Sanjay Rana

Exploring multiple viewshed analysis using terrain features and optimisation techniques

The calculation of viewsheds is a routine operation in geographic information systems and is used in a wide range of applications. Many of these involve the siting of features, such as radio masts, which are part of a network and yet the selection of sites is normally done separately for each feature. The selection of a series of locations which collectively maximise the visual coverage of an area is a combinatorial problem and as such cannot be directly solved except for trivial cases. In this paper, two strategies for tackling this problem are explored. The first is to restrict the search to key topographic points in the landscape such as peaks, pits and passes. The second is to use heuristics which have been applied to other maximal coverage spatial problems such as location-allocation. The results show that the use of these two strategies results in a reduction of the computing time necessary by two orders of magnitude, but at the cost of a loss of 10% in the area viewed. Three different heuristics were used, of which Simulated Annealing produced the best results. However the improvement over a much simpler fast-descent swap heuristic was very slight, but at the cost of greatly increased running times.

The automatic definition and generation of axial lines and axial maps

Space syntax is a technique for measuring the relative accessibility of different locations in a spatial system which has been loosely partitioned into convex spaces. These spaces are approximated by straight lines, called axial lines, and the topological graph associated with their intersection is used to generate indices of distance, called integration, which are then used as proxies for accessibility. The most controversial problem in applying the technique involves the definition of these lines. There is no unique method for their generation; hence different users generate different sets of lines for the same application. In this paper we explore this problem, arguing that to make progress, there need to be unambiguous, agreed procedures for generating such maps. The methods we suggest for generating such lines depend on defining viewsheds, called isovists, which can be approximated by their maximum diameters, these lengths being used to form axial maps similar to those used in space syntax. We propose a generic algorithm for sorting isovists according to various measures, approximating them by their diameters and using the axial map as a summary of the extent to which isovists overlap (intersect) and are accessible to one another. We demonstrate our techniques for the small French town of Gassin used originally by Hillier and Hanson in a 1984 book to illustrate the theory, exploring different criteria for sorting isovists, and generating different axial maps by changing the level of resolution.

Frontiers of Geographic Information Technology

Geographic Information Technologies - Overview (Sanjay Rana and Jayant Sharma).- Soft Computing and Geographical Information Systems (Yingjie Yang, David Gillingwater and Chris Hinde).- Using Geospatial Information for Autonomous Systems Control (Ayanna Howard and Edward Tunstel).- Simulating Spatial Dynamics Using Cell & Agent-Based Urban Models (Mike Batty).- Distributed Geospatial Information Services (Chaowei (Phil) Yang and C. Vincent Tao).- Geospatial Grid (Liping Di).- Geospatial Semantic Web (Yaser Bishr).- The Role of DBMS in New Generation GIS Architecture (Sisi Zlatanova and Jantien Stoter).- Multimodal Interfaces for Representing and Accessing Geospatial Information (Reg Golledge, Matthew T. Rice and R. Daniel Jacobson).- Way Finding With Mobile Devices Decision Support (Sabine Timpf).- Augmented Reality Visualisation of Geospatial Data (Daniel Holweg and Ursula Kretschmer).- GeoICT and Development - The Inverted Pyramid Syndrome (Satya Prakash, Ayon Kumar Tarafdar and Ravi Gupta).- Privacy Issues in Geographic Information Technologies (Alastair Beresford).- Frontiers of Geographic Information Technology (Sanjay Rana and Jayant Sharma).

A framework for augmenting the visualization of dynamic raster surfaces

Animated sequences of raster images that represent continuously varying surfaces, such as a temporal series of an evolving landform or an attribute series of socio-economic variation, are often used in an attempt to gain insight from ordered sequences of raster spatial data. Despite their aesthetic appeal and condensed nature, such representations are limited in terms of their suitability for prompting ideas and offering insight due to their poor information delivery and the lack of the levels of interactivity that are required to support visualization. Cartographic techniques aim to assist users of geographic information through processes of abstraction, by selecting, simplifying, smoothing and exaggerating when representing an underlying spatial data set graphically. Here we suggest a number of transformations and abstractions that take advantage of these techniques in a specific context–that of addressing the limitations associated with using animated raster surfaces for visualization, and propose them in the context of a framework that can be used to inform practice. The five techniques proposed are spatial and attribute smoothing, temporal interpolation, transformation of the surfaces into a network of morphometric features, the use of a graphic lag or fading and the employment of techniques for conditional interactivity that are appropriate for visualization. These efforts allow us to generate graphical environments that support visualization when using animated sequences of images representing continuous surfaces and are analogous to traditional cartographic techniques, namely, smoothing and exaggeration, simplification, enhancement and the various issues of design. By developing a framework for considering cartography in support of visualization from this particular type of data and phenomenon we aim to highlight the utility of a generically cartographic approach to information visualization. A number of particular techniques originating from computer science and conventional cartography are used in an application of the framework. A suitably interactive software tool is offered for evaluation–to establish the results of applying the framework and demonstrate ways in which we may augment the visualization of dynamic raster surfaces through animation and more generally aim to offer opportunity for insight through cartographic design.

Use of Plan Curvature Variations for the Identification of Ridges and Channels on DEM

This paper proposes novel improvements in the traditional algorithms for the identification of ridge and channel (also called ravines) topographic features on raster digital elevation models (DEMs). The overall methodology consists of two main steps: (1) smoothing the DEM by applying a mean filter, and (2) detection of ridge and channel features as cells with positive and negative plan curvature respectively, along with a decline and incline in plan curvature away from the cell in direction orthogonal to the feature axis respectively. The paper demonstrates a simple approach to visualize the multi-scale structure of terrains and utilize it for semiautomated topographic feature identification. Despite its simplicity, the revised algorithm produced markedly superior outputs than a comparatively sophisticated feature extraction algorithm based on conic-section analysis of terrain.

Two approximate solutions to the Art Gallery Problem

In this proposal, a dense mesh of potential observers is placed in the open space that surrounds/immerses the obstacles in the gallery (Figure 1a) which basically discretises the open space. This aspect highlights the central assumption in this approach that the open space of the art gallery is covered with dense arrangement of observers. Each observer i v has a visual connectivity expressed as a set called isovist ai where ai = {vi,vj,...,vn}; 1≥i,j,...,n≤N, vj,...,vn are observers visible from vi and N is the set of all observers in the gallery. |ai| is referred as the rank of the observer. With this premise, the following algorithms are proposed to solve the art gallery problem.