You can search for "Earth Projection 1," in a projection string (the comma is important) and you'll know it's lat/lon. You could try setting the current projection to something appropriate for area analysis (i.e. an equal-area projection) and use CartesianArea. Also, coordinate precision comes into play, since the points where a soil layer objects intersects a paddock layer object presumably aren't in the boundaries of either object. Coordinate precision of objects in a MapInfo table is derived from the BOUNDS clause of the projection when you captured the data or imported it, and the precision of an analysis is depenednt on that and all of the projections it filters through "on the fly").
I'm not sure if the Area function takes eccentricity into account when the projection is not based upon a sphere. These are the reasons why we never claim that GIS analysis measurements are legally binding, and property owners need to hire surveyors. :) Hope this helps Spencer ________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Robert Crossley Sent: Tuesday, March 21, 2006 8:21 PM To: [email protected] Subject: [MI-L] Area, CartesianArea, SphericalArea? Hi, I am doing some work with intersecting layers at present, and getting some quirky results when analysing the areas from the splits. When I intersect a paddock with a soil layer, I calculate the area of the resultant intersected areas. I usdate the result with the original paddock ID and the original Paddock area (as Floats). In theory, the sum of the resultant areas from each paddock should equal the total area of the original paddock. I also calculate the proportion of the original paddock for each resultant area (result area/original area). Sum these up for each paddock and you should get 1. If I use the function Area(obj,"hectare"), the sum of the proportion of the areas ranges from 0.990 to 1.01 with a few more extreme outliers. I get better results from using CartesianAreas(Obj,"hectare"), but can only use this for non lat/long projections, but actually get mainly 0.996 as the sum of the proportions (which is similar to the AMG factor of 0.9996) I have read some of the posts about this a while ago (although they were mainly concerned with distance), and they suggested a reasons for this being the scaling factor used in the projections - although I didn't quite understand it. I have some questions: Is there something I should do to get a better result? Can I test for the projection being a lat/ long or not easily? Or should I use a function that does something like take the projection string and search for the word latitude? Is anyone else worried about how inaccurate these values are? Thanks R _______________________________________________ MapInfo-L mailing list [email protected] http://www.directionsmag.com/mailman/listinfo/mapinfo-l
