Effects of Weather on the Determination of Heights by Aneroid Barometer in Great Britain

Abstract

Although techniques for the correction of readings taken with the sur? veying aneroid are well known, the instrument has not been much used by geographers. This is no doubt due to the variable errors introduced by changes in the general atmospheric pressure. It is unfortunate that the aneroid barometer has been so neglected, as the rapidity with which heights may be determined by it is a real advantage. There are of course many problems in which it would be totally wrong to sacrifice accuracy to speed of operation, but in others it is immaterial if an error of a few feet is made. In such geomorphological problems as the mapping of old erosion surfaces, which because of dissection are unlikely to be at their original elevations, pre? cise levelling may give a false air of accuracy. The mapping of these features probably requires an accuracy of + or ?10 feet if the landforms are old and + or ? 5 feet if they are younger and better preserved. Although the tech? nique described below has been used mainly in such problems, it probably has a wider application. Undergraduates in the Department of Geography at Cambridge have measured the heights of wells with it in the course of a survey of the water-table in the Chalk south of Cambridge. It may possibly interest the microand local climatologists if they wish to determine quickly the height relations of certain phenomena. The mistrust of the aneroid has been reflected in the remarks made about it at various times. As an example one may quote a paper by George * in which the author claimed an accuracy of + or ? 5 feet, and a contributor to the discussion considered that errors of 15-20 feet would be likely to occur. Neither of these opinions was supported by concrete evidence. On the other hand, the aneroid has been used in the Tropics with much more confidence. The major pressure change in these regions is diurnal, so that a reasonable degree of accuracy can be obtained by applying a correction based on the assumption that this change is always the same. Hinks a for instance quotes the use of the aneroid in Southern Nigeria for plotting fairly accurate con? tours, and Sutton Bowman 3 has made aneroid traverses in Nepal of up to six weeks and found closing errors of as little as 4 feet. In our latitudes however conditions are by no means so settled. The daily change of pressure is small and usually completely dwarfed by pressure changes due to the development, movement and dissipation of lowand high-pressure systems. Corrections for these largely unpredictable changes can be made from the pressure trace of