In a feeble attempt to amuse his dog, Ken Tenaka wrote:
"does anyone know of other spatially enabled databases that can also do
this, either free or commercial?"
This comes from an Oracle forum where somebody else asked exactly the same
question:
From: Oracle, Gary Smith 02-Mar-04
21:00
Subject: Re : Oracle Spatial and
geodec behavior
Oracle Spatial's geodetic model allows
customers to specify geodetic
geometries that span the 180 meridian,
and also geometries that encompass a
pole (for example a polygon geometry
that follows the 70 degree latitude
line could represent a polygon that
looks like a hat that covers a pole).
When associating a geodetic SRID with
a geometry, Oracle uses great circle
distances between consecutive points
that define the geometry.
For linestrings, Oracle will always
choose the smaller great circle
distance when evaluating consecutive
points in a geodetic linestring.
Oracle Spatial rules enforce that no
two consecutive points in a geodetic
linestring can be equal to or greater
than 1/2 the distance of the great
circle apart. You can define a
linestring where the sum of the
distances of all the linesegments that
make up the linestring exceeds 1/2 the
Earth's great circle distance.
Similar for polygons. A polygon
element associated with a geodetic
SRID will choose the direction around
the world that is associated with less
than 1/2 the surface area of the
Earth. Outer rings still need to
maintain counter clockwise direction.
Inner rings or voids are specified in
a clockwise direction.
Oracle Spatial rules enforce that no
polygon element in a geodetic polygon
can have a surface area equal to or
greater than 1/2 the surface area of
the Earth. You can define a polygon
where the sum of the polygon element
areas that make up the geodetic
polygon exceeds half the surface area
of the Earth.
Hope this helps. Thanks.
Gary
|---------+--------------------------------------------->
| | "Ken Tanaka" <Ken.Tanaka at noaa.gov>|
| | Sent by: |
| | postgis-users-bounces at postgis.refr|
| | actions.net |
| | |
| | |
| | 10/20/2004 02:12 PM |
| | Please respond to PostGIS Users |
| | Discussion |
| | |
|---------+--------------------------------------------->
>----------------------------------------------------------------------------------------------|
| |
| To: postgis-users at postgis.refractions.net |
| cc: |
| Subject: [postgis-users] Anti-meridian and polar polygon question |
>----------------------------------------------------------------------------------------------|
Hello,
I'm new to this list and have a general question on PostGIS's
capabilities. I spent a little time searching the list archives, but
didn't see this question asked before. Are polygons that cross the
anti-meridian (international date line) or encircle the poles allowed?
For example is it valid to define a 2x1 degree Pacific box with lon,lat
coordinates of 179,0 -179,0 -179,1 179,1? Could I then expect to get
correct area calculations or select other geometry within 100km of this
area?
(text diagram, use monospaced font)
180
179,1 : -179,1
+----------+
| : |
+----------+
179,0 : -179,0
I was also wondering if PostGIS can similarly deal with a polygon that
goes around a pole, like the coastline of Antarctica?
The Informix DB with the Geodetic Datablade option handles these, but
does anyone know of other spatially enabled databases that can also do
this, either free or commercial? I'm involved with a system design team
that is monitoring GIS capabilities available.
Thanks,
Ken
--
= Geospatial Data Services Group ==================
| CIRES, National Geophysical Data Center / NOAA |
| 303-497-6221 |
= Ken.Tanaka(a)noaa.gov ===========================
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