[postgis-users] How do I manipulate a polygon?

[Steve Lehr]

Steve:

Hey thanks for the post.  You can't read my mind?  Sorry about that folks.

What I'm hoping to do is take a polygon shape file and generate a perimeter
polygon, so I can use it with a reference image.  I've been thinking its a
union, but perhaps the faster way is to compute a perimeter?  I thought this
group might have the answer, hoping it was already a function in postgis.

parcels.shp load into postgis
generatePermiterPolygon(parcels)
write polygon to new .shp
write a single line .dbf

Thanks
Steve

Steven Lehr
Assistant Professor
Freshmen Program
College of Engineering
Embry-Riddle Aeronautical University (LB159)
600 S. Clyde Morris Blvd.
Daytona Beach, FL 32114-3900
386-226-7740

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of
Stephen Woodbridge
Sent: Wednesday, March 16, 2005 2:56 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] How do I manipulate a polygon?


Hi Steve,

I think you need to tell us more about the data you are using from which
you expect to extract a polygon. For example:

1) is it street data, then how do you want to define the bounding polygon
2) is it tiger data? what layers, other data? what layers?
3) etc

If the data is topologically connected then it is a "simple" process of
identifying all the segment that belong to only one polygon and then
chain these together into a polygon/multipolygon. only the outer edge
will only be associated to a single polygon. If the there are islands,
rivers, lakes, etc. then you might have holes in the polygon or even
multiple disjunct polygons depending on how the data is organized.

This is exactly what you have to do to generate polygons from tiger data.

-Steve W.

Steve Lehr wrote:
> Can you make a an outline of a county only?  I'd like to take a shape file
> and compute a union shape to generate a single polygon for a whole county
> any one have any code they'd share or a thought on how to union all.
>
> Thanks
>
> Steven Lehr
> Assistant Professor
> Freshmen Program
> College of Engineering
> Embry-Riddle Aeronautical University (LB159)
> 600 S. Clyde Morris Blvd.
> Daytona Beach, FL 32114-3900
> 386-226-7740
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of
> strk at refractions.net
> Sent: Thursday, February 24, 2005 11:48 AM
> To: postgis-users at postgis.refractions.net
> Subject: Re: [postgis-users] How do I manipulate a polygon?
>
>
> On Thu, Feb 24, 2005 at 09:35:52AM -0700, Matt Lynch wrote:
>
>>Hi,
>>
>>I have a table that has a field populated with multipolygon.  How would
>>I go about manipulating the polygons (e.g. shifting the polygon,
>>add/drop vertices)?
>>
>>I did look through the online documentation and list archives, but did
>>not find anything.  Please accept my appologies if  I have missed the
>>obvious.
>
>
> For shifting you can use the translate() function - see chapter 6.2.6
> of the postgis manual.
>
> There is currently no direct interface for adding/dropping vertices
> of geometries.
>
> --strk;
>
>
>>Thanks,
>>
>>Matt
>>
>>
>>--
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>>
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