FW: IANA time zone registration - proposal

Markus Kuhn Markus.Kuhn at cl.cam.ac.uk
Tue Jan 18 15:29:52 UTC 2005

Doug Royer:
> I am adding the POLYGON property targeted for the -01 version that has an
> ADD/DELETE parameter to allow for the optional inclusion of
> latitude/longitude geographic areas to be included that include (add) and
> exclude (delete) areas from the time zone geographic region (again from the
> CALSCH mailing list discussions).

The ADD/DELETE parameter will seem superfluous to anyone with a bit of
background in computational geometry or computer graphics.

Out-line font formats have well-established efficient ways of handling
the problem of distinguishing between what is "inside" and "outside" a

For example, see the section "3.5 Direction of Paths" in

  Adobe Type 1 Font Format

Included polygons must be coded in counter-clockwise direction and
excluded polygons clockwise.

In other words, as you draw the boundary of the polygon, "inside" is
always "left" of the pen direction, and "outside" is always "right".
Couldn't be simpler.

With such a path-direction convention in place, the ADD/DELETE
parameter becomes redundant and most algorithms operating on such
polygons become slightly simpler.

However, standard polygon handling algorithms might need to be slightly
adopted for spherical geometry, where, contrary to planar geometry,
polygons may also go around the poles of the polar coordinate system


Markus Kuhn, Computer Lab, Univ of Cambridge, GB
http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__

More information about the tz mailing list