The Dispersion of Geomorphic Data around Measures of Central Tendency and Its Application

摘要

A voluminous amount of data were gathered from a random sample of over 200 USGS and AMS topographic maps of the United States. As a first step in the analysis of this information, means of several terrain factors were computed. The mean relief of a 5/16 square mile area is 240 feet, and for a 160 square miles area is 1,420 feet. A successive doubling of areas from the smallest of the limits to the largest produces means of relief which fall into a regular progression. This permits the construction of a nomograph for determining the mean relief of areas of any size lying between 5/16 and 160 square miles. Other information of interest computed from these data include an estimate of about 2,300 feet for the average elevation of the United States. A 20 foot contour is crossed on the average of every 290 feet of random traverse. On a similar traverse, 3 ridges averaging 150 feet above valleys would be encountered about every mile. Hilltops rising sufficiently above other land to be represented by a closed contour occur on the average of three every square mile. The above mean values probably seem extreme, but modes and medians have values of a smaller magnitude. The strong skewness of geomorphic data indicated by the spacing between the means and medians calls for a kind of analysis different from that used with normal distributions. (Author)