The altimeter gives a reasonably accurate estimation of altitude. With the introduction of the wrist or watch altimeter, altimeters are becoming more frequently used in outdoor recreation. For example, when walking up a spur to the summit, you can check your position on the spur by matching the altitude given by the altimeter to the contours shown on the map.
Altimeters have certain limitations which can easily mean that the information they give you is misleading. An altimeter is a barometer which measures the atmospheric pressure and compares it to a known pressure—sea level. As the air pressure is constantly changing, even at sea level, the altimeter needs to be adjusted to the known altitude every now and then so that it has an accurate reference point. Depending on the weather this can be necessary several times a day. If the weather is stable, an adjustment will be required less frequently. It is good practice for the navigator to recalibrate the altimeter whenever a known altitude is reached to ensure greater accuracy.