One of the challenges associated with using air photos as sources of map data is that the scale of an aerial image varies from place to place as a function of the elevation of the terrain shown in the scene. As discussed in Chapter 6, large scale maps are typically derived from aerial imagery. This isn't true for aerial imagery, however, except for images that have been orthorectified. One of the defining characteristics of topographic maps is that scale is consistent across each map and within each map series. This tendency is illustrated below in Figure 2.5.1. In general, the larger the map scale, the more detail is shown. When we talk about large- and small-scale maps and geographic data, then, we are talking about the relative sizes and levels of detail of the features represented in the data. If we were to change the scale of the map such that the length of the section of road on the map was reduced to, say, 0.1 units in length, we would have created a smaller-scale map whose representative fraction is 0.1:100,000, or 1:1,000,000. The representative fraction 1:100,000, for example, means that a section of road that measures 1 unit in length on a map stands for a section of road on the ground that is 100,000 units long. The proportion, or ratio, is also typically expressed in the form 1 : D g rather than 1 / D g. Map scale is the proportion between a distance on a map and a corresponding distance on the ground:īy convention, the proportion is expressed as a "representative fraction" in which map distance (D m) is reduced to 1. When people who work with maps and aerial images use the word "scale," they usually are talking about the sizes of things that appear on a map or air photo, relative to the actual sizes of those things on the ground. These are fun, thought-provoking exercises to help you better understand the ideas presented in the chapter. Take a minute to complete any of the Try This activities that you encounter throughout the chapter. classify projected graticules by projection family.interpret distortion diagrams to identify geometric properties of the sphere that are preserved by a particular projection and. ![]()
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