[postgis-users] calculating area

[David Haynes]

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.pieri at 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 <ahmettemiz88 at 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
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>>
>>
>> ________________________
>>
>> Ahmet Temiz
>> Geological Eng.
>> Information Systems - GIS Group
>> Disaster and Emergency Management
>> of Presidency
>>
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>
>
>
> --
> Rogério De Pieri  (Lelo)
> MBA em Gerenciamento de Projetos - FGV
> SCJP 5
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>
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