RE: [MI-L] MapInfo versus Oracle Area

[Robert Crossley]

Title: MapInfo versus Oracle Area

David,   Thanks for your explanation.  But now I am confused.  Are you saying that the correct answer for the area of a one km square piece of ground is 0.997 sq km?   This is probably related to the issue that I had a problem with, in that if you split the 1 km square polygon, calculated the proportion of the original area covered by each piece, and then summed up the proportion of those areas, you would expect to get 1, but actually got 0.997.   To a surveyor this may make sense.   R     From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Llewellyn, David Sent: 04 April 2006 11:31 To: MapInfo-L@lists.directionsmag.com Subject: [MI-L] MapInfo versus Oracle Area   Dear List, A response you may find interesting regarding a question I had earlier about different areas being reported by MapInfo and Oracle Spatial: My original question related to the area being reported for a Mapinfo table (Lat/Long GDA94) containing a single square shaped polygon of 1km sides centred at 141 degrees East, 22 degrees South. In MapInfo the Area() or SphericalArea() function applied to this object returns: 0.999969 square kilometres. After loading the table to Oracle Spatial and running the following SQL+ query the area returned is as shown: SELECT A.ID, SDO_GEOM.SDO_AREA(A.GEOLOC, 0.0000001, 'UNIT=SQ_KM') FROM TEST1 A ID SDO_GEOM.SDO_AREA(A.GEOLOC,0.0000001,'UNIT=SQ_KM') --- -------------------------------------------------- 0 .997368877 The Oracle area is significantly smaller than the MapInfo area. Why? The following response was provided by a member of the Oracle Spatial development team: “Oracle's result is smaller than the MapInfo result by a factor of .997. This is precisely the ratio expected if MapInfo is returning spherical area using the authalic sphere for this GRS80 ellipsoid. This authalic sphere has radius 6371007 meters. The Oracle result is an ellipsoidal area, corrected for meridional and prime normal radii of curvature at the average location of the polygon. At -22 degrees latitude ( gleaned from an earlier post), this ellipsoidal correction is precisely a factor of .997”   Best regards, David Llewellyn ***This Email, together with any attachments is intended for the addressee only. If you have received this Email in error please inform the sender immediately by return Email. Alternatively you can ring the National Native Title Tribunal on the toll free number 1800 640 501 and report the error. Please delete the Email and any copies from your computer and/or computer network. The Tribunal does not warrant that this email is virus free. The Tribunal has also implemented content filtering of Emails to restrict inappropriate and spam emails being received. Other emails which may contain viruses may also be blocked. In either case, and in any case the email may not reach its intended recipient. If the email is important you may wish to see confirmation of its receipt by placing that request in the text of the email. Please resend the email if you haven't received that confirmation as requested. ***

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