[postgis-users] Bounding Box Clipping

[Chris Faulkner]

Good idea. Is there any easy way of creating a grid suitable for overlaying
? I could script something but is there anything built-in ? Second, how
would I do overlay and what would the results be ? I can see various
functions but not all are documented.

Thanks

Chris

> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of
> chodgson at refractions.net
> Sent: 10 November 2003 17:45
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] Bounding Box Clipping
>
>
> If you are using this for a dynamic mapping application, using
> geos may not be
> ideal - I believe it would take longer to calculate the
> intersection of your
> box and the polygon than to just transfer all of the points and
> throw away
> those outside of the box (as mapserver would do when rendering the map).
>
> A better solution would be to chop your coastline into smaller
> pieces, small
> enough that a "normal-sized" bounding box would select a few of
> the pieces.
> Even though there would now be more geometries to select, each
> would be much
> smaller and the total number of coordinates would be much less.
> In addition,
> the spatial index would be able to locate which pieces are inside
> the bounding
> box very quickly.
>
> In the case of lines, this can be achieved by "chopping" the line
> at every 50th
> vertex, or something like that (depending on the resolution of
> your data and
> the intended viewing resolution). For polygons, you can overlay a
> grid onto the
> polygon and chop it into little squares - on the edges of the
> polygon, they
> won't be square, they will have a section of the edge of the
> original polygon,
> as well as some straight edges where the grid cut it up. Again,
> the size of the
> grid is relatively flexible, but you want to achieve a balance
> between not
> having to deal with too many excess points, and not having to
> select too many
> individual pieces.
>
> Chris
>
> Quoting Chris Faulkner <chrisf at oramap.com>:
>
> > Hello
> >
> > I am selecting a large polygon (coastline of Great Britain) from my
> > geometry_field, using a clause like this
> >
> > geometry_field &&GeometryFromText('BOX3D(189858.0 746543.0,195858.0
> > 750543.0)'::box3d,-1)
> >
> > This obviously retrieves the whole geometry where it overlaps with the
> > coastline - a lot of coordinates ! Are there any functions or
> operators I
> > can retrieve so that this polygon is clipped to my box and yet
> retain the
> > polygon ?
> >
> > Thanks
> >
> > Chris
> >
> >
> >
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