Snapplot can display reference features such as coastlines and roads to provide a visual reference for stations and observations. Reference features are defined in simple text files. These are loaded either by include the files in the snap command file, or by specifing them when snapplot is started using the -b command line option.
Background files are specified in the command file with the command
plot background file_name
The format of the background file is very simple. It consists of two commands to specify the name of the background layer, and the coordinate system used for coordinates, and a series of coordinates defining the features. Each coordinate has a pen code of 1 (to start a new feature) or 0 (to continue the feature). The following example illustrates this:
#layer coast #coordsys nzmg 1 2125878.0 5329937.0 0 2126545.0 5330337.0 0 2127211.0 5329802.0 0 2127745.0 5329668.0 0 2128812.0 5330201.0 0 2129613.0 5331000.0 0 2129614.0 5332067.0 1 2670951.0 6040404.0 0 2670836.0 6040318.0 0 2670167.0 6038718.0 0 2669099.0 6037519.0 0 2668000.0 6036480.0 0 2666562.0 6035121.0 0 2666562.0 6034587.0 0 2666961.0 6033787.0 0 2668028.0 6033785.0 0 2672167.0 6037915.0 0 2672969.0 6039381.0 0 2673769.0 6039914.0 0 2673370.0 6040715.0 0 2671370.0 6040717.0 0 2670951.0 6040404.0