Early altimeter were nothing more than a modified aneroid barometer, displaying altitude rather than air pressure. Following in the footsteps of pioneering balloonists aviators simply changed the scale on the barometer from a measure of pressure to a measure of height above the ground, subsequently giving birth to the first aviation altimeters in the process. Bimetallic strips were incorporated inside the sealed unit of the altimeter to compensate for changes in temperature that could cause inaccurate readings. Later the simple dial and needle style altimeters grew more complex. Some made use of movable dials to compensate for daily variations in atmospheric pressure, and others allowed the pilot to input a pressure compensation factor (Mears, 1923). However, the overall indicator design changed very little-a simple rotating needle from zero to the maximum operating altitude of the aircraft, or the current theoretical safe maximum altitude. The pilot now had a tool that could be used in any weather and at any time of day accurately to indicate his altitude. For example, the altimeter in conjunction with navigational charts dramatically improved the pilot's SA by making him ware of potential collision hazards.