There are many ways to determine area in PostGIS, but I will simplify this to 3 groups. 1 consider the earth as an ellipsoid, where the radius changes, 2 consider the world to a perfect sphere, 3 your area is small enough to fit in a defined equal area projection. For work, I have generated a number of area datasets for country boundries using the functions in PostGIS. You will notice that the 3rd function uses a global equal area projection. They are all different and in someways they are all accurate. The first function uses a mathematical model that tries to model the earth and takes a bit longer. The second uses a sphere and is the fastest, while the third flattens the earth and its accuracy is based on the projection.
1) Spherical Area ST_Area(p.geog, false) as ellipsoid / spheroid The current World Geodetic System <http://en.wikipedia.org/wiki/World_Geodetic_System> model uses a spheroid whose radius is 6,378.137 km at the equator <http://en.wikipedia.org/wiki/Equator> and 6,356.752 km at the poles <http://en.wikipedia.org/wiki/Geographical_pole>. 2) Spheroid AREA ST_Area(p.geog) as sphere, 3) Projected Area ST_Area(ST_Transform(p.geog::geometry,3410)) proj_area In general, I would recommend the function 1 as an accurate calculation. I have document of country areas I can share with you if you like. It shows a collection of country polygons that have the area determined using all three different methods. In this document, I am assuming that spheroid will be the truth. The interesting thing is that there are major differences based on the latitude. The area of a geography goes from over to under projection at the 35 degree. This is due to shrinking and expansion of the ellipsoid and equal area projection. On Wed, Feb 25, 2015 at 6:53 AM, Lelo - Luiz Rogério De Pieri < lelo.pi...@gmail.com> wrote: > Hi, > > I think you always have to transforme to UTM to have area in square meters. > > Best Regards, > > 2015-02-25 9:10 GMT-03:00 Ahmet Temiz <ahmettemi...@gmail.com>: > >> Hello, >> >> How do we calculate the area (polygon) in country ( Turkey) level ? >> >> Which one does give better result ? transforming to UTM or using >> geography ? >> >> regards >> >> -- >> Ahmet Temiz >> Jeoloji Müh. >> Afet ve Acil Durum Yönetimi Başkanlığı >> Planlama ve Zarar Azaltma Dairesi Başkanlığı >> >> >> ________________________ >> >> Ahmet Temiz >> Geological Eng. >> Information Systems - GIS Group >> Disaster and Emergency Management >> of Presidency >> >> _______________________________________________ >> postgis-users mailing list >> postgis-users@lists.osgeo.org >> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users >> > > > > -- > Rogério De Pieri (Lelo) > MBA em Gerenciamento de Projetos - FGV > SCJP 5 > Buscando melhorar a cada dia > Áudio, Hardware & Software > www.twitter.com/lelopieri > blogdolelo.wordpress.com > > _______________________________________________ > postgis-users mailing list > postgis-users@lists.osgeo.org > http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users >
_______________________________________________ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users