Plotting apparatus for use in conjunction with surveying instruments

Claim 1

A plotting apparatus, for use in conjunction with a surveying instrument which is traversed over the ground and produces a varying electrical signal, comprising means for playing back a recording of the signal from the instrument to produce a varying output signal, a drum sheet carrier mounted for rotation about its own axis, a first electric motor for driving the drum, a marking device adapted to produce a marking on a sheet secured around said drum, a carriage movable in a direction parallel to the axis of the drum, and carrying said marking device, a second electric motor for driving the carriage, variable pulse generator means controlled by the output signal and arranged to supply pulses of a frequency dependent on the strength of the output signal to actuate the marking device to produce marks at a corresponding rate, a shaft which rotates with the drum carrying a first cam which is arranged to close a first contact-beaker to energize the carriage-driving electric motor and thereby to cause the carriage to move one step each time the drum completes a revolution, and a second cam which is arranged to close a second contact-breaker momentarily at the beginning of each revolution of the drum to trigger operation of the play back means, the play back means being adapted to produce a tone as well as the recorded signal, and a relay responsive to said tone to maintain operation of the play back means so long as the recording of a traverse is being played back.