# Geometry :

Subject: [ggl] Snapping
From: Stephen Leary (sleary)
Date: 2009-11-30 15:55:55

Not 100% sure my snapping document got through so here is a link to the PDF.

Stephen

On Mon, Nov 30, 2009 at 2:29 PM, Stephen Leary <sleary_at_[hidden]> wrote:

> Attached is a PDF with the beginnings of a snapping spec.
>
> Thoughts welcome.
>
> Stephen
>
>
> On Fri, Nov 27, 2009 at 8:50 AM, Stephen Leary <sleary_at_[hidden]> wrote:
>
>> All of the above.
>>
>> The way this is usually done (in GIS) is to have the entire portion of
>> line A that is within snapping tolerance of line B snap down onto line B.
>> Vertices are inserted n line A at all threshold points (points which are
>> exactly snapping tolerance from a line) and then all vertices within or at
>> snapping tolerance on line A get moved to the nearest point on line B.
>>
>> There are exceptions for when two lines cross at close to 90 degrees.
>>
>> Its a little odd and I admit and I didnt like it when i first encountered
>> it but eventually I was convinced of the business case for it,
>>
>> I will definately try and find time to get some diagrams together,
>>
>> Stephen
>>
>>
>>
>>
>> On Fri, Nov 27, 2009 at 8:43 AM, Barend Gehrels <Barend.Gehrels_at_[hidden]
>> > wrote:
>>
>>> Hi Stephen,
>>>
>>> OK, I'm curious. I didn't understand your diagram completely. You
>>> mentioned that another would come, one so didn't react on this earlier. I
>>> understand that you've two lines, and basically want to snap one line on
>>> another line, based on mutal vertex distances. Do you also snap between
>>> vertices? According to your diagram, yes.
>>> Do you also need to snap one line to multiple lines? Or more lines to
>>> multiple lines?
>>>
>>> Regards, Barend
>>>
>>>
>>>
>>> Stephen Leary wrote:
>>>
>>> Thanks Barend,
>>>
>>> I will do some simple snap tests at the weekend.
>>>
>>> Stephen
>>>
>>> On Sun, Nov 22, 2009 at 10:29 PM, Stephen Leary <sleary_at_[hidden]>wrote:
>>>
>>>> This is a very crude first attempt at a diagram (I will do another soon)
>>>> but the left hand scenario the line at an angle snaps to the other side with
>>>> the result being on the right.
>>>>
>>>> In GIS we usually have rules for what snaps to what. Most common new
>>>> features snap to existing ones.
>>>>
>>>> If we get this snapping in place it would allow us to do ISO 19107
>>>> topology further down the line.
>>>>
>>>> Cheers,
>>>> Stephen
>>>>
>>>> On Sun, Nov 22, 2009 at 6:48 PM, Barend Gehrels <
>>>> Barend.Gehrels_at_[hidden]> wrote:
>>>>
>>>>> Hi Stephen,
>>>>>
>>>>>
>>>>>
>>>>> You mean something like: ggl::snap(geometry_from, geometry_to,
>>>>>> snapped_geometry);
>>>>>>
>>>>>
>>>>> Something like that yeah. Most of the actual work involved is in
>>>>> finding points on a line that are at the snapping tolerance distance from
>>>>> "snap to" feature.
>>>>>
>>>>>
>>>>> Ah, of course, specifying a tolerance is essential :-)
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> I'll have a look at the extension model then and see how this can fit.
>>>>> For me a snap feature is just as important as a "buffer" feature.
>>>>>
>>>>>
>>>>> OK, you'll need the account then, I'll send it to you privately.
>>>>>
>>>>>
>>>>>
>>>>> poly/poly breaks down to multiple line/line snaps. line line is a
>>>>> complicated beast because all parts of the line that are within a snap
>>>>> tolerance of the "snap to" line get snapped. I'll produce some diagrams of
>>>>> this and post them.
>>>>>
>>>>> I see. Points/lines might be snapped potentially to multiple lines or
>>>>> holes. I'll wait your diagrams.
>>>>>
>>>>> Regards, Barend
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> ggl mailing list
>>>>> ggl_at_[hidden]

>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Stephen Leary
>>>>
>>>
>>>
>>>
>>> --
>>> Stephen Leary
>>>
>>> ------------------------------
>>>
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>>>
>>>
>>> --
>>> -------------------------------------
>>> Barend Gehrels
>>> -------------------------------------http://www.erplex.comhttp://trac.osgeo.org/gglhttp://www.geodan.nl
>>> -------------------------------------
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>>>
>>>
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>>>
>>>
>>
>>
>> --
>> Stephen Leary
>>
>
>
>
> --
> Stephen Leary
>

```--
Stephen Leary
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