[postgis-devel] Re: Postgis Extension

[Craig Miller]

Roop Goyal,PhD and Max Egenhofer, PhD at the NCGIA wrote a really good
article on this subject.  I believe you can get a copy of the paper from
Max's website at the University of Maine - Department of Spatial Information
Science and Engineering.  http://www.spatial.maine.edu/~max/RJ36.html

Here is the article summary info from Max's website:

Cardinal Directions between Extended Spatial Objects
Roop Goyal, Max Egenhofer
IEEE Transactions on Knowledge and Data Engineering (in press)

Abstract
Cardinal directions complement topological relations and approximate
relations as qualitative spatial relations. They are frequently used as
selection criteria in spatial queries or for assessing similarities of
spatial scenes. Current models for cardinal directions use quite crude
approximations in the form of the objectsÕ minimum bounding rectangles or
their generalizations to points. To overcome the limitations of these models
so that cognitively more plausible inferences can be made, the
direction-relation matrix is introduced. It partitions space around a
reference object and records into which direction tiles an extended target
object falls. At a finer level of granularity it offers the option to
capture how much of the target falls into each direction tile. Further
details due to the particular shape of the target object may be captured for
tiles that contain multiple separations of the target. This multi-resolution
representation provides a better approximation for direction relations of
complexly structured spatial objects, including such shapes as concave
regions or objects with holes, and leads to a multi-level model for
processing spatial queries with direction relations

Happy reading,
--Craig


-----Original Message-----
From: postgis-devel-bounces at postgis.refractions.net
[mailto:postgis-devel-bounces at postgis.refractions.net] On Behalf Of
strk at refractions.net
Sent: Monday, July 04, 2005 12:10 PM
To: Joel da Silva
Cc: postgis-devel at postgis.refractions.net
Subject: [postgis-devel] Re: Postgis Extension

On Mon, Jul 04, 2005 at 03:55:59PM -0300, Joel da Silva wrote:
> strk,
> 
> I talk about  twelve possible positional relationships
> between two objects: (atNorth_of, atSouth_of, at_East_of,
> at_West_of, at_North-Est_of, at_North-West_of, at_South-
> East_of, at_South-West_of, Above, Below, at_Left_of and
> at_Right_of.
> For the first eight ones it is possible to specialize their
> meaning, that is, each of them can have the following prefix:
> Completely, Only, Partially, Also.
> 
> Seems to me that The postgis provides only above, below, Right and left
> operators.
> I am right?

Also Over{Right,Left,Above,Below}.
Combining them can give you the others.
For example, A North-West-of B could be expressed with:

	A |>> B AND A << B

Let's keep the discussion on postgis-devel so you to get other
opinions on this.

--strk;
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