Mounted on boards in front of the pilot and observer are various instruments to indicate the performance of engine and plane (Fig. 2). Those of interest to the photographic observer are the compass, the altimeter, the air speed indicator, the inclinometers.

The compass is usually a special airplane compass, with its "card" immersed in a damping liquid. Like most of the direction indicating instruments on a plane its indications are only of significance when the plane is pursuing a steady course. On turns or rapid changes of direction of any sort perturbations prevent accurate reading.

The altimeter is of the common aneroid barometer type. On American instruments it is usually graduated to read in 100-foot steps. While somewhat sluggish, it is quite satisfactory for all ordinary determinations of altitude in photographic work. Were primary map making to be undertaken, where the scale was determinable only from the altitude and focal length of the lens, the ordinary altimeter is hardly accurate enough.

The air speed indicator consists of a combination of Venturi and Pitot tubes, producing a difference of pressure when in motion through the air which is measured on a scale calibrated in air speed. This instrument is important for determining, in combination with wind speed, the ground speed of the plane, on the basis of which is calculated the interval between exposures to secure overlapping photographs. Its accuracy is well above that necessary for the purpose.

Inclinometers for showing the lateral and fore and aft angle of the plane with the horizontal, are occasionally used, and have also been incorporated in cameras. The important point to remember about these instruments is that they are controlled not alone by gravity but as well by the acceleration of the plane in any direction. They consequently indicate correctly only when the plane is flying straight. On a bank the lateral indicator continues to indicate "vertical" if the bank is properly calculated for the turn.