On Tue, Feb 24, 2009 at 7:29 AM, Barend Gehrels <barend@geodan.nl> wrote:
What should the generic "area" algorithm return for point and linestring? - 0 (zero) - compiler error (so this function is not defined for them at all) - exception
I can't decide between compiler error and 0. To be formally correct, I believe it should be compiler error because it doesn't exist, but 0 also signals non-existance.
What should the generic "length" algorithm return for polygon? - 0 (zero) - its perimeter - compiler error - exception
Compiler error. Provide functions like perimeter, chord_length, length_along_axis, or whatever else.
What should the generic "centroid" algorithm return for point and linestring? - the point / the average its points - compiler error - exception
To be formally correct, I believe linestring should be compiler error. Centroid is the point on which it would balance when placed on a needle, and obviously that doesn't often happen with a non-straight linestring. You could provide average_point (not sure I like the name) that returns the average of the points and have centroid call it, only with a stricter interface.
and for an invalid polygon (e.g. no points at all, or not closed)? - exception - return "false" (if centroid is passed as a reference parameter) - ? (we've not always the compiler error option here because points are usually stored in a runtime collection)
This is a much bigger question than you ask. Where do you want the error checking to occur? Do you provide an is_closed, or is_overlapped, or things of that nature for checking the validity of a polygon? Is a valid polygon a precondition of all functions that work on polygons, or do you intend to check for validity in every function? If I *know* my polygon is valid, I don't want to pay for the error checking. --Michael Fawcett