### 7 Projections

PONGO is capable of plotting astronomical coordinate data in one of several ‘projections’ or, more strictly, in one of several geometries (not all of the ‘projections’ are true projections onto a plane, but are geometries that have other properties – e.g. the equal area property where areas are preserved in the transformation from the celestial sphere to a two dimensional plot). Table 1 lists the geometries available in PONGO.

All the main point plotting commands will work in any of the available geometries. They assume that the data in the XCOL and YCOL areas are stored internally in radians and refer to the longitude and latitude respectively of the point to be plotted. Where the position values are to be entered on the command line, the latitude and longitude are normally given in degrees.

In addition to the point plotting commands, there are also three commands whose only use is when plotting in a particular projection type:

GRID
– Plot a coordinate grid in the current projection. The default parameters of this command are to draw a grid over the whole sky with a longitude line every 30 degrees and a latitude line every 10 degrees.
GT_CIRCLE
– Draw a great circle between two specified points on the celestial sphere.
PVECT
– Draw proper motion vectors from each plotted point as small great circle arcs.

 Projection Description NONE No projection is used. AITOFF The Aitoff projection (has an equal area property). ARC Move an equal distance on the tangent plane as a great circle on the celestial sphere. GLS Right Ascension intervals are multiplied by $cos\delta$ (has an equal area property). MERCATOR The Mercator projection. SIN Drop a perpendicular onto the tangent plane. STG The Stereographic projection. TAN Projection onto the tangent plane through the centre of the celestial sphere.

Table 1: Projection geometries available in PONGO