[postgis-users] optimal textbased geometry format

[Willy-Bas Loos]

thanks.
yes it helps :)

On Tue, Jul 29, 2008 at 4:46 PM, Obe, Regina <robe.dnd at cityofboston.gov>wrote:

>  Haven't done any benchmarks, but I know the following
>
> the ::text display you get is basically the string representation of the
> native binary format that PostGIS actually stores the geometries.
>
> It is not WKB nor WKT.  Its a PostGIS proprietary format if you will.  I
> would guess casting from that format is the most efficient since in theory
> there is very little processing that PostgreSQL needs to do to reconstitute
> from that format.
>
> Keep in mind EWKT and WKT also may result in floating point errors since
> they will round the decimals so be careful using those.
>
> So your best formats to maintain precision (and not have lossiness) are
> EWKB and the ::text representation (which I shall dubb the native PostGIS
> binary notation (which I guess we can call light-weight something or
> other)).
>
> Hope that helps,
> Regina
>
>  ------------------------------
> *From:* postgis-users-bounces at postgis.refractions.net [mailto:
> postgis-users-bounces at postgis.refractions.net] *On Behalf Of *Willy-Bas
> Loos
> *Sent:* Tuesday, July 29, 2008 8:43 AM
> *To:* postgis-users at postgis.refractions.net
> *Subject:* [postgis-users] optimal textbased geometry format
>
>  Sometimes i build queries inside plpgsql scripts, that i then run using
> EXECUTE.
> Is there a preferred way to include geometries in those?
>
> I use postgis 1.1 (for now). Are there are any changes in later versions
> that influence this aspect of PostGIS?
> The SRID is a factor for me, so asText(the_geom) is no option.
> I eiter use asEWKT(the_geom), or just the_geom::text .
> Just casting the geometry directly into text results in a very long text
> value (what is the name of this format?). The length of the string might be
> a network transfer drawback.
> asEWKT results in human readable format, possibly shorter, but it might
> require more processing power (back and forth!).
>
> I've tested both difference in length and performance, all on a limited
> dataset.
> LENGTH:
> The difference in length varies greatly. I use SRID 28992, which is a
> national grid that has coordinates in meters, so it is possible to have
> coordinates with 0 decimal places (1 meter precision). Simple geometries
> seem to benefit most from the WKT format. The EWKT is 40% of the number of
> characters for some cases (square polygons from coordinates without
> decimals). Most polygons score 50-60% of length, but in some cases the WKT
> representation is up to 10% longer than the direct cast (geom::text).
> code:
>   --select length(the_geom::text), length(asewkt(the_geom)),
> round(((length(asewkt(the_geom))::float)/(length(the_geom::text))::float)*100)
> as perc
>   select
> avg(((length(asewkt(the_geom))::float)/(length(the_geom::text))::float)*100)
> as perc
>   from atable
>   order by perc
>
> PERFORMANCE:
> I've tested these "warm" (not the first run), and averaged the scores from
> 3 measurements each. The fluctuations were minimal.
> The diference in performance is small for converting from geometries to a
> text representation. Casting directly is a factor 1.1 faster (90% of the
> time needed)
> The other way around is a diferent story. Converting a directly casted
> geometry back to a geometry is a factor 6 faster than a EWKT string (16% of
> the time needed)!
> I hope my method (code below) is adequate?
>
> code:
>   select area(the_geom::text::geometry)
>   --select area(asewkt(the_geom)::geometry)
>   from atable
>
> Regarding my tests, i would say that the format that results from a direct
> cast is the better string representation for intra-application
> communication. Are there any drawbacks?
> Of course my test was limited to a small dataset on a windows pc (mem 2GB,
> athlon 64 3200+) with postgis 1.1. Does anyone else have different
> results/ideas about this?
>
> Cheers,
>
> WBL
>
>
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