Specifically, these techniques assist in the search for associations and dependencies. among the data and in the identification of patterns (or randomness), clusters, outliers, anomalies, or trends in the data. The techniques may be considered, therefore, as effective ways to look at the data from a variety of perspectives and to explore the data base. To enhance the use of GIS technology, standard statistical methods for exploratory data analysis, including statistical graphics, and methods for smoothing and filtering data in both space and time need to be incorporated.

Preliminary research findings suggest that efficient global-change research involving land-surface monitoring and dynamic-process modeling requires the capability for interactive analysis based on a variety of software applications with high-speed computer response times. Such capabilities are necessary to efficiently investigate complex data structures; determine various time and space scales for analysis; explore data; search for associations; and develop, test, and use models in various scenarios. In this framework, GIS modeling would allow scientists to go one step further into scientific inquiry by permitting effective visualization of the many complex data sets involved, while simultaneously providing capabilities for quantified investigation of spatial and temporal patterns in the data.

These quantified analysis capabilities will further enhance the usefulness of GIS's for global-change research. Current GIS technology provides a comprehensive set of advanced functional tools for processing, managing, manipulating, and analyzing spatial data. GIS technology is ideally suited to integrating and analyzing spatial data themes such as base cartographic data, satellite-derived data on Earth surface and weather conditions, gridded outputs from general circulation models, and irregularly spaced data sets on climate and hydrology. The ability to integrate these diverse themes and to examine their interrelationships promises to make modeling in the GIS environment a premier method for investigating the complex questions of global change.

The National

Gazetteer of the

United States

By Roger L. Payne

The National Gazetteer series is being published in cooperation with the U.S. Board on Geographic Names as USGS Professional Paper 1200. A separate volume is being prepared for each State and territory. Two phases of compilation are required for each volume. Phase 1, which is now complete for all States and territories, includes the compilation of most of the 2 million named features (except roads and highways) found on the large-scale topographic maps of the USGS. Phase 2 includes research and compilation from other Federal and State sources and historical materials.

Each volume contains official and historical geographic names listed in alphabetical order. Variant names are cross-referenced to official or primary name listings. Each entry includes information on the type of feature, the official status of the name, the county in which it is located, the geographic coordinates (including sources of linear features), the elevation of the place of the feature, and the name of the topographic map on which the feature is found.

New Jersey was the first volume published in the gazetteer series in 1982. Volumes for Delaware, Kansas, Arizona, and Indiana have been published, and volumes for South Dakota, North Dakota, Florida, and Massachusetts are scheduled for publication in the near future. An abridged, concise volume containing the major populated places, physical features, and other key entries for all States is also being prepared. Currently, research and compilation of volumes for Oregon, Iowa, Alabama, Mississippi, Pennsylvania, North Carolina, Tennessee, Missouri, and Utah are underway.

Expert Map Projection Selection System

By Doyle G. Smith and John P. Snyder

Cartographers at the U.S. Geological Survey are developing a system for the computerized selection of map projections for special-purpose map and chart design applications. This system, named the Expert Map Projection Selection System (EMPSS), is a computer software package that uses object-oriented structures and artificial-intelligence programming methods. The EMPSS has been developed for use in the design of thematic maps and other customized map products of the type generally constructed within a geographic information system environment.

The map projection is one of the most important factors of any map or chart design. The validity and utility of

Map Projections

A map projection is used to portray all or part of the surface of the Earth on a flat surface. Every flat map misrepresents the surface of the Earth in some way. No map can rival a globe in truly representing the Earth's surface. Every projection has its own set of advantages and disadvantages; there is no "best" projection. Mapmakers must select the one best suited to their needs, so that the map will distort least the things they want to show

most.

Mapmakers and mathematicians have devised almost limitless ways to project the image of the globe onto paper. Scientists at the U.S. Geological Survey have designed some projections for their specific needs, such as the Space Oblique Mercator, which allows them to produce maps from satellite images with little distortion. The examples on the facing page show several historically important projections and projections frequently used by mapmakers today.

any set of spatial geographic information are dependent upon the characteristics of the coordinate reference system or map projection in which the information is drawn. For this reason, it is important that the map designer select the map. projection that most closely satisfies the usage requirements of each product to be created. Hundreds of map projections are available for product design. Each projection is defined by a set of physical attributes that describes its characteristic features and usages. Many of these attributes are common to several projections but in different combinations and at varying levels of applicability or utility. Selecting the best projection requires the indepth knowledge and practical experience of a human expert in the map projection domain. Human reasoning is the only process capable of weighing the subjective values and merits of different combinations of projection attributes to determine the best projection for any map or chart design.

