Re: Transverse cylidrical map projection.

[James Kuyper <kuyper(at)wizard.net>]

Liam Gumley wrote:
> 
> James Kuyper <kuyper@wizard.net> wrote in message
> 38DA1B88.7A690089@wizard.net">news:38DA1B88.7A690089@wizard.net...
> > 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

It turns out I was mistaken. In both cases, it was tilting the map,
rather than the axis of the projection.

> > 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
...
> James,
> 
> 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.

I don't want to do it that way. I'm conveying an extra level of
information by my choice of boundaries; they corresponding to the
farthest distance the satellite could see in each direction at any time
during the data collection, if it had been pointed in that direction.
This provides a context for displaying the images it saw in the
direction it was actually pointing.

In any event, I resolved the problem. The feature I wanted is controlled
by the 'central_azimuth' parameter, rather than by the 'rot' parameter.
Looking at their descriptions, I can't figure out why they work the way
they do. 'rot' has basically the effect I would expect given the
description of 'central_azimuth', and vice-versa. Once I figured out the
correct central_azimuth angle, MAP_SET had no problems with my limit
points.