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Re: Transverse cylidrical map projection.

James Kuyper <kuyper@wizard.net> wrote in message
> I want to plot data using a transverse cylindrical map projection. An
> equal-area one would be best, but equidistant or mercator would be
> almost as good, just so as long as it's transverse version of one of the
> cylindrical projections. MAP_SET accepts a tilt angle, which doesn't do
> what I want for most of the cylindrical projections. The user's guide
> contains an example command:
>         map_set,0,0,45,londel=20,latdel=20,/grid,$
>         /continent,/cyl,title='Oblique Cylindrical Equidistant'
> which is shown in the book as producing a map with the projection axis
> tilted by 45 degrees: the lines of constant latitude and longitude are
> curved. When I try it, I get a map tilted by 45 degrees, which is a very
> different thing: The lines of constant latitude and longitude are
> strait, tilted by 45 degrees. This suggests that the book was printed
> using a different (hopefully later) version of IDL than I'm using. I saw
> the problem first in version 5.0.3, but I've recently discovered where
> they've hidden version 5.2 on our machine, and I still see the same
> results using it.
> Luckily, I've found that the transverse mercator projection does
> implement the tilt properly. However, in large maps it often considers
> one or more of my limit points unmappable, for reasons that escape me.
> For example,
> map_set,-15.7970,-90.4190,260.1820, limit=[78.548,-31.494, $
> -27.66,-64.441, -64.066,103.55, -0.792,-114.296],$
> /continents,/grid,/label,/isotropic,/transverse_mercator
> Produces the complaints:
> % MAP_SET_LIMITS: Unmappable limit point:      -31.4940      78.5480
> % MAP_SET_LIMITS: Unmappable limit point:       103.550     -64.0660
> The only points that should be unmappable in a mercator projection are
> points near the projection axis, and of the four points I gave, those
> two are the ones farthest from the axis.
> If you use /lambert instead of /transverse_mercator, you'll get a feel
> for the kind of plot I want. /stereo and /azimuthal also work, each with
> their own distortion properties. However, I want a cylindrical
> projection, not an azimuthal one - the projection properties aren't
> quite what I want.


I haven't bothered with the LIMIT keyword to MAP_SET since I discovered the
SCALE keyword:

map_set, -15.7970, -90.4190, 260.1820, /transverse, scale=50e6
map_continents, /hires
map_grid, /label

The map is always isotropic, and it always fills the current display. You
usually need to experiment a little with the scale factor, but it beats
trying to guess map limits.