Re: PostGIS

Sandeep Kumar Jakkaraju wrote:
>
> I use postgis to wipe my ASS ...
> no good documentation ...

The docs are a bit technical, but the product is excellent, I use it to
manage a variety of spatial datasets, with up to hundreds of millions of
features, on half a dozen systems. I'd highly recommend it, & would also
say that the online support from the developers and users is of a very
high standard.

Note that you should have some sort of understanding of projections
before you start doing things like measuring or reprojecting, to
understand the issues involved, but these are not just a Postgis issue.

In answer to the original question (I'd have replied earlier but just
got back from 2 weeks at sea - using Postgis amongst other things)
If you look at the case studies Paul Ramsey is accumulating at
http://postgis.refractions.net/news/ it might help.

> in what units is the result of distance(geometry,geometry)

As in the docs, the distance is returned in the units specified in the
projection definition of your geometries.

> what is distance_sphere(geometry,geometry).. when u give same geometry
> as both parameters it gives a non-zero value ....
Hmm. just tried this. Within floating point rounding errors, this seems
to contradict this claim, unless you suggest a floating point non-zero
represenation of zero is a postgis error. In which case I suggest
CompSci 101 might be educational.

corax=# select distance_sphere(the_geom,the_geom) from bivouac_pnt limit 10;
distance_sphere
----------------------
0
0
0
0
0
7.07321632848846e-10
7.07321632848846e-10
7.07321632848846e-10
0
7.07321632848846e-10
(10 rows)

Cheers,

Brent Wood

>
> On 10/19/06, *James Cradock* <jcradock(at)me3(dot)com
> <mailto:jcradock(at)me3(dot)com>> wrote:
>
> On Oct 19, 2006, at 11:08 AM, Bob Pawley wrote:
>
>> Can anyone point me to an overview of PostGIS and how it is being
>> used???
>
> Bob,
>
> PostGIS provides extensions to PostgreSQL's procedural language
> and geometric data types so that it can store geo-referenced data,
> and so that you can query this data using geo-spatial coordinates.
>
> Software like MapServer (web) and Qgis (desktop) use PostGIS to
> provide data in map applications.
>
> The Refractions site already noted is a good start. The MapServer
> site is another:
>
> http://mapserver.gis.umn.edu/gallery
>
> Jim
>
> -----
>
> James Cradock, jcradock(at)me3(dot)com <mailto:jcradock(at)me3(dot)com>
>
>
>
>
>
> --
> Sandeep Kumar Jakkaraju
> WeBlog:
> http://jakkarajus.blogspot.com