Interactively Measure Distance
Measures the distance between two points which the user has clicked on.
This function allows the user to measure the distance between two spatial locations, interactively, by clicking on the screen display.
clickdist() is called, the user is expected to
click two points in the current graphics device. The distance
between these points will be returned.
This function uses the R command
input the mouse clicks. It only works on screen devices such as
‘X11’, ‘windows’ and ‘quartz’.
A single nonnegative number.