[postgis-users] TR: strange behavior of ST_Area(geography)

[Regina Obe]

That is very strange.  I tested on 2.4.0 PostgreSQL 10 Windows 64-bit

POSTGIS="2.4.0 r15853" PGSQL="100" GEOS="3.6.2-CAPI-1.10.2 4d2925d" PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 2.2.2, released 2017/09/15 GDAL_DATA not found" LIBXML="2.7.8" LIBJSON="0.12" LIBPROTOBUF="1.2.1" RASTER PostgreSQL 10.0, compiled by Visual C++ build 1800, 64-bit

WITH poly AS (
 SELECT
  ST_SetSRID(ST_GeomFromText(
   'POLYGON((138.002 37.001, 138.003 37.0010, 138.003 37.0013, 138.002 37.001))'
   ),4326) g1,
  ST_SetSRID(ST_GeomFromText(
   'POLYGON((138.002 37.001, 138.003 37.0013, 138.003 37.0010, 138.002 37.001))'
   ),4326) g2
)
SELECT ST_Equals(g1,g2),
 ST_Area(g1)                     = ST_Area(g2) AS eq_area,
 ST_Area(g1::geography)          = ST_Area(g2::geography) AS geog1,
 ST_Area(g1::geography, false)   = ST_Area(g2::geography, false) AS geog2,
 ST_Area(g1::geography)          = ST_Area(ST_Reverse(g2)::geography) AS geog3,
 ST_Area(ST_Transform(g1,32654)) = ST_Area(ST_Transform(g2,32654)) AS geom
 FROM poly;

And get all trues as expected.  

st_equals | eq_area | geog1 | geog2 | geog3 | geom
-----------+---------+-------+-------+-------+------
 t         | t       | t     | t     | t     | t
(1 row)


What does this query return you

WITH poly AS (
 SELECT
  ST_SetSRID(ST_GeomFromText(
   'POLYGON((138.002 37.001, 138.003 37.0010, 138.003 37.0013, 138.002 37.001))'
   ),4326) g1,
  ST_SetSRID(ST_GeomFromText(
   'POLYGON((138.002 37.001, 138.003 37.0013, 138.003 37.0010, 138.002 37.001))'
   ),4326) g2
)
SELECT ST_Equals(g1,g2),
 ST_Area(g1)                     - ST_Area(g2) AS eq_area,
 ST_Area(g1::geography)          - ST_Area(g2::geography) AS geog1,
 ST_Area(g1::geography, false)   - ST_Area(g2::geography, false) AS geog2,
 ST_Area(g1::geography)          - ST_Area(ST_Reverse(g2)::geography) AS geog3,
 ST_Area(ST_Transform(g1,32654)) - ST_Area(ST_Transform(g2,32654)) AS geom
 FROM poly;

I get 
st_equals | eq_area | geog1 | geog2 | geog3 | geom
-----------+---------+-------+-------+-------+------
 t         |       0 |     0 |     0 |     0 |    0
(1 row)


-----Original Message-----
From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of IIDA Tetsushi
Sent: Monday, October 16, 2017 9:58 AM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Subject: Re: [postgis-users] TR: strange behavior of ST_Area(geography)

Thanks all.

I tested on another PostGIS 2.4 environment (CentOS7.4 + EPEL repo.) but it returned same bad result.

-------
POSTGIS="2.4.0 r15853" PGSQL="96" GEOS="3.5.0-CAPI-1.9.0 r4084" SFCGAL="1.2.2"
PROJ="Rel. 4.8.0, 6 March 2012" GDAL="GDAL 1.11.4, released 2016/01/25"
LIBXML="2.9.1" LIBJSON="0.11" RASTER
-------

I have not enough time to build full environment from source now.
probably this behavor is not caused by PostGIS itself.
anyway, I wrote it as a record for those using the same environment.

--
hogeman
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