft?). For my GPS tracks I ended up running  
v.clean tool=rmdangle three times with threshold of 10, 20 and 30 ft.

>> It may be worth having a note called "What is a dangle?", but
>> regardless it would be good to have a statement that says something
>> like...
>>
>> "The rmdangle tool processes dangles sequentially by internal Id,
>> which may result in short lines with high Id values remaining after
>> lines with lower Id values have been deleted from the nearest branch
>> point."
>>
>> Does any of this look incorrect? Can the processing by internal Id be
>> confirmed?
> According to the source code, the rmdangle tool processes nodes by
> internal id, not lines. As soon as it finds a node with only one line
> connected, it looks at the length and the other end node. If only one
> other line is connected at that other end node, the length of that one
> is added. That continues until a node is found with more than two  
> lines
> of the desired type connecting to it. If the accumulated length is
> smaller than threshold or threshold is < 0, all lines from the start
> node up to that node are removed.
>
> I have updated the manual in grass-6.4.svn, please have a look at it  
> [1]
> and give feedback if it is now better explained what the different  
> tools
> do. You can safe a local copy of the html file for nicer display. I  
> did
> not modify the tooltip description in the GUI, that's long enough  
> already.
>
> [1]
> https://trac.osgeo.org/grass/browser/grass/branches/releasebranch_6_4/vector/v.clean/description.html
>
> Markus M

I looked at the updated manual, and I like the description for the  
rmdangle tool (big improvement). There are several somewhat surprising  
(but predictable) behaviors that it might be useful to point out.

A) It sometimes removes lines that are not part of a dangle in the  
original vector (line B in my test case). Specifically, if two branch  
points are connected by a short line, this connecting line may end up  
deleted as part of a newly created dangle. This probably won't happen  
very often without a very large or negative thresh, because it  
requires a single node with 3 short lines attached.

B) It sometimes doesn't remove lines that are part of a dangle in the  
original vector (line E in my test case). Specifically, if the  
original vector has a node with two dangles shorter than threshold,  
one of them (the one with the larger internal Id) will not be deleted.  
This will only happen where there is a single node with 2 short lines  
attached (e.g. near a terminus).

C) It sometimes ends up with a vector that still has dangles, which  
require a second pass (and, conceivably even a third pass).

D) Use of a negative threshold removes a somewhat surprising number of  
lines, but in a useful way (leaving loops and lines connecting loops).  
I have attached two screenshots showing this with my averaged GPS  
trail data (gray is original vector, red is vector after v.clean tool- 
rmdangle thresh=-1).

All of these quirks have the same underlying cause... the rmdangle  
tool does not remove all dangles in the original vector, it  
sequentially removes dangles one at a time and doesn't go back to  
lines it has already processed. If run repeatedly, the final vector  
will have no dangles shorter than threshold, but this is different  
than removing all of the dangles in the original. Perhaps this  
underlying behavior can be communicated with something like the text  
below.

Existing text...
"Threshold gives maximum line/boundary length in map units, degrees  
for latlon. Dangles shorter than thresh will be removed. All dangles  
will be removed if thresh < 0."

Suggested text...
"Threshold gives maximum line/boundary length in map units, degrees  
for latlon. Dangles shorter than thresh are removed sequentially.  
Lines that are not either part of a loop or connecting two loops will  
be removed if thresh < 0."

And perhaps under Notes, something like...

"The lines removed by the rmdangle tool will sometimes not be the same  
as the dangles in the original vector. For example, if the original  
vector has a node with two dangles shorter than threshold, one of them  
(the one with the larger internal Id) will not be deleted, because it  
is no longer part of a dangle. If the tool is run repeatedly, the  
final vector will have no dangles shorter than thresh. To  
preferentially remove the shortest dangles, use a small thresh value  
in the first pass, and a larger value in the second pass."

Thanks, -Dwight

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