[postgis-users] polygons crossing 0/360 longitude

[Stephen Woodbridge]

Shane,

If you take the polygon bbox and split that into two bboxes on opposite 
sides of the -180/180 line, then you should be able to intersect the 
original polygon with each of the right/left bbox polygons to split it 
into the respective right/left components.

-Steve

Shane Byrne wrote:
> ok
> I was thinking more of that polygon (170 20, -170 20, 175 5, 170 10, 170 
> 20) becoming:
> 
> Polygon 1:
> 170 20, 180 20, 180 10, 175 5, 170 10, 170 20
>         ^^^^^^  ^^^^^^
> Polygon 2:
> -180 20, -170 20, -180 10
> ^^^^^^^           ^^^^^^^
> 
> i.e. introducing two new vertices on the dividing meridian and having 
> the area of the two polygons add up to the total area of the original.
> 
> It sounds like there is no easy way to do this in PostGis though. I 
> think your approach of using perl to figure it out would probably be the 
>   easiest way.
> 
> My polygons are pretty simple and only cross the meridian once, I guess 
> a complicated polygon could do that several times which will confuse 
> things.
> 
> Cheers,
> Shane
> 
> 
> Carlos Ferrão wrote:
>> To calculate the two polygons I am using a PERL script on data
>> ingestion to build the insert statement..
>> If I have a polygon which is (lon/lat)
>> 170 20, -170 20, 175 5, 170 10, 170 20
>> it becomes
>> polygon 1:
>> 170 20, (-170+360=190) 20, 175 5, 170 10, 170 20
>>
>> polygon 2:
>> (170-360=-190) 20, -170 20, (175-360=-185) 5, (170-360=-190) 10,
>> (170-360=-190) 20
>>
>> Just insert the two polygons into a single row using the MULTIPOLYGON 
>> object.
>>
>> Carlos.
>>
>> On 6/21/06, Shane Byrne <shane at quake.mit.edu> wrote:
>>> Thanks Carlos, this is a good idea.
>>>
>>> Finding the problem polygons is easy enough, but how did you split them
>>> into two polygons along this meridian? Can I define a line and with that
>>> use some postgis function to split the polygon?
>>> I'd also like to copy over all the information in the other fields to
>>> each new polygon.
>>>
>>>
>>> Shane
>>>
>>> Carlos Ferrão wrote:
>>> > Hi,
>>> > I recently had a similar problem with products crossing the -180/180
>>> > longitude. The problem is that postgis doesn't connect the -180 to the
>>> > 180 as, for instance, Oracle Spatial Module.
>>> > The way I found to solve it was to do an algorithm to find the
>>> > polygons that cross the date line. For each one, I calculate 2
>>> > polygons, one in the negative coordinates and another in the positive
>>> > coordinates (you need to add/subtract 360). I add a multipolygon with
>>> > the information of the two polygons in a single row in the database
>>> > and it works perfectly.
>>> >
>>> > Hope it helps,
>>> > Carlos Ferrao
>>> > EOP - ESRIN - European Space Agency
>>> > Critical Software - www.criticalsoftware.com
>>> >
>>> > On 6/21/06, Shane Byrne <shane at quake.mit.edu> wrote:
>>> >> Hi,
>>> >> I have a postgres/postgis database with many (~80k) polygons, the
>>> >> vertices of which are stored in longitude (0-360) and latitude space.
>>> >> These polygons are typically small, but a few hundred of them span 
>>> the
>>> >> prime meridian. This creates a problem for me when I search for 
>>> polygon
>>> >> intersections. i.e. a polygon with a longitude range -5 to +5 is
>>> >> actually stored as +355 to +5 and so postgis thinks it intersects a
>>> >> whole bunch of polygons that it really doesn't.  I could change to 
>>> the
>>> >> -180 to 180 longitude system but that only moves the problem to a
>>> >> different longitude.
>>> >>
>>> >> Is there a graceful way around this?
>>> >> I'm thinking of just making two tables, the other table would have
>>> >> longitudes ranging from -180 to +180 and doing two searches with
>>> >> longitude ranges chosen to avoid the problem area on each table. Does
>>> >> postgis have a way to handle cyclical coordinates?
>>> >>
>>> >> Thanks for any help,
>>> >> Shane
>>> >>
>>> >>
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>>>
>>> -- 
>>>
>>> ______________________________________________________________
>>> Shane Byrne  -  University of Arizona  - Lunar & Planetary Lab
>>> ______________________________________________________________
>>> Email: shane at lpl.arizona.edu              Phone: (928)556-7235
>>> Web  : http://www.gps.caltech.edu/~shane  Fax  : (928)556-7014
>>> ______________________________________________________________
>>> Mail : USGS - Astrogeology Division,
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