Answer:
There are different types employed on Cameras ( very practical) surveying instruments, aircraft, some types of Cranes, and submarines. There is a circular dial with a horizontal bar or bars on the center of the scale. movements of this bar correspond to actions of the vessel or vehicle. down is up- at least on aircraft and submarine models. Up is down-there are cross hairs akin to engineering instruments and rifle sights. it is quite involved- a stabilized platform is the tilting table reference- and is rotated by a gyroscope type instrument at a high rate of speed. More than that would plunge into the High Yellow- sensitive stuff. The type used on some cameras has a regular cross-haired sight but has a gravity spirit level superimposed in the view-finder so the operator can see both before he squeezes off- these things, even non-Gyro type are Expensive. They type used on some forms of hammerhead crane must be astronomical in cost- but look at the stakes!