Jonathan Gregory wrote:
>Dear Magi et al
>
>As far as I can tell, polar stereographic and general stereographic are the
>same operation, but perhaps the longitude doesn't play quite the same role.
>With a general stereographic projection, the longitude and latitude of the
>projection origin must be specified, and the meridian passing through the
>projection origin (along its longitude) is the central meridian of the
>projection, and appears as a vertical line in the middle of the map. For a
>polar stereographic projection, *all* meridians pass through the projection
>origin. The way to orientate the map is therefore arbitrary, so we specify
>one of the longitudes (the straight vertical longitude) to be the vertical
>line running upwards from the middle of the map.
>
>But in both cases the latitude_of_projection_origin is needed, so I think
>this ought to be added to the definition of polar_stereographic. It is
>allowed to be -90 or +90. This could be avoided by having separate projections
>of north_polar_stereographic and south_polar_stereographic but no-one does
>that apparently.
>
>I am not very well informed about map projections so if anyone else can
>correct or comment on the above that will be welcome.
>
>Cheers
>
>Jonathan
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>
yes, i think you are right, that polar stereographic is a special case
of stereographic projection
so your stereographic parameters look good to me:
longitude_of_projection_origin
latitude_of_projection_origin
scale_factor_at_projection_origin
false_easting
false_northing
but your "polar stereographic"parameters are confusing:
straight_vertical_longitude_from_pole
latitude_of_projection_origin - Either +90. or -90.
Either standard_parallel or scale_factor_at_projection_origin
false_easting
false_northing
since there is no explanation of the
straight_vertical_longitude_from_pole or standard_parallel (FGDC is
pretty deficient sometimes)
i would prefer to remove polar stereographic, and just explain to use
the general case with lat = +/- 90. also i would advocate that
false_easting, false_northing are in all cases optional and are assumed
= 0, 0 when not specified.
Received on Fri Jun 18 2004 - 05:55:36 BST
