A vertically driving marking robot includes a robot body; at least one magnet constraining the robot to move parallel to a vertical, magnetically responsive surface; a drive configured to displace the robot relative to the surface while the robot is held to the surface; a holder configured to hold a marker; an accelerometer measuring a gravity vector; a computing device in communication with the optical sensors, the accelerometer, and the drive. The computing device includes a processor and computer-readable memory, wherein the computer-readable memory includes non-transitory program code for at least one of the following actions: (a) generating a drift correction to compensate for drive slippage drift in response to and as a function of the gravity vector and (b) commanding the drive to displace the robot along a desired trajectory in response to the drift correction.