By D H Maling
A vintage textual content and reference paintings for college students, teachers and execs coming near near cartometry from quite a lot of backgrounds - geography, cartography, forestry and stereology. the writer hasd transcended those topic obstacles to supply a definitive and coherent advisor to the idea and means of measuring distance and components on maps. to extend this book's accessibility, at least mathematical wisdom is assumed.
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Additional info for Measurements from Maps. Principles and Methods of Cartometry
Sample text
Similarly measurement of the same line divided into four rectilinear elements is longer than those measured in only one or two steps. Since an increase in the number of rectilinear elements is equivalent to a reduction in the separation of the dividers' points, we may state the invariable rule that measured length increases as the span of the dividers is made smaller. This is seen in Fig. 1(b) where the rectilinear elements are now only onetwelfth of the length of the line. Nevertheless their total length is appreciably less than that of the line which is to be measured, for we can still see that some corners are cut and the broken line defined by the dividers misses some of the major irregularities.
PRECISION DIVIDERS (Fig. 5). These are usually smaller than the typical half-set. There is an adjustable screw on one leg which alters the width of a split in that leg. The purpose of this screw is to introduce a small change the distance between the points by bending that leg. This allows fine measurements and settings to be made against a scale. The maximum spread of the instrument is usually of the order of 150 mm, but because the points are fixed FIG. 6. Spring-bow dividers. The Methods of Measuring Distance 37 parts of the legs, they meet the plane of the m a p at an acute angle if the legs are too widely separated.
The Nature of the Medium 15 Scale The scale of a m a p is its most important mathematical property. This is often defined as: The ratio of the distance on the map to the actual distance that it represents on the ground. 1) The fraction 1/S is correctly called the representative fraction by many m a p users. M a p makers tend to use the work "scale" without any qualification to mean the same thing. 1) represents the commonest calculation which occurs in cartometric work, for it is used every time that it is necessary to convert from the measured distance (ab) into the corresponding ground distance (AB) with equivalent transformations to area measurements.
