What is the actual altitude if you take off with an altimeter setting of 30.12 and later read 30.02 at 5,000 feet?

To determine the actual altitude when there is a difference in altimeter settings, one must understand how altimeter settings affect indicated altitude. An altimeter measures atmospheric pressure, which decreases with an increase in altitude. An altimeter doesn't provide a direct reading of altitude; rather, it gives an indication based on the local pressure setting.

When you take off with an altimeter setting of 30.12 inches of mercury, the altimeter is calibrated to that pressure. If you later read an altimeter setting of 30.02 inches at a height of 5,000 feet, the discrepancy between the two settings indicates a change in atmospheric pressure.

The pressure difference can be calculated; for every 0.01 inches of mercury, an altimeter typically indicates a difference in altitude of approximately 30 feet. This means that the 0.10 inch variation between the two settings (30.12 - 30.02 = 0.10) would lead to an error in altitude indication of approximately 300 feet (0.10 inches x 30 feet per inch).

Since the altimeter reads higher at the original setting of 30.12, when transferring to a lower setting of 30.02 at the same point, the actual altitude