[GRASS-user] snap point to line/break line at given points

[Johannes Radinger]

Am 25.04.2011 um 15:51 schrieb Moritz Lennert:

> On 25/04/11 12:04, Micha Silver wrote:
>
>>   On 04/24/2011 03:36 PM, Johannes Radinger wrote:
>>
>>> Hi Micha,
>>> Hi other GRASS users!
>>>
>>> Am 23.04.2011 um 18:01 schrieb Micha Silver:
>>>
>>>
>>>
>>> So if I understand you correctly, it is not really possible to  
>>> split a
>>> river network at one point into to parts. I attached a picture to
>>> illustrate what I want (e.g. two break points in a rivernet). I  
>>> think
>>> that this kind of breaking I want to do is quite common, so there's
>>> probably another solution in grass gis...
>>>
>>>
>>
>> Well, in QGIS you can merge all the line segments of the original
>> shapefile into one, using the "Vector->Geometry Tools->Singleparts to
>> Multipart" tool. This will leave you with a single  "MULTILINE"   
>> feature.
>>
>> However when importing to GRASS all the lines will be "re-broken" at
>> each intersection and new nodes created in order to enforce topology.
>> Referring to the attached image (I altered yours) the stars represent
>> topological "errors" and as far as I know GRASS will not allow you to
>> ignore these intersections when you import. (The only vector feature
>> that can be imported without topology is points).
>>
>> So to recap:
>> * You can split line features at arbitrary points using v.edit  
>> tool=break
>> * You cannot, AFAIK, cause GRASS to merge line segments where
>> topological rules require there to be a node.
>>
>
> You can, however, attribute the same category number (or the same  
> other arbitrary attribute) to those segments which belong to the  
> same subnetwork.
>
>
For sure that would be a solution to have the same category number  
for all single segments in a subnetwork. But how to define the  
subnetwork between the breakpoints, or how can I tell grass which  
segments belong to a subnetwork, so far I don't know any tool in  
GRASS which I can use in a python script.



> Your "physical" (i.e. on disk in GRASS format) representation of  
> the network(s) does not necessarily have to be the same as the  
> semantical (content-oriented) representation. So you can represent  
> the subnetworks through attribute info. Or you can play with  
> layers: the entire river network with the same cat value in layer  
> 1, different cat values for subnetworks in layer 2.
>
Either with layers or with additonal attributes both can be solutions  
but the problem is still the "how", the problem to define this  
subnetworks (see picture in former posts)

/johannes


>
> If you need to extract only parts of the network, almost all of the  
> modules allow you to either use cat values or a where clause on  
> your attribute as a filter.
>
> Moritz
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