# [postgis-users] Can GEOGRAPHY object cross the date-line

Dale Noss noss at asu.edu
Wed Feb 24 10:38:28 PST 2010

```I'm confused about whether the new Geography type correctly handles shapes
that cross the date-line. The documentation in
http://postgis.refractions.net/documentation/manual-1.5/ch04.html#id2802139

"What about the date-line and the poles?

All the calculations have no conception of date-line or poles, the
coordinates are spherical (longitude/latitude) so a shape that crosses the
dateline is, from a calculation point of view, no different from any other
shape."

"What is the longest arc you can process?

We use great circle arcs as the "interpolation line" between two points.
That means any two points are actually joined up two ways, depending on
which direction you travel along the great circle. All our code assumes that
the points are joined by the *shorter* of the two paths along the great
circle. As a consequence, shapes that have arcs of more than 180 degrees
will not be correctly modelled."

The st_distance() function returns a sensible result for points that lie on
either side of the
date-line.

g4wd=# select st_distance('POINT(179.5 1)'::geography, 'POINT(-179.5
1)'::geography)/1000;
?column?
------------------
111.302645148245
(1 row)

The st_area() function also returns a sensible result for a square degree
near the equator.

g4wd=# select st_area(g)/1000000, st_area(transform(geometry(g), 4326)),
astext(geometry(g)) from (select 'POLYGON((178.5 2, 178.5 1, 179.5 1, 179.5
2, 178.5 2))'::geography) as foo(g);
?column?     | st_area |                       astext
-----------------+---------+----------------------------------------------------
12304.814950073 |       1 | POLYGON((178.5 2,178.5 1,179.5 1,179.5 2,178.5
2))
(1 row)

If the GEOGRAPHY object crosses the date-line, I see results similar to what
I'd get using a POLYGON. In other words, the calculations cannot be choosing
the shorter of the two possible paths along a great circle. (See query
results below) 359 implies choosing the longer path, and I have no idea why
the area in square kilometers is roughly 4.5x instead of 359x larger than
the non-dateline case above.

g4wd=# select st_area(g)/1000000, st_area(transform(geometry(g), 4326)),
astext(geometry(g)) from (select 'POLYGON((179.5 2, 179.5 1, -179.5 1,
-179.5 2, 179.5 2))'::geography) as foo(g);
?column?     | st_area |                        astext
------------------+---------+------------------------------------------------------
55910.8880691158 |     359 | POLYGON((179.5 2,179.5 1,-179.5 1,-179.5
2,179.5 2))
(1 row)

/////////////////////////////////
My setup:
g4wd=# select postgis_full_version();
postgis_full_version
-----------------------------------------------------------------------------------------------------------
POSTGIS="2.0.0SVN" GEOS="3.2.0-CAPI-1.6.0" PROJ="Rel. 4.7.1, 23 September
2009" LIBXML="2.6.16" USE_STATS
(1 row)

g4wd=# select version();
version
--------------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 8.4.2 on powerpc-apple-darwin9.8.0, compiled by GCC
powerpc-apple-darwin9-gcc-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5490), 32-bit
(1 row)

Running OS X 10.5.8

Work arounds, clarifications, links to solutions will be much appreciated.

--
View this message in context: http://old.nabble.com/Can-GEOGRAPHY-object-cross-the-date-line-tp27714275p27714275.html
Sent from the PostGIS - User mailing list archive at Nabble.com.

```