On May 7, 2012, at 3:44 PM, Stephen Woodbridge wrote:
> Hi Puneet,
>
> First if you intersect a line with a polygon you should expect to get back:
>
> 1. a line if it crosses the polygon or lies alont an edge of it
> 2. or a point if it touches a vertex
> 3. or a collection of these if there are multiple instances along the line
>
> The results are always returned in a GEOMETRYCOLLECTION.
>
> Also you should expect to get one result row for every row in TABLE
>
> If you only want, results for rows the intersect then add a WHERE clause
>
> SELECT ST_AsText(ST_Intersection(ST_GeomFromText('LINESTRING(<p0>,<p2>)',
> 4326), the_geom))
> FROM table
> WHERE ST_Intersects(ST_GeomFromText('LINESTRING(<p0>,<p2>)', 4326), the_geom);
>
>
Thanks... the above works well. But, now I am confused by a slightly different
problem. How do I guarantee that the returned results are in the same order as
the direction of the line? For example, given the line below (<p0> .. <pn>),
how do I guarantee that I will get back "a, b" and not "b, a"?
Suggestions?
> -Steve
>
> On 5/7/2012 1:36 PM, Puneet Kishor wrote:
>> Given a line, I want to get the points at which it intersects a polygon
>> dataset.
>>
>> +--------------+
>> | |
>> +--------+ |
>> | a | b |
>> ====o========o==============o=======
>> p0 |p1 |p2 |p3 pn
>> +--------+ |
>> | |
>> | |
>> +--------------+
>>
>> In the figure above, I want to get the following
>>
>> poly_id point1 point2
>> ------- ------ ------
>> a p1 p2
>> b p2 p3
>>
>> SELECT
>> ST_AsText(ST_Intersection(ST_GeomFromText('LINESTRING(<p0>,<p2>)', 4326),
>> the_geom))
>> FROM table;
>>
>> But I back N 'GEOMETRYCOLLECTION EMPTY' where N = number of rows in the
>> 'table'. From what I understand, that indicates "the geometries do not share
>> any space (are disjoint)". But, I know that is not correct. So, my query is
>> formulated incorrectly. What am I doing wrong?
>>
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