The documentation for the cm class explains the structure
of cm objects, and also outlines the other functions dealing with them.
Other things related to cm data:
[[,cm-method,
[[<-,cm-method,
applyMagneticDeclination,cm-method,
as.cm(),
cm,
cm-class,
plot,cm-method,
read.cm(),
rotateAboutZ(),
subset,cm-method