CaGIS, Vol. 27, No. 2

Equal Area Map Projection for Irregularly Shaped Objects

Yang Cheng and Jean J. Lorre

Approximately half of the planetary bodies in our solar system imaged by spacecraft have irregular shapes. Since maps are used to record, interpret and display these irregularly shaped bodies, a special map projection, which can display them faithfully, is desirable. Unfortunately, no mathematical approach that permits true conformal or equal-area projections has been developed. In this paper, a novel approach that permits true conformal or equal-area projection has been developed. In this paper, a novel approach to construct a special equal area map projection for irregularly shaped objects is suggested. Using this innovative approach, equal area map projections for two Martian satellites-Phobos and Deimos-are developed.

Automated Name Placement With High Cartographic Quality: City Street Maps

Francois Chirie

As maps are more and more frequently produced from geographic databases using mapmaking software there is an increasing need for automated cartography. In particular, much research into automated name placement has been done, yielding systems that produce useful results but do not rival classical cartography. An automated street name placement program is proposed in this paper, which yields cartographic quality comparable to that obtained by traditional cartographers. We improved the modeling of cartographic rules in quantity and quality by addressing all the rules about street names, and analyzing each rule in detail. The program implements a mathematical model of these cartographic rules, and uses procedural programming. The results show that one type of traditional paper map can be produced using automated name placement.

KEYWORDS: Automated name placement, traditional cartography, high cartographic quality, modeling of cartographic rules, city street map.

Reducing Linear and Perimeter Measurement Errors in Raster-based Data

David M. Theobald

An important issue in cartography and GIS is determining the appropriate resolution or cell size when converting vector data to raster. The general consensus is to make the cell size as small as possible to resolve geographic features and provide the most accurate estimates of measurements. Finer resolution results in more accurate estimates of polygon area; however, the raster data structure introduces an artifact that causes errors in the estimation of the length of linear features and of the perimeter of polygon features to increase with increasing resolution. Over-estimation as high as 41 percent is theoretically possible and was found to be around 26 percent for representative polygon maps. A method is described that uses a correction coefficient to reduce overestimation error to less than 3 percent.

Spatial Accuracy Assessment of Wetland Permit Data

Nina M. Kelly

A collection of permits granted for alterations to coastal wetlands in North Carolina from 1984 through 1992 was examined to determine the spatial accuracy of the data. Each permit site for which a precise location existed in its associated permit file was surveyed using a Global Positioning System, and the error was identified between the location described in the permit file and true location. The error was analyzed with respect to direction of error, accompanying map type, and time. Results suggest that the spatial error found in the Permit Record for coastal North Carolina was too large to perform spatial analysis. Only 50 percent of the permit sites were found within 250 meters of their true location, and the rest were in error by as much as 45 kilometers. Errors were uniformly distributed in direction and not biased in any direction. The inclusion of maps with greater detail did not significantly reduce error in locating the permit site. There was a slight decrease in error over time, but the fit was not sufficiently strong to indicate an improvement in accuracy over time. The results suggest a need for better standards for gathering future data and call for more stringent spatial data quality controls on environmental permit data of this kind.

Medial Axis Generalization of River Networks

Michael McAllister and Jack Snoeyink

We examine some benefits of using the medial axis as a centerline for rivers and lakes. One benefit, automatic centerline generation, has been used for many years. We show that additional benefits can be derived from the geometric relationships between the medial axis and the riverbanks or lakeshores. These include area estimates, association of centerline analysis to banks, and definition of opposite for riverbanks. We also report on our experience at approximating the medial axis with a Voronoi diagram of point sites.

Institutionalization of Geographic Information Technologies: Unifying Concepts?

1. H. Erik de Man

This paper studies the role, impact and effectiveness of geographic information technologies such as GIS, Contemporary frameworks

such as economic evaluation, organization context, or diffusion -- deal with some aspects only. Institutionalization of geographic information technologies is proposed as a paradigm for studying the impact and effectiveness more comprehensively. It refers to the ongoing process within a group or society whereby this technology itself is becoming Institutionalized and gaining a strong (normative) impact on common perceptions of spatial problems and, subsequently, on collective actions to remedy these problems. Hence, it is a paradigm that encompasses value, as well as organizational and societal issues, and links these to the level of individual behavior patterns. The paradigm therefore may provide context for economic evaluation, organizational considerations, and diffusion, as well as for other interpretive perspectives. The paper outlines the concept of institutionalization of geographic information technologies and some of its salient factors and conditions. The focus is on feedback and participatory approaches in the design, and choice and implementation of geographic information technologies; whether these are institutionalized or not. Further (empirical) research is needed to explore the practical usefulness of this concept.

KEYWORDS: GIS, geographic information technologies, effectiveness, economic evaluation, organizational context, diffusion, institutionalization, paradigm, common problem perception, society.

Modeling Dynamic Polygon Objects in Space and Time: A New Graph-based Technique

David J. Wilcox, Matthew C. Harwell, and Robert J. Orth

The analysis of dynamic spatial systems requires an explicit spatio-temporal data model and spatio-temporal analysis tools. Event-based models have been developed to analyze discrete change in continuous and feature-based spatial data. In this paper, a spatio-temporal graph model is described that supports the analysis of continuous change in feature-based polygon spatial data. The spatio-temporal graph edges, called temporal links, track changes in polygon topology through space and time. The model also introduces the concept of a spatial-interaction region that extends a model’s focus beyond short-term local events to encompass long term regional events. The structure of the spatio-temporal graph is used to classify these events into five types of local polygon events and two types of spatial-interaction region events. To illustrate its utility, the model is applied to the ecological question of how patch size influences longevity in underwater plant communities in Chesapeake Bay, USA. Both a short local analysis and a longer-term regional analysis showed that patches of plants, or groups of patches, larger than one or two hectares in size were more likely to persist than smaller patches or groups of patches. Overall, the spatio-temporal graph model approach appears applicable to a variety of spatio-temporal questions.

KEYWORDS: Spatio-temporal analysis, temporal GIS, spatial-interaction region, spatio-temporal graph model, patch dynamics, submersed aquatic vegetation.

Visual and Statistical Comparisons of Surface Modeling Techniques for Point-based Environmental Data

Xiaojun Yang and Thomas Hodler

Existing studies on spatial interpolation tend to overplay statistical perspective, paying little attention the locality and the visual performance of generated surface models. In an attempt to bridge these gaps in literatures, the authors compares the performance of five surface modeling methods, using a set of integrative criteria including absolute and relative statistical accuracy, visual pleasantness and faithfulness of generated surface models, sensitivity to changing sample size and search conditions, and computational intensity. The modeling methods used were: inverse distance, kriging, linear triangulation, minimum curvature, and radial basis functions. Because terrain relief is one of the few environmental attributes whose continuous surfaces can be directly observed through appropriate procedures, we used as input data two sets of elevation points sampled irregularly from a USGS 1:24,000 topographical map covering a hilly area. We found that surface modeling methods, even if statistically accurate, may not always ensure a graphically faithful representation of reality. The surprising result of this study was that the surface models generated from a larger were less stati stically accurate than those generated from a smaller sample.

KEYWORDS: Scattered data, surface modeling, relative accuracy, visual faithfulness, sensitivity.

A Visual Basic Algorithm for the Winkel Tripel Projection

Fritz C. Kessler

National Geographic Society (NGS) has made several changes throughout the years in their choice of map projection for their world reference maps. The van der Grinten I map projection was used from 1922 to 1988. Then, in 1988, it was replaced by the Robinson projection. Beginning in 1998, the Winkel Tripel became the map projection of choice for NGS’ world maps. Given this change, cartographers and others who make maps may be interested in using the Winkel Tripel for custom applications. The goal of this paper is to show how Winkel Tripel’s complex projection equations can be programmed using Visual Basic. Those who use other languages such as C++ can use this programming example to help them create a similar algorithm in the language of choice.

KEYWORDS: Map projection, Winkel Tripel, algorithm.