# [gdal-dev] Polygonizer / Lines

Peter Halls p.halls at york.ac.uk
Fri Mar 15 06:23:40 PDT 2013

```Alisson,

in vector mode, where you are working with linestrings, the procedure
is as described in Stephen Wise's book, GIS Basics (Taylor & Francis, 2002)
chapters 2 - 4.  You will need to compute the intersection of all the lines
(Chapter 3), split them at the intersections and either duplicate the
common boundary segments or (my preferred way) build a simple topology
table that lists each line segment required for each polygon.  That way you
can still output the set of complete polygons to a shapefile (no
topology).  You will need to take account of the fact that some line
segments must be reversed when building the polygon.

Does that help?

Best wishes,

Peter

On 15 March 2013 12:50, Alisson Barbosa <alisson.uece at gmail.com> wrote:

> Thank you Chaitanya,
>
> I had read the explanation of Peter Halls. But that was *raster
> polygonization*. I am currently working with another type: *polygonization
> of OGRLineStrings*.
>
> My question was because i was not able to polygonize using the mentioned
> method. I created an OGRMultiLineString or an OGRGeometryCollection and
> addGeometryDirectly methods. But the polygonization always returned null.
>
> I solved using this:
>
> OGRMultiLineString *mls = new OGRMultiLineString();
> OGRMultiLineString* pTemp = NULL;
> while(...) //iteration of lines
> {
>         //creation of line
>         pTemp = static_cast<OGRMultiLineString*>(mls->Union(line));
>         if (pTemp != NULL)
>         {
>             if( mls != NULL)
>             {
>                 if( mls->getNumGeometries() > 0)
>                     delete mls;
>             }
>             mls = pTemp;
>         }
> }
>
> Thanks,
>
> 2013/3/14 Chaitanya kumar CH <chaitanya.ch at gmail.com>
>
>> Alisson,
>>
>> You can refer to the method's documentation here:
>> http://www.gdal.org/ogr/classOGRGeometry.html#a674319670e735bf6d4049300096157ec
>>
>> The mechanism of this operation was already explained Peter Halls in this
>> mailing list[1]. GEOS does a good job identifying line strings touching at
>> end points and then combining them to make rings and then polygons.
>>
>> However, it's your job to ensure that all the line strings passed to the
>> method are part of a single polygon.
>>
>> [1]: http://lists.osgeo.org/pipermail/gdal-dev/2012-November/034793.html
>>
>>
>> On Thu, Mar 14, 2013 at 11:58 PM, Alisson Barbosa <alisson.uece at gmail.com
>> > wrote:
>>
>>> Can you explain a bit better?
>>>
>>>
>>> 2013/3/14 Chaitanya kumar CH <chaitanya.ch at gmail.com>
>>>
>>>> Allison,
>>>>
>>>> Check out OGRGeometry::Polygonize()
>>>>
>>>> The result depends on the type of intersections in your lines.
>>>>
>>>> --
>>>> Best regards,
>>>> Chaitanya Kumar CH
>>>> On 14 Mar 2013 21:53, "Alisson Barbosa" <alisson.uece at gmail.com> wrote:
>>>>
>>>>> Hi friends,
>>>>>
>>>>> I have a set of OGRLineStrings and an OGRLinearRing with many
>>>>> intersections. I would like to polygonize this set of lines. Can anybody
>>>>> help me?
>>>>>
>>>>> Best regards,
>>>>> --
>>>>> Alisson Barbosa
>>>>> Systems Analyst - FUNCEME
>>>>> M.Sc. in Computer Science - MACC
>>>>> Graduate in Computer Science  - UECE
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> gdal-dev mailing list
>>>>> gdal-dev at lists.osgeo.org
>>>>> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>>>>>
>>>>
>>>
>>>
>>> --
>>> Alisson Barbosa
>>> Systems Analyst - FUNCEME
>>> M.Sc. in Computer Science - MACC
>>> Graduate in Computer Science  - UECE
>>>
>>>
>>
>>
>> --
>> Best regards,
>> Chaitanya kumar CH.
>>
>> +91-9494447584
>> 17.2416N 80.1426E
>>
>
>
>
> --
> Alisson Barbosa
> Systems Analyst - FUNCEME
> M.Sc. in Computer Science - MACC
> Graduate in Computer Science  - UECE
>
>
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>

--
----------------------------------------------------------------------------------------------------------------
Peter J Halls, GIS Advisor & Team Leader Applications Support and Training,
Information Directorate, University of York
Telephone: 01904 323806     Fax: 01904 323740
Snail mail: Harry Fairhurst Building, University of York,
Heslington, York YO10 5DD
This message has the status of a private and personal communication
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