<div dir="ltr"><div>There are many ways to determine area in PostGIS, but I will simplify this to 3 groups.</div><div>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.</div><div><br></div><div>1) Spherical Area ST_Area(p.geog, false) as ellipsoid / spheroid<br></div><div>The current <a href="http://en.wikipedia.org/wiki/World_Geodetic_System" title="World Geodetic System">World Geodetic System</a> model uses a spheroid whose radius is 6,378.137 km at the <a href="http://en.wikipedia.org/wiki/Equator" title="Equator">equator</a> and 6,356.752 km at the <a href="http://en.wikipedia.org/wiki/Geographical_pole" title="Geographical pole">poles</a>.<br></div><div>2) Spheroid AREA ST_Area(p.geog) as sphere,</div><div>3) Projected Area ST_Area(ST_Transform(p.geog::geometry,3410)) proj_area<br></div><div><br></div><div>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.</div><div>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.</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Feb 25, 2015 at 6:53 AM, Lelo - Luiz Rogério De Pieri <span dir="ltr"><<a href="mailto:lelo.pieri@gmail.com" target="_blank">lelo.pieri@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Hi,<div><br></div><div>I think you always have to transforme to UTM to have area in square meters.</div><div><br></div><div>Best Regards,</div></div><div class="gmail_extra"><br><div class="gmail_quote">2015-02-25 9:10 GMT-03:00 Ahmet Temiz <span dir="ltr"><<a href="mailto:ahmettemiz88@gmail.com" target="_blank">ahmettemiz88@gmail.com</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="h5"><div dir="ltr"><div class="gmail_default" style="font-size:small">Hello,</div><div class="gmail_default" style="font-size:small"><br></div><div class="gmail_default" style="font-size:small">How do we calculate the area (polygon) in country ( Turkey) level ?<br></div><div class="gmail_default" style="font-size:small"><br></div><div class="gmail_default" style="font-size:small">Which one does give better result ? transforming to UTM or using geography ?</div><div class="gmail_default" style="font-size:small"><br></div><div class="gmail_default" style="font-size:small">regards</div><div><br></div>-- <br><div>Ahmet Temiz<br>Jeoloji Müh.<br>Afet ve Acil Durum Yönetimi Başkanlığı<br>Planlama ve Zarar Azaltma Dairesi Başkanlığı<br><br><br>________________________<span><font color="#888888"><br><br>Ahmet Temiz<br>Geological Eng.<br>Information Systems - GIS Group<br>Disaster and Emergency Management<br>of Presidency</font></span></div>
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