Thank you, very detailed explanation
On Wednesday, February 25, 2015, David Haynes <[email protected]> wrote: > 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 model uses a spheroid whose radius is 6,378.137 km at the equator and 6,356.752 km at the poles. > 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 < [email protected]> 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 <[email protected]>: >>> >>> 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 >>> [email protected] >>> 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 >> [email protected] >> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users > > -- 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
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