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### 8.2 Surface Types

The different `plot2*` commands correspond to different plot surface geometries. The different commands come with their own specific axis configuration parameters. Some of the plot layer types are specific to certain surface types. When supplying data from input tables to plot layers, the coordinate values you need to supply (and hence the corresponding parameter names) are determined not by the layer type, but by the surface type. For instance, point positions for layer N on a 2-d Cartesian surface (`plot2plane` command) are given using parameters `xN` and `yN`, but when plotting to the celestial sphere (`plot2sky` command) you supply `lonN` and `latN`).

The following list summarises the available surface types and their corresponding positional coordinates.

Plane (`plot2plane`)
  2-dimensional Cartesian axes. Positional coordinates are supplied as `x`, `y` pairs. Note that this command can also be used to draw histograms.
Sky (`plot2sky`)
  Celestial sphere. Positional coordinates are supplied as `lon`, `lat` pairs, giving longitude and latitude in decimal degrees. A number of different projections are available, and conversion between different celestial coordinate systems can also be performed. You could use it for other spherical coordinate systems too (like the surface of a planet).
Cube (`plot2cube`)
  3-dimensional Cartesian axes. Positional coordinates are supplied as `x`, `y`, `z` triples.
Sphere (`plot2sphere`)
  3-dimensional isotropic space with spherical polar coordinates. Positional coordinates are supplied as `lon`, `lat`, `r` triples, giving longitude and latitude in decimal degrees, and radius in an arbitrary unit. The plotting surface (space) is similar to Cube, except that the unit distance is always the same in all three directions.
Time (`plot2time`)
  2-dimensional axes, but the horizontal axis represents time. The axis may be labelled in various ways (ISO-8601 dates, decimal year, MJD etc). Positional coordinates are supplied as `t`, `y` pairs. Time can be represented in input data in various ways; if sufficient metadata is provided in the input format the epoch can be determined automatically, otherwise it may be necessary to specify the time representation being used.

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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk