Between the equator and the poles of the earth g varies by only 1% and, if it is assumed that instrument manufacturers have used the value at a latitude of 45 degrees, the error would only be 0.1% if the instrument were to be used at a latitude of 52 degrees. In fact g changes with both but only by very little. Sources of error derived from assumptions in the equation Acceleration due to gravity, gįirstly, it is assumed that the acceleration due to gravity, g, is constant with latitude and with height. Instead it looks at the assumptions made in the application of the equation and therefore these errors are general and applicable to all instruments. Note that what follows does not deal with sources of error associated with the construction of the instrument and which are therefore specific to that instrument or design. We have made a statistical assessment of the errors achievable after making appropriate corrections to the height reading and tested the theory on the hill. Unfortunately for the manufacturers of altimeters and for those of us using them, these terms are not constant and so may affect height measurements. To convert this change to height, the other terms in the equation are assumed constant. The problems arise because the detector responds only to changes in pressure. The equation is merely expressing this change in a quantifiable way. Consequently, at the top of the mountain, the total amount of air pressing down on you, the pressure, is less than at the bottom of the mountain. As you climb to the top of the mountain the column above you is now shorter, but also the air around you is thinner, or less dense. Above you is a column of air several miles thick pressing down on you. So what is this equation saying? Imagine yourself standing at the foot of a mountain at sea level. The derivation of this relationship is given in Appendix 1.Įlectronic altimeters have this relationship programmed into the chip while for hand-held instruments the graduated height scale is calculated from it. Where z is the height difference between the starting height and the measurement height, R is the gas constant, T is temperature of the air measured in Kelvin, g is the acceleration due to gravity, M is the molar mass of the gas (in this case air), p o is the atmospheric pressure at the starting height and p is the atmospheric pressure at the measurement height. The mathematical relationship that relates them is: The altimeter works on the principle that the pressure within a column of air varies in a known way with height. The principle of the altimeter and sources of error This article will show that both barometric drift and temperature may cause large errors to altimeter measurements and corrections must be made for these. That the authors have experienced errors of over 50m on the summits of hills on days when atmospheric pressure changes were equivalent to changes in height reading of only 10 to 20m prompted this study, in which the accuracy of the instrument was tested. With the arrival or departure of a weather front, displayed altitude can change 20 to 50 metres, and a storm can cause a change of more than 50 metres'. However, just how accurately do altimeters measure height? The makers of one of the popular models claims 'On a typical day, minor atmospheric pressure changes may cause the displayed altitude to vary from the actual altitude by 20 metres.
These claims have also helped to bring these instruments to the attention of hillgoers. The inclusion of Knight's Peak in Munro's tables was justified on height measurements made with an altimeter and recently altimeter measurements have led to the suggestion that Leathad an Taobhain, a Corbett in Glen Feshie ( ref. The use of altimeters in height measurement Graham Jackson and Chris CrockerĪltimeters have been used by mountaineers for many years, but the recent development of electronic wristwatch type instruments has made the altimeter easier to use and more popular with walkers and climbers.
0 kommentar(er)
