p Nicely documented example - but can you provide the full geography of the dam (that's Australian for pond!) maybe using st_astext() so other's can try on their local machines?
cheers Ben On 21/07/2011, at 8:42 PM, p valdes wrote: > Hi, > I'm trying to calculate the area of a polygon, with strange results... > > First a description of the problem, the ugly details at the end of the post > > I have measured the perimeter of a pond. An irregular > polygon vaguely 'chop shaped' without holes or spikes > and closed (beginning and ending in the same point). > For simplification purposes let's suppose that I was > expecting something like the left shape... > well, I'm obtaining instead this: > > expected obtained > ------- --- > | | | | > | | | | > | |__ | | > | | | | > ----------- | | > | | > | | > | |_ > ----- > > The right shape is "not right". The polygon is clearly > stretched in the vertical North-South axis in the screen. The > satellite google map support that the real shape of > the pond is the first represented and I think the same... > > Well I could live with this, but there's two more nasty consequences. > > 1) If I take a point inside or related to the polygon > in a separated map layer the point is drawn OUTSIDE > and far away below the figure of the pond (note also that the > pond is printed much more big than expect, because the relative distance > between a and b is the same in both, and seems right). Graphically: > > expected obtained > > --*a --- > | | | | > | | | | > | | | | > | | | | > | | | | > | | | | > | | | | > | |_ | |_ > -----*b ------ > > > > > > > *a > *b > > Maybe a problem of the map viewer software, yes but the second > question is more serious: > > ST_distance calculates a distance between the "a" and "b" points of > about 160 m for this rectangle. A max length of 160 m for this pond > seems reasonable to me. > > but ST_area calculates an area for this polygon of almost 7 ha!, for a > polygon that could fit in a rectangle of about 160x50 m!. > I'm obtaining a faked area about seven times BIGGER than real, uh-oh... > > Its clear to me that I'm doing a very obvious and stupid mistake. I > greatly appreciated if you could take a look at the problem or suggest > any possible source of the error > > Thanks in advance > > pvaldes. > > > > ..................................................................... > Details > .................. > > The points were registered with a garmin GPS as coordinates in degrees > (ie: 43.254 N -4.839 W) > > I'm using PostgreSQL 9.0.4 > on i486-pc-linux-gnu, compiled by GCC gcc-4.6.real (Debian > 4.6.0-6) 4.6.1 20110428 (prerelease), 32-bit > (obtained as Debian package from an official repository) > > postgis-1.5.3 > (downloaded and compiled from the tarfile at postgis.refractions.net) > > This is the table > > CREATE TABLE mytable( > ref serial PRIMARY KEY, > name varchar(80), > perimeter geography(POLYGON,4326) > ); > > The polygon with lat/lon coords was loaded with something like: > > INSERT INTO mytable (perimeter, name) > VALUES ( > ST_GeogFromText( > 'POLYGON(( > -5.3850 43.230, > -5.3858 43.231, > -5.3859 43.245, > ... > -5.3850 43.230 > ))', 4326),'myname'); > > And this is the very simple query returning strange results > > SELECT st_area(mytable.perimeter)::double precision AS square_meters > FROM mytable > WHERE name = 'myname'; > > I use pgsql2shp for obtaining a shapefile that I'm loading with thuban > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
