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

[Paul Ramsey]

FYI, this is now resolved and will be in the 1.5.1 release.

http://trac.osgeo.org/postgis/ticket/450

P.

On Wed, Feb 24, 2010 at 2:32 PM, Paul Ramsey <pramsey at cleverelephant.ca> wrote:
> OK, this looks like a bug...
>
> The area of 1 versus the area of 359 is expected, since the geometry
> object does not handle the dateline. However, the geography area
> should be identical in both cases. So, I have a bit of work to do.
>
> More information as I find it.
>
> P.
>
> On Wed, Feb 24, 2010 at 10:38 AM, Dale Noss <noss at asu.edu> wrote:
>>
>> 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
>> reads:
>>
>> "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."
>>
>> Section 4.2.3.3.        reads:
>>
>> "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.
>>
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>