[ed_hyer]

@Nathaniel: The “scale ranks” is in fact what I wanted. It means that, for instance, I have to process all the points through 459 polygons in order to determine which points are and are not in Canada, but it works!

This is a capability I have longed for in IDL for some time, and now it seems I can have it, thanks to this *excellent* free data set!

Thanks again,

–Edward H.

[ed_hyer]

Nathaniel: Thanks for the quick reply! OK, the problem I am describing is definitely not a precision issue, and hopefully it is user error. I am directly extracting the vertices from the shapefile, where they are stored as a (2,N) array. I break this into a list of longitude coordinates (0,*) and a list of latitude coordinates (1,*). These coordinates should describe a polygon, BUT– they appear to be somewhat out of order. So, when I try to draw Canada, for instance, it draws a segment along the eastern coast of Canada, and then jumps over and draws a segment along the western coast. The result is a fragmented polygon with multiple intersections. It is these intersections that are the problem– they break up the polygon, and make it impossible to cleanly calculate inside/outside. I suspect, but I am not sure, that this would be a problem in any software.

I do still have some ESRI product, but I haven’t used it in a while, because most of my work is with automated analysis of large datasets, which I used to do with good ol’ AML, but now do mostly with IDL and command-line tools like GDAL.

Am I explaining the problem well enough? I might have to find a space online to upload the graphic, it’s instantly clear from the picture.

–Edward H.

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