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FIGURE 25.-Base map used with oil and gas overlay. From the USGS Beeville, Tex
1:250,000-scale topographic quadrangle.

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ADVANTAGES AND LIMITATIONS

Overprints can be used to keep a map from becoming cumbersome, in cases where more than one overlay would be required. However, because an overprint is a permanent part of a map, the entire map may have to be revised if the overprint becomes undesirable. Overprinting requires at least one additional press run and register may not be exact because of printing on top of published maps whose size may have changed with time. Overprinting must be limited to data that will not cause cluttered detail or confusion.

The greatest advantage of overlays is that they avoid crowding map detail. Overlays, inoping with the feature-separation system, show unlimited Information related to the same base map r. Production costs for small quantiless than that

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FIGURE 25.-Base map used with oil and gas overlay. From the USGS Beeville, Texas,
1:250,000-scale topographic quadrangle.

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creasing the scale could cause data crowding, which can be avoided by removing a number of guides from the original map materials before reduction. For example, the primary and secondary road guides from several 1:24,000-scale feature-separated maps could be reduced and combined to compile a road map at 1:50,000 or 1:100,000 scale.

Figures 26 through 35 show various sample guides of a feature-separated map.

AUTOMATION

Computers and related machinery have eased the task of preparing maps and extracting map data. Although automated cartography still is in its infancy, some steps have been taken toward the goal of automation.

One approach is an automated coordinatograph, an instrument that plots points or lines by their x and y coordinates. Map data can be stored on punched cards or magnetic tapes to be recalled later and fed to the coordinatograph to plot a graphic from the stored information. The plotted data can be edited by correcting, deleting, or adding, and the edited data can be returned to storage. Some editing can be done automatically.

The usefulness of stored data can be enhanced by feature separation. Information can be stored in a data bank for selective recall. For example, a user can select all towns with a specific population or all contours within a stated range. The user can make his own feature separation from the available information and can combine separate features as he chooses. Sometimes he can select the scale or even the projection on which the graphic output is drawn.

If the user's goal is to compile statistics, he can extract the data from published maps and compile his lists manually. However, digitized map data can be retrieved in list form, eliminating the need for map interpretation and saving the time needed to compile a list by hand.

LAND USE AND LAND COVER
CLASSIFICATION AND MAPPING

To make sound decisions, a modern nation needs adequate information about many complex, interrelated activities. Land use has become increasingly important in overcoming problems of haphazard, uncontrolled development, deteriorating environmental quality, loss of prime and unique agricultural lands, destruction of important wetlands, and loss of fish and wildlife habitat. Land use and land cover

data are needed to analyze environmental processes and problems that must be understood to improve living conditions and standards or to maintain them at current levels.

Because of increasing emphasis on coastal development, various agencies are collecting land use and land cover data. The handbook should stimulate more uniformity and standardization in this data collection.

DEFINITION OF LAND USE

Although most people have a general idea of what constitutes "land use," the term is surrounded by confusion and misunderstanding. In the handbook, land use is comprised of two componentsland use and land cover. Land use is activity related, that is, how a parcel of land or water is used. Land cover refers to vegetation or artificial structures on the land. The land cover of an area might be classified as nonforested wetland, whereas the actual use may be as a wildlife refuge, an oil extraction operation, or a waste disposal operation. Aircraft or satellite remote sensing used to map these data makes the distinction more important because activity is not always discernible. Land cover can be used sometimes instead of activity, but not always.

CLASSIFICATION THEORY AND PRINCIPLES

There are no ideal methods for inventorying or classifying natural resources or land use and land cover. A detailed inventory is adequate only for a relatively short time because land use patterns change with resource demands. Users of this type of information want an inventory that reflects change and satisfies most of their needs.

Depending on needs, a classification system can be oriented to people, resources, or both. Wide acceptance and use shows that a combination is preferred.

As remote sensing technology improves, more and more inventories are made with remote-sensor source material obtained from both aircraft and satellites. Whether it is an aerial photograph or a Landsat image in digital tape format, the source material requires interpretation for use in the inventory. Although supplemental material may be needed, the inventory will include only what can be identified with acceptable accuracy. Remote sensors using relatively inexpensive and generally unbiased source material provide many desirable features that may outweigh their limitations. The informa

tion they provide is gathered at a specified time and is usually obtained under conditions favorable to identification.

Classification systems can be derived inductively or deductively. In inductive classification, where observed features are recorded and categorized, the number of observable features is limited to what the remotely sensed source material can detect. Therefore, the classification and inventory will be limited. In deductive classification, where preconceived categories are established, using remote sensors limits some of the categories because they may not be observable. Usually, a miscellaneous category is used for unclassifiable features.

A land use and land cover classification system that can effectively use orbital and high-altitude remote-sensor data should meet the following criteria (Anderson, 1971).

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The minimum level of interpretation accuracy in the identification of land use and land cover categories from remote-sensor data is at least 85 percent.

The accuracy of interpretation for the categories is approximately equal.

Repeatable or repetitive results are obtainable from one interpreter to another and from one time of sensing to another.

The system is applicable over extensive areas. The categories permit vegetation and other land cover to be used to replace activity.

The system is usable with remote-sensor data obtained at different times of the year. Subcategories that can be obtained from ground surveys or from larger scale or enhanced remotesensor data are used.

Aggregation of categories is possible.

Comparison with future land use data is possi

ble.

Multiple uses of land are recognized when possible.

THE DECISION PROCESS

Clearly defined categories are rarely found in land use classification. Fine points, such as how to separate water and land, are debated. At first the class selection seems obvious, but becomes clouded when one considers problems of seasonally wet areas or coastal tidal flats, or marshes, swamps, and bogs with various types of plants providing some degree of crown cover.

Most types of land use classified in the inventory

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