[postgis-users] calculating area

[Ahmet Temiz]

Thank you,

very detailed explanation

On Wednesday, February 25, 2015, David Haynes <haynesd2 at gmail.com> 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 <
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
>> Buscando melhorar a cada dia
>> Áudio, Hardware & Software
>> www.twitter.com/lelopieri
>> blogdolelo.wordpress.com
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>
>

-- 
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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