Maps are graphical models or simplified views that represent the distribution of a collection of features on the Earth's surface. To provide a consistent spatial data representation of a feature there must be agreement on how to classify it, how to delineate it, the criteria for inclusion, valid attributes to assign to it, and the appropriate way to symbolize it. In the previous chapter we examined the federal response to the creation and maintenance of a data base of street centerlines for the nation. Such a file is composed of a relatively unambiguous and easily identifiable set of geographic features. These features are very tangible and can be graphically represented as a set of line segments that form the links of an integrated network. For example, while differences of opinion exist among agencies regarding the appropriate classification of roads and the set of attributes that should be assigned to them, the task of delineating and representing them is straightforward and can be achieved from a wide variety of source materials. In contrast, the natural environment presents a much more complex set of challenges. Naturally occurring features such as wetlands are not only difficult to classify, but they also have poorly defined edges, tend to change through time, appear and disappear at different scales, and often can only be delineated with field work and skilled interpretation based on specialized source materials, such as color infrared photography. Furthermore, the areal extent or quantity of natural features has important ramifications for economic policy, land use controls, taxation, and environmental concerns.

To monitor whether the mandate for no net loss of wetlands is being followed, there needs to be accurate data regarding the spatial distribution of existing wetlands through a time interval. In other words, there is a need for a series of wetland maps, and there really is no other acceptable alternative. Armed with GIS and the appropriate series of digital maps, it would be relatively easy to evaluate the distribution of wetlands. For example, an important tool of modern GIS is the ability to measure the area of features. In fact once a wetland area is encoded into the system, its area is automatically calculated. In a digital world it is also easy to automatically compare wetland maps at different times to measure and to display changes.

Unfortunately, the fact that the tools exist to monitor wetland change does not mean that governments have been able to implement such systems. From a mapping science perspective several problems exist. The most fundamental issues are classification, delineation, resolution, and representation of wetland data. Although the FGDC is beginning to address



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