[postgis-users] exterior rings in multipolygons

[Mr. Puneet Kishor]

On Jul 25, 2011, at 3:44 PM, Sandro Santilli wrote:

> On Mon, Jul 25, 2011 at 03:40:34PM -0500, Mr. Puneet Kishor wrote:
>> Please help me understand the following
>> 
>> ====
>> SELECT 
>>    ST_Nrings(the_geom) nr, 
>>    ST_NumInteriorRings(the_geom) nir, 
>>    ST_AsText(the_geom) WKT 
>> FROM table 
>> WHERE objectid = 280;
>> 
>> nr  nir  WKT
>> --  ---  ----------------------------------------------------------------
>> 4    3  "MULTIPOLYGON(((..),(..),(..),(..)))"
>> 
>> ====
>> 
>> Looking at the above, I am assuming the first (..) is the exterior ring, and the remaning three (..) are the 3 interior rings. However,
>> 
>> ====
>> SELECT ST_ExteriorRing(the_geom) er  
>> FROM table 
>> WHERE objectid = 280;
>> 
>> ERROR: ExteriorRing: geom is not a polygon
>> ====
>> 
>> 
>> So, what is going on? Of course, the docs say that ST_ExteriorRing doesn't work on a MULTIPOLYGON. So, is the above only an implementation anomaly, and I can treat the first ring as an exterior ring? Or, is it really something else?
> 
> All your rings are wrapper in another pair of parens, which identify
> a component of the MULTIPOLYGON, that is a POLYGON. You must extract
> the Polygon to use ST_ExteriorRing against it. So your query would be:
> 
> SELECT ST_ExteriorRing(ST_GeometryN(the_geom,1)) er
> FROM table
> WHERE objectid = 280;
> 


Thanks. The above works. So, does this mean that given "MULTIPOLYGON(((1),(2),(3),(4)))", 1 indeed is an exterior ring, and 2-4 are interior rings, and the "entity" does have 4 rings kinda like below?

+-------------------+
|                   |
|  +-----+   +---+  |
|  |     |   | 3 |  |
|  |  2  |   +---+  |
|  +-----+          |
|       +--------+  |
|       |        |  |
|   1   |   4    |  |
|       |        |  |
|       +--------+  |
+-------------------+

Puneet.