The calculation of viewsheds is a routine operation in GIS 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 ten percent 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.

1 Introduction: the problem Space syntax provides a method for partitioning a spatial system into relatively independent but connected subspaces so that the importance of these subspaces can be measured in terms of their relative nearness or accessibility (Hillier and Hanson, 1984). It is similar to a wide class of models for measuring spatial interaction, developed over the last fifty years as part of social physics, which derive relative accessibility from the underlying graphtheoretic structure of relations, usually based on the Euclidean distances between small areas (Wilson, 1998). It differs from this class, however, in three significant ways. First, the subspaces or small areas which compose the basic representational elements in space syntax are ill defined. The spatial elements used are not directly observable and measurable and, although they depend upon the geometric properties of the space, there is no agreed or unique method for their definition. Second, spaces are not collapsed to nodes or points but are first defined by lines which are then considered as nodes. Third, the relations between these components or nodes are defined in terms of their topology and, although Euclidean distance is implicit, relations are measured in binary termsöwhether they exist or not.In this paper we will focus entirely on the first problem which involves defining the spatial components used in subsequent relational analysis. We will introduce methods which resolve the problem of deriving a unique set of elements, thus enabling their automatic definition. These methods extend quite naturally to the second and third problems in that representing lines as nodes is no longer necessary. The method we introduce suggests that the relative importance of lines associated with subspaces is

BackgroundSnow’s work on the Broad Street map is widely known as a pioneering example of spatial epidemiology. It lacks, however, two significant attributes required in contemporary analyses of disease incidence: population at risk and the progression of the epidemic over time. Despite this has been repeatedly suggested in the literature, no systematic investigation of these two aspects was previously carried out. Using a series of historical documents, this study constructs own data to revisit Snow’s study to examine the mortality rate at each street location and the space-time pattern of the cholera outbreak.MethodsThis study brings together records from a series of historical documents, and prepares own data on the estimated number of residents at each house location as well as the space-time data of the victims, and these are processed in GIS to facilitate the spatial-temporal analysis. Mortality rates and the space-time pattern in the victims’ records are explored using Kernel Density Estimation and network-based Scan Statistic, a recently developed method that detects significant concentrations of records such as the date and place of victims with respect to their distance from others along the street network. The results are visualised in a map form using a GIS platform.ResultsData on mortality rates and space-time distribution of the victims were collected from various sources and were successfully merged and digitised, thus allowing the production of new map outputs and new interpretation of the 1854 cholera outbreak in London, covering more cases than Snow’s original report and also adding new insights into their space-time distribution. They confirmed that areas in the immediate vicinity of the Broad Street pump indeed suffered from excessively high mortality rates, which has been suspected for the past 160 years but remained unconfirmed. No distinctive pattern was found in the space-time distribution of victims’ locations.ConclusionsThe high mortality rates identified around the Broad Street pump are consistent with Snow’s theory about cholera being transmitted through contaminated water. The absence of a clear space-time pattern also indicates the water-bourne, rather than the then popular belief of air bourne, nature of cholera.The GIS data constructed in this study has an academic value and would cater for further research on Snow’s map.

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Live specimens of benthic foraminiferal species Rosalina leei were subjected to a combination of temperature (25°C, 30°C and 35°C) and salinity (25‰, 30‰ and 35‰) to assess its differential response to the annual range of seawater temperature and salinity reported at the sampling site. A total of 216 specimens were used for the experiment. Within the range of temperature and salinity, to which R. leei specimens were subjected as part of the present experiment, growth increased with increasing salinity, whereas increase in seawater temperature resulted in retarded growth. Maximum growth was reported in the specimens kept at 25°C temperature and 35‰ salinity while the rest of the specimens maintained in 30‰ and 25‰ saline water, showed comparatively less growth. The specimens kept at 30°C and 35°C temperature and different salinities showed much less growth as compared to the specimens maintained at 25°C temperature. However, none of the R. leei specimens subjected to the present experiment reproduced during the course of the experiment. The absence of reproduction under the present set of temperature and salinity conditions, probably indicates that R. leei reproduces at a very narrow range of temperature and salinity which is different from the temperature and salinity conditions in the present experiment. It is further inferred that under the present set of temperature–salinity conditions, 25°C temperature and 35‰ saline water is most suitable for the growth of R. leei. Results are significant as the responses of benthic foraminifera to different temperatures and salinity are being used for palaeoclimatic reconstruction.