The EMPSS package includes a base of information that contains humanexpert technical descriptions and evaluations of the physical attributes of more than 50 of the most commonly used map projections. The computer accesses and organizes this human knowledge and expertise and makes it available to map and chart designers.

The EMPSS operates by asking the user a series of general questions about specific needs or applications. The answers to this series of 10 to 24 questions enable the system to construct an idealized set of specifications for the proposed application. Once this ideal model has been constructed, the system evaluates each of the map projections in the knowledge base against the ideal model, using the stored evaluative information provided by the human expert. The name of the map projection that compares most favorably to the specifications of the idealized model is provided to the user as the optimal projection for the proposed application or usage. The EMPSS also provides a numeric rating of the ability of this projection to satisfy the stated requirements, as well as the rating of other possible choices.

Figure 2. The concepts of feature,

entity, and object.

An Enhanced Digital
Line Graph Design

By Stephen C. Guptill and
Robin G. Fegeas

The U.S. Geological Survey has been producing digital cartographic data in digital line graph format for almost a decade. During this time, the tasks for which the data are being used have become increasingly complex, placing information demands on the data that were not planned for in their initial design. To respond to these requirements, the Survey has designed an enhanced version of the digital line graph, termed Digital Line Graph— Enhanced (DLG-E).

The underlying philosophy behind. DLG-E is to view cartography as an information transfer process centered on a spatial data base that can be considered, in itself, a multifaceted model of geographic reality. DLG-E data form the contents of the spatial data base. The DLG-E features must be representative of (and indeed must define) this multifaceted model of geographic reality.

In the DLG-E model, the phenomena of geographic and cartographic data are considered, in totality, as entities. An entity is a real-world phenomenon that is not subdivided into phenomena of the same kind. An entity (and its digital representation) is termed a feature. The concept of "feature" encompasses both entities and objects. The common attributes and relationships used to define the feature also apply to the cor

responding entities and objects. Figure 2 illustrates the relationships among concepts of features, entities, and objects.

For example, one could declare a "bridge" to be a feature. A bridge is one of a number of built-up structures on or near the surface of the earth. Its definition may be further refined to a structure erected over a depression or obstacle to carry traffic. Thus, the feature "bridge" is an element of a set of phenomena (an "erected structure"), with the common attributes of function ("to carry traffic") and location. It also has the common relationship of spanning another feature ("over a depression or obstacle"). The real-world entity "bridge" is defined by the common attributes and relationships used to define the feature. For the bridge example, the entity "bridge" is the erected unit in the real world (fig. 3).

The underlying philosophy ...is to view cartography as an information transfer process centered on a spatial data base that can be considered...a multifaceted model of geographic reality.

An object is the representation of all or part of an entity. For the bridge example, the objects include point, line, and area symbols and associated text on a map graphic, or a collection of records in a file of digital data (fig. 4). The concept of "object" encompasses both feature object and spatial object.

A feature object is an element used to represent the nonpositional aspects of an entity. In the example, the feature object represents the entity described by the definition of "bridge." Spatial objects are elements used to represent the position of an entity. Spatial objects are defined for zero-, one-, and twodimensional objects. To be mapped, a feature object must be composed of one or more spatial objects.

Attributes are the spatial and nonspatial characteristics of the entities represented by the objects or of an attribute value. Spatial attributes describe an

object's geographic location or geometric characteristic. The nonspatial characteristics of a feature include such concepts as shape, size, material composition, form, and function. The attributes for the bridge feature are given below (fig. 4). For every feature in the domain, each associated attribute must have an attribute value(s).

Features have relationships both with other features and with the spatial objects that represent them. Relationships are the spatial and nonspatial links between the objects. Nonspatial relationships are used to define the classes of objects that correspond to cartographic features. Spatial relationships include the topological relationships among objects. Taken collectively, these objects, attributes, and relationships make up the DLG-E data model.