Spatio-temporal GIS Design for Exploring Interactions of Human Activities
Hongbo Yu
An effective representation of human activities in geographic information systems (GIS) presents a challenging research topic. Recent developments of information and communication technologies (ICT), which enable a virtual space and allow people to interact with others remotely, make such efforts even more difficult. This paper first extends the space–time path concept of Hägerstrand’s time geography to represent both physical and virtual activities in a space–time context. A spatio-temporal GIS design which can accommodate the extended space–time path concept is proposed to support the exploration of spatio-temporal patterns of human interactions in physical and virtual spaces. Using time as a linear referencing system, a temporal dynamic segmentation method was developed to dynamically locate physical and virtual activities on space–time paths. The GIS design supports the identification of four different spatio-temporal patterns (i.e., co-existence, co-location in space, co-location in time, and no co-location in either space or time) of human interactions through their space–time paths. Using a hypothetical activity dataset, a prototype system is implemented as a three-dimensional GIS (i.e., two-dimensional space + one-dimensional time) in ArcGIS. The prototype system demonstrates the feasibility and potential of applying spatio-temporal GIS concepts to extend Hagerstrand’s time–geographic framework for the representation and analysis of human interactions in both physical and virtual spaces. The proposed GIS design can be useful in such applications as understanding traffic congestion, tracking terrorist activities, and modeling the spread of infectious diseases. Keywords: Human activities and interactions, spatio-temporal GIS, temporal dynamic segmentation, spatio-temporal relationships

A Uniform Sky Illumination Model to Enhance Shading of Terrain and Urban Areas
Patrick J. Kennelly and A. James Stewart

Users of geographic information systems (GIS) usually render terrain using a point light source defined by an illumination vector. A terrain shaded from a single point provides good perceptual cues to surface orientation. This type of hill shading, however, does not include any visual cues to the relative height of surface elements. We propose shading the terrain under uniform diffuse illumination, where light arrives equally from all directions of a theoretical sky surrounding the terrain. Surface elements at lower elevations tend to have more of the sky obscured from view and are thus shaded darker. This tinting approach has the advantage that it provides more detailed renderings than point source illumination. We describe two techniques of computing terrain shading under uniform diffuse illumination. One technique uses a GIS–based hill-shading and shadowing tool to combine many point source renderings into one approximating the terrain under uniform diffuse illumination. The second technique uses a C++ computer algorithm for computing the inclination to the horizon in all azimuth directions at all points of the terrain. These virtual horizons are used to map sky brightness to the rendering of the terrain. To evaluate our techniques, we use two Digital Elevation Models (DEMs)—of the Schell Creek Range of eastern Nevada and a portion of downtown Houston, Texas, developed from Light Detection and Ranging (lidar) data. Renderings based on the uniform diffuse illumination model show more detailed changes in shading than renderings based on a point source illumination model. Keywords:  Illumination model, point source illumination, uniform diffuse illumination, shading, shadowing

Conflict Removal between B-spline Curves for Isobathymetric Line Generalization Using a Snake Model
E. Guilbert, E. Saux, and M. Daniel

This paper describes a method for suppressing distance conflicts between isobathymetric lines modeled with B-spline curves for cartographic generalization. The initial B-spline curves are obtained by compressing a set of polygonal lines representing the raw data. The conflict removal is realized by respecting strong cartographic constraints, such as the navigation safety and the legibility of the map, and weak constraints, such as the preservation of the submarine relief. A polygonal line representing the minimal displacement that corrects a conflict is first computed. Then, the curve is deformed using a snake model. The constraints are expressed via different energy terms, and the curve is deformed until a valid solution is reached. In order to apply the method without user intervention, the shape parameters of the snake are set automatically. The advantage of the method is that it ensures the respect of strong constraints in every case while taking into account the weaker ones. The method can also be applied to the correction of visual self-intersections. The use of a snake model combined with B-spline curves permits a better control of the deformation than polygonal lines, better preserving of the shape of the curves. Examples of real case studies issued from bathymetric databases are provided, and the benefits and drawbacks of the method are discussed.

Placing Text and Icon Labels Simultaneously: A Real-Time Method
Qingnian Zhang and Lars Harrie

This paper presents a method of combining text and icon label placement in a real-time computing environment. The method computes label configurations based on placement properties, cartographic disturbance, and label overlap. The process is divided into four phases. In the first phase, candidate positions of the text labels are chosen. In the second phase, the same is done for the icon labels. The choice of candidate positions is based on cartographic preference and cartographic disturbance. The removal of overlap between labels is solved, in the third phase, by means of a combinatorial optimization technique (simulated annealing). When there are label pairs in conflict that could not be resolved, the fourth and final step is executed to remove one label in the pair. The success of the proposed method lies in the ability to effectively reduce the search space for the combinatorial optimization. A number of strategies for reducing search space have been evaluated in a case study. The results show that a good search-space-reduction strategy will lead to acceptable solutions for text and icon labeling within a limited processing time. Keywords:  Label placement, map labeling, real-time map, combinatorial optimization, simulated annealing

Shuttle Radar Topography Mission Elevation Data Error and Its Relationship to Land Cover

Ashton Shortridge

The Shuttle Radar Topography Mission (SRTM) has resulted in the construction of the first publicly available near-global high resolution digital elevation model (DEM). The utility of this DEM, as for any geospatial data set, is a function of its quality. This paper is concerned with the assessment of SRTM accuracy and its relationship to land cover. Two methods—one raster-based and one point-based—are compared to match “finished” three-arc-second SRTM data to high precision, high accuracy surveyed elevations, as well as a corresponding DEM from the USGS National Elevation Dataset (NED). Differences between the two methodologies were not found to be significant. Error for the study site is substantially less than the mission objective, but substantially more than that for the NED. Significant overestimation of actual elevations pervades the SRTM DEM, and the overestimation is significantly higher in forested areas. This systematic error has implications both for applications employing SRTM data and for research on elevation data error modeling.

Evaluating Decision Support Systems for PPGIS Applications
Graeme Aggett and Chris McColl

A number of decision support systems (DSS) were evaluated prior to commencing public participatory geographic information systems (PPGIS) workshops within a small rural town undergoing rapid development. A paucity of existing information regarding the capabilities of these tools and their utility in supporting various PPGIS applications was noted. In order to improve this situation and to better understand the strengths and limitations of the selected DSS CommunityViz, we monitored and measured its utility in supporting community-based spatial decision-making processes from both a technologist and citizen’s perspective. Results indicate that CommunityViz has several strengths; however, the software exhibits some weaknesses in the context of our study. In assessing these results we realized that the utility of such findings could be much enhanced if contained within a structure that enables comparison between various DSS. Thus, a major objective was to develop and apply a DSS Evaluation Matrix (DSSEM) that can be used by PPGIS practitioners and participants to evaluate DSS in various project settings. The application of DSSEM will help formalize evaluations of DSS and encourage dissemination of results. Furthermore, it will assist practitioners, citizens, and agencies to make better choices when selecting DSS for PPGIS applications and provide DSS developers with the appropriate information to improve tool design. Keywords: Public participation, decision support, evaluation matrix, community support