An Isarithmic-Dot Population Map

Abstract

^TT HE two most common symbols employed in the mapping of population distribution are isarithms and dots. Each of these has advantages, but there also are disadvantages which leave something to be desired in each type of map. However, these disadvantages are not mutual but are complementary to each other. 1. Isarithms: One advantage of isarithms is that they allow one to read directly from the map the number of people in an area. Since, on population maps, isarithms are usually lines delimiting ratios (population density), they are most satisfactory for maps of areas where the important thing is the ratio of number of people to the area that they occupy-in other words, for regions of dispersed population. Isarithms do not satisfactorily treat agglomerated populations for a number of reasons. (a) The important thing about an agglomeration is the actual number of people in it rather than its density. For instance, in a region of dispersed population a city of 50,000 spread over four square miles and another city of 50,000 occupying eight square miles are equally important as clusters of people, yet an isarithmic map would render the first much more conspicuous than the second because its density is twice as great. (b) From an isarithmic map one cannot determine the actual population of an urban unit. (c) The fact that isarithms are lines across the map means that they show gradual transitions better than abrupt changes in densities. The transition in density from rural to urban areas is not gradual but abrupt; isarithms are not adapted to showing such a change very satisfactorily. (d) On an isarithmic map a large city surrounded by dispersed population would be represented by numerous concentric isarithms. This is especially true on maps where the areal units are small enough to bring out sharply the contrast between densities of urban and adjoining rural localities. These peaks of concentric isarithms coinciding with urban agglomerations interrupt the general patterns on the map and make it difficult to get a clear picture of population distribution-they clutter up the map. It can be concluded that isarithms represent dispersed population better than they do agglomerated. 2. Dots: A dot map is the best map for showing the pattern in which population is scattered over an area. However, there are some disadvantages in using dots. (a) One cannot tell from a dot map whether a dot represents dispersed population or a small agglomeration. For instance, on a large-scale dot map where one dot equals 500 people one cannot ascertain whether a certain dot represents 500 scattered people or a village of 500 inhabitants. (b) The placing of dots is a difficult task especially in areas of dispersed population where each dot represents a large number of people scattered over an area larger than that actually covered by the dot on the map. One must decide where within those areas to place the dots. No matter where the dots are located, a false impression is given of population being clustered