CaGIS vol. 31, no. 4

A New Method for the Specification of Geographic Footprints in Digital Gazetteers

John P. Wilson, Christine S. Lam, and Deborah A. Holmes-Wong

This paper presents the strategy used to add neighborhood names and footprints to the Los Angeles Digital Gazetteer. The gazetteer database currently contains 4,500 features and is needed to: (1) facilitate the specification of geographic footprints in the Qualified Dublin Core metadata records that are used to describe digital assets; and (2) support the search for and retrieval of selected objects based on location, time, format, and/or keyword. The role of the digital gazetteer and a new browser which will offer the library patron a web-based query form with an interactive map is explained. The interface can be used to draw a query on a map, and it provides a series of pull down menus that can be used to specify time periods, formats, collections, and key words of interest. A new method for specifying neighborhood footprints in the digital gazetteer is described in some detail, and opportunities are highlighted for generalizing the method to help with search and retrieval using the map browser.

Keywords: Geolibraries, geographic footprints, maps, gazetteers, interface design, search and retrieval, digital libraries

The Discontinuous Nature of Kriging Interpolation for Digital Terrain Modeling

Thomas H. Meyer

Kriging is a widely employed method for interpolating and estimating elevations from digital elevation data. Its place of prominence is due to its elegant theoretical foundation and its convenient practical implementation. From an interpolation point of view, kriging is equivalent to a thin-plate spline and is one species among the many in the genus of weighted inverse distance methods, albeit with attractive properties. However, from a statistical point of view, kriging is a best linear unbiased estimator and, consequently, has a place of distinction among all spatial estimators because any other linear estimator that performs as well as kriging (in the least squares sense) must be equivalent to kriging, assuming that the parameters of the semivariogram are known. Therefore, kriging is often held to be the gold standard of digital terrain model elevation estimation. However, I prove that, when used with local support, kriging creates discontinuous digital terrain models, which is to say, surfaces with “rips” and “tears” throughout them. This result is general; it is true for ordinary kriging, kriging with a trend, and other forms. A U.S. Geological Survey (USGS) digital elevation model was analyzed to characterize the distribution of the discontinuities. I show that the magnitude of the discontinuity does not depend on surface gradient but is strongly dependent on the size of the kriging neighborhood.

Keywords: Kriging, digital terrain model, discontinuity

Frequency Histogram Legend in the Choropleth Map: A Substitute to Traditional Legends

Naresh Kumar

This article presents the use of the frequency histogram legend (FHL) as a substitute to traditional legends in both classed and unclassed choropleth maps. Great variation in the size of mapping units can hinder readers’ ability to comprehend statistical distributions from a choropleth map. Replacing conventional legends with FHL can aid readers in their understanding of spatial as well as statistical distributions of the mapped data simultaneously. A customized mapping application was designed in ArcInfo 9.0 to test the use of FHL in both classed and unclassed choropleth maps. Frequency histogram legends were tested on different types of statistical distributions. Although the comparison of the results shows that the FHL works best for a Gaussian or close to a Gaussian distribution for eight or fewer classes, the customized application permits users to generate choropleth maps with frequency histogram legends for any type of statistical distribution with any number of classes. The analysis reveals that readers’ background in statistics helped them to effectively utilize and interpret frequency histogram legends in the choropleth maps.

Keyword: Frequency histogram legends, choropleth map, Gaussian distribution, classed map, unclassed maps

The Distance–Similarity Metaphor in Network-Display Spatializations

Sara Irina Fabrikant, Daniel R. Montello, Marco Ruocco, and Richard S. Middleton

Dimensionality reduction algorithms are applied in the field of information visualization to generate low-dimensional, visuo-spatial displays of complex, multivariate databases—spatializations. Most popular dimensionality reduction algorithms project relatedness in data content among entities in an information space (e.g., semantic similarity) onto some form of distance among the entities, such that semantically similar documents are placed closer to one another than less similar ones. In previous studies of point-display spatializations we have shown that people indeed associate metric straight-line inter-point distances with the semantic dissimilarity of documents depicted as points in two-dimensional space. In this paper we investigate the strategies viewers employ when conflicting notions of distance (straight-line metric vs. network metric vs. topological proximity) are jointly shown in a spatialized network display of Reuters news articles depicted as points connected by links. We report empirical results of an experiment where viewers are asked to assess document similarity, depending on various distance types. We also investigate how cartographic symbolization principles (the use of visual variables, such as size, color hue, and value) influence similarity judgments. These findings provide rare empirical evidence for generally accepted design practices within the cartographic community (e.g., the effects of visual variables). In addition, empirical results from this and related studies can be used to develop design guidelines for constructing cognitively adequate spatializations for knowledge discovery in very large databases. We conclude by presenting design guidelines for network spatializations within the context of cartographic practice and theory.

Keywords: Visualization, spatialization, perception, network, distance, similarity, visual variables.