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At 09:06 24/02/2015 -0700, Nobody Noname wrote:
If I create a list of Y-values for the shape boundary for each X-coordinate in the grid I end up with:

Y-value of the shape boundary Corresponding X-coordinate of the grid
9,10,11,12,13,14                                        7

Does this mean that the shape is re-entrant and that 9 to 10, 11 to 12, and 13 to 14 are inside? Well, that's the same as saying that 9 to 14 is inside, and you need better resolution of your grid if you are to treat the shape appropriately. Note that it may be the case that if you chose, arbitrarily, to list the boundary X-values for each Y-value instead, it's possible that your shape might not be re-entrant and the shape could be expressed more simply.

6,11,12,17                                                  12

If I now look at my list of bird coordinates and take one example of a birds X-coordinate being 12, its within the ranges of the shape boundary Y-values above, but if I check its position (at X=12, Y=19) I can see its not within the boundary. So I'm still missing something.

Again, if this means that 6 to 11 and 12 to 17 are within the shape, then that is the same as 6 to 17 and you need better resolution. But in any case, a Y-value of 19 is clearly outside the 6 to 17 range for X=12, which coincides with your claim that this bird is outside the shape. No problem.

Are you perhaps confusing X and Y?

Brian Barker

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