CaGIS Vol. 28, no. 4

Reflections on Mapping Census 2000

Cynthia A. Brewer

Mapping Census 2000: The Geography of U.S. Diversity is an atlas containing 75 maps and presenting county- and state-level data for Census 2000. Cynthia Brewer and Trudy Suchan rapidly produced the atlas in the Population Division of the U.S. Census Bureau using ArcInfo 8.1 prerelease software. Redistricting data, on which the atlas was based, were released in March 2001 and the full-color atlas went to press in July 2001. It is the first decennial atlas produced by the Census Bureau in 80 years. Race maps in the atlas are designed to present the new Census 2000 data that include counts of people who indicated more than one race. Choropleth classifications are constructed to facilitate comparison among maps in series for all race/ethnicity groups, both large and small. Map series share breaks that are adjusted to include meaningful breaks based on overall U.S. percentages for individual groups. Additional shared meaningful breaks, such as “no change” and “50 percent of the population” also anchor classifications. Maps of prevalent groups and diversity provide syntheses of race/ethnicity data. Maps were evaluated at numerous levels in the Census Bureau, with issues from topic selection to page layout to title wording debated by many people. The essay describes the production processes, agency standards, and cartographic principles that come together to present these new data in an atlas that is a colorful showpiece.

KEYWORDS: Census 2000, atlas, map design, choropleth classification, population, diversity

On the Uncertainty of Local Shape of Lines and Surfaces

Bernhard Schneider

Modeling line or surface phenomena digitally involves two tasks: discretization of the phenomenon, which yields a finite set of data, and subsequent interpolation, which reconstructs the continuum. Many mathematical techniques exist for the latter task, and most methods require a number of parameters to be specified. The shape of digital line or surface models between the data points (that is, the local shape) and the information derived from these models both depend on the selected method and, possibly, on the specification of parameters. The reconstruction of the continuum thus introduces uncertainty. This paper examines the sources and effects of this type of uncertainty. For this purpose, the modeling of lines and surfaces is separated into an abstraction, an implementation, and measurement. The individual factors affecting uncertainty of local shape in each step are identified and discussed. The paper concludes that local shape uncertainty, unlike positional uncertainty of given data, cannot be numerically assessed. Instead, measures of plausibility have to be used to denote the quality of digital models of lines and surfaces. Finally, the concept and potential problems of future empirical investigations are discussed.

KEYWORDS: Spatial modeling; digital line and surface models; uncertainty of local shape of line and surface models; model quality; abstraction; interpolation; scale

Developing Web-based Mapping Applications Through Distributed Object Technology

Xuan Zhu

Emerging web-based mapping technologies use the World Wide Web (WWW) and Internet protocols to provide the ability to distribute, access, and visualize geospatial information over the Internet. Many web-based mapping applications have been developed to deliver geospatial information within and across organizations and even to the public at large. A major technological challenge is to achieve interoperability amongst web-based mapping applications so that mapping and geoprocessing resources distributed over the Internet can be shared and integrated. This paper presents an approach to the development of web-based mapping applications using distributed object technology in order to enable interoperability. Distributed object technology combines object technology, which utilizes reusable software components (called objects) that model real-world entities to build software systems, and distributed computing, which allows computing resources to be distributed and accessed over computer networks. The paper introduces a distributed object technology, the Common Object Request Broker Architecture (CORBA); proposes an architecture for web-based mapping using CORBA; and presents a prototype implementation.

KEYWORDS: Web-based mapping, the World Wide Web, distributed object technology, CORBA

Extracting High Points from Statistical Surfaces: A Case Study Using Digital Elevation Models

Elizabeth A. Wentz, Michael Kuby, and Brandon J. Vogt

We introduce and test an algorithm for extracting high-point locations from statistical surface data. The algorithm uses map algebra and local neighborhood analysis via three key parameters: minimum vertical gain, vertical gain neighborhood, and horizontal separation neighborhood. Though the method is applicable to any x,y,z data set, we tested it on 1:250,000 digital elevation models (DEMs) for Arizona. The resulting high points were compared quantitatively with an independent data set of named summits from the USGS Geographic Names Information System (GNIS). The comparison showed that, on an aggregate basis, the extraction method can approximate the number and spatial pattern of high points when compared to the GNIS points. However, extraction by neighborhood analysis may consistently misdiagnose certain features, such as the edges of troughs (e.g., canyon rims).

KEYWORDS: Statistical surface, digital elevation model (DEM), feature extraction, high point, summit