PDL::Transform::Cartography 2.4.3

PDL::Transform::Cartography Perl module contains useful cartographic projections.

SYNOPSIS

# make a Mercator map of Earth
use PDL::Transform::Cartography;
$a = earth_coast();
$a = graticule(10,2)->glue(1,$a);
$t = t_mercator;
$w = pgwin(xs);
$w->lines($t->apply($a)->clean_lines());

PDL::Transform::Cartography includes a variety of useful cartographic and observing projections (mappings of the surface of a sphere), including reprojected observer coordinates. See PDL::Transform for more information about image transforms in general.

Cartographic transformations are used for projecting not just terrestrial maps, but also any nearly spherical surface including the Sun, the Celestial sphere, various moons and planets, distant stars, etc. They also are useful for interpreting scientific images, which are themselves generally projections of a sphere onto a flat focal plane (e.g. the t_gnomonic projection).

Unless otherwise noted, all the transformations in this file convert from (theta,phi) coordinates on the unit sphere (e.g. (lon,lat) on a planet or (RA,dec) on the celestial sphere) into some sort of projected coordinates, and have inverse transformations that convert back to (theta,phi). This is equivalent to working from the equidistant cylindrical (or "plate caree") projection, if you are a cartography wonk.

The projected coordinates are generally in units of body radii (radians), so that multiplying the output by the scale of the map yields physical units that are correct wherever the scale is correct for that projection. For example, areas should be correct everywhere in the authalic projections; and linear scales are correct along meridians in the equidistant projections and along the standard parallels in all the projections.

The transformations that are authalic (equal-area), conformal (equal-angle), azimuthal (circularly symmetric), or perspective (true perspective on a focal plane from some viewpoint) are marked. The first two categories are mutually exclusive for all but the "unit sphere" 3-D projection.

Extra dimensions tacked on to each point to be transformed are, in general, ignored. That is so that you can add on an extra index to keep track of pen color. For example, earth_coast returns a 3x< n > piddle containing (lon, lat, pen) at each list location. Transforming the vector list retains the pen value as the first index after the dimensional directions.