Message-Id: <199807120453.OAA19842@mustang.sdc.com.au>
To: "Thomas G. Lockhart" <lockhart@alumni.caltech.edu>
cc: dgross@lri.fr,
	Postgres Interfaces Mailing List <pgsql-interfaces@postgreSQL.org>
Subject: Re: [INTERFACES] postgres for spatial data 
In-reply-to: Your message of "Sat, 11 Jul 1998 14:09:16 GMT."
	<35A7720C.B254F09@alumni.caltech.edu>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Date: Sun, 12 Jul 1998 14:23:21 +0930
From: Stephen Davies <scldad@sdc.com.au>

> >         I'm looking for a graphical interface that can display the
> > geometric types like polygon,...
> 
> Don't know about pre-built utilities for graphical display of geometric
> objects. Let us know what you discover.
> 
> >         I'm also looking for SQL extension in order to make
> > spatial queries (for example, a function that tells if a point
> > belongs to the interior of a polygon)
> 
> There are already some operators available; for example, the "@"
> operator is for "on or inside":
> 
> postgres=> select '(0,0)'::point
> postgres->  @ '((0,1),(1,0),(0,-1),(-1,0))'::polygon;
> ?column?
> --------
> t
> (1 row)
> 
> Look in the hardcopy or html doc set which is included in v6.3.x. You
> need to do a "make install" from the doc directory to get them unpacked.
> Good luck...
> 
>                          - Tom

The "point in poly" code in the release package looks rather complicated and 
uses lots of calls. The fastest PinP that I have found is the following (coded 
here in basic but readily translatable to anything else).
(The underlying algorithms are the same - just the coding differs).

Cheers,
Stephen.

Function TestPoly(X As Double, Y As Double) As Boolean
' check if point X,Y is inside a polygon. Return true if ins%<>0
'
' Polygon coordinates are in global arrays PX and PY.
' The number of coordinates is in global nP.
'
Dim X1 As Double, X2 As Double, Y1 As Double, Y2 As Double
Dim ins As Boolean

    ins = False
    j% = nP - 1
    For i% = 0 To nP - 1
        X1 = PX(j%)
        X2 = PX(i%)
        Y1 = PY(j%)
        Y2 = PY(i%)
        If X1 < X And X <= X2 Then
                If (Y - Y1) * (X2 - X1) < (Y2 - Y1) * (X - X1) Then
                    ins = Not ins
                End If
        ElseIf X2 < X And X <= X1 Then
            If (Y - Y2) * (X1 - X2) < (Y1 - Y2) * (X - X2) Then
                ins = Not ins
            End If
        End If
        j% = i%
    Next i%
    TestPoly = ins
End Function


========================================================================
Stephen Davies Consulting	    		                            scldad@sdc.com.au
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