Mitchel Langford

Generating and mapping population density surfaces within a geographical information system

Some alternative ways of presenting maps of spatial population distribution are examined using northern Leicestershire, England, as an example. The authors conclude that dasymetric and isarithmic maps are a desirable alternative and can be relatively easily generated using satellite imagery in a geographical information system.

Modelling the errors in areal interpolation between zonal systems by Monte Carlo simulation

Areal interpolation involves the transfer of data (often socioeconomic statistics and especially population data) from one zonation of a region to another, where the two zonations are geographically incompatible. This process is inevitably imprecise and is subject to a number of possible errors depending on the assumptions inherent in the methods used. Previous analysts have had only limited information with which to compare the results of interpolation and so assess the errors. In this paper a Monte Carlo simulation method based on modifiable areal units is employed. This allows multiple interpolations of population to be conducted from a single set of source zones to numerous sets of target zones. The properties of the full error distribution associated with a particular interpolation model can then be examined. The method based on dasymetric mapping consistently gave the highest accuracy of those tested, whereas the areal weighting method gave the lowest. More important than the results presented is the potential for future testing of other methods in increasingly complex situations.

Measuring Potential Access to Primary Healthcare Services: The Influence of Alternative Spatial Representations of Population

Abstract Spatial accessibility measures are an important policy tool for managing healthcare provision and reducing health inequality. The two-step floating catchment area technique, in common with many alternative methodologies, requires that demand-side population be estimated using spatial interpolation techniques. This article studies the implications of adopting differing spatial representations of population on healthcare accessibility modeling outcomes. Results indicate that a dasymetric model yields lower accessibility scores than a standard pro rata model. More important, the difference is spatially disproportionate, suggesting that the degree of disadvantage experienced in rural areas may be greater than has previously been recognized.

Obtaining population estimates in non-census reporting zones: An evaluation of the 3-class dasymetric method

Interpolating population data between incompatible spatial zones is an important task in many GIS applications. This paper investigates whether regional regression models between population and land cover outperform a global approach, and whether the 3-class dasymetric method improves upon the binary dasymetric approach. In the experiments conducted, regional regressions resulted in better areal interpolation, but also highlighted spatial non-stationarity in the relationship between population and land cover. The benefits of a 3-class dasymetric model over a binary model were inconclusive. However, it is suggested that greater flexibility in model calibration to more fully incorporate spatial non-stationarity could improve 3-class dasymetric performance. Accurate urban residential density mapping is also important since the 3-class dasymetric method seems less robust than the binary approach to land cover classification error.

Rapid facilitation of dasymetric-based population interpolation by means of raster pixel maps

Areal interpolation between one partitioning of geographical space and another remains an important topic, particular in terms of population counts and related statistics which are often required in order to compute an incidence ratio. Despite numerous recent developments in intelligent areal interpolation methods, and studies that have demonstrated their clear advantage over simple areal weighting, there is little evidence to suggest widespread usage amongst the GIS user community. It is argued that to encourage greater uptake such methods must offer simplicity and convenience. Areal interpolation based on binary dasymetric mapping is conceptually simple, but examples to date tend to use information extracted from multi-spectral satellite imagery which limits its perceived convenience. This paper examines a simple method to extract equivalent information from a raster pixel map. It is shown to offer comparable areal interpolation performance at considerably less cost in terms of both time and complexity.

Refining Population Surface Models: Experiments with Northern Ireland Census Data

This paper compares and contrasts alternative methods for the construction of discontinuous population surface models based on the census and remotely sensed data from Northern Ireland. Two main methods of population distribution are employed: (1) a method based on redistribution from enumeration district (ED) and postcode centroids, and (2) a method based on dasymetric redistribution of ED population counts to suitable land cover zones from classified remotely sensed imagery. Refinements have been made to the centroid redistribution algorithm to accommodate an empirical measure of dispersion, and to allow redistribution in an anisotropic form. These refinements are evaluated against each other and the dasymetric method. The results suggest that all of the methods perform best in urban areas, and that while the refinements may improve the statistical performance of the models, this is at the expense of reduced spatial detail. In general, the techniques are highly sensitive to the spatial and population resolution of the input data.

Urban population distribution models and service accessibility estimation

This paper examines the influence of alternative population distribution models on GIS-based spatial accessibility analyses using the two-step Floating Catchment Analysis technique. Two population models were tested: the de facto standard of even-distribution within census tracts and a dasymetric-based approach. The latter builds on previous research through the use of a novel methodology that integrates raster map data with a recently built mailing information database in order to enhance the precision with which residential areas are identified. Analysis was conducted for a case study area (Cardiff, South Wales) in order to examine variations in accessibility to a number of public services in the city. The dasymetric model showed a general tendency to report lower accessibility scores, but detailed patterns depended on local factors and, to some extent, on modelling assumptions and methodological issues. A paired T-Test analysis demonstrated that significant differences in outcomes were dependant on the population model adopted. Accessibility-based measures are increasingly being incorporated into deprivation indicators and the paper concludes by suggesting that, if such analysis is to inform urban planning, local service provision and the spatial allocation of financial resources, greater attention needs to be given to the method of population representation utilised in such models.

Measuring transit system accessibility using a modified two-step floating catchment technique

Previous research has drawn attention to the importance of measuring accessibility to public transit services for transport planning and decision-making purposes and to the use of GIS to produce accessibility maps. Existing measures have been criticised for their lack of sophistication and reliance on simple operations such as Euclidean buffering. This article introduces an accessibility measure based on enhanced ‘floating catchment’ techniques, which have to date not been applied to measure access to public transport opportunities. We demonstrate that, after essential modifications to reflect the characteristics of transit service provision, this approach captures many detailed aspects of accessibility including proximity, the balance between service supply and demand, cumulative opportunity and temporal availability. We illustrate its application using a case study based in South Wales, integrating publicly available digital transport timetables and information on bus stop locations with geospatial analysis tools to provide a realistic appraisal of bus transit accessibility. Such measures could clearly form the basis of future studies of social exclusion and access to public transport opportunities. We conclude by drawing attention to the potential for including more intelligent demand-side and supply-side modelling inputs and by reiterating the policy importance of developing more accurate evaluations of the current and future state of the public transport network.

Investigating the implications of using alternative GIS-based techniques to measure accessibility to green space

A large body of research has examined relationships between accessibility to green space and a variety of health outcomes with many researchers finding benefits in terms of levels of physical activity and relationships with levels of obesity, mental health, and other health conditions. Such studies often use spatial analytical techniques to examine relationships between distance to such spaces and health data collated at an individual survey respondents home address or, more commonly, derived from area-based census measures summarised at a centroid. Generally, such measures are becoming more sophisticated and have moved on from the use of straightforward Euclidean-based measures to those based on network distance. However, few studies tend to use a combination of approaches or seek to establish the implications of incorporating alternative measures of accessibility on potential relationships. Using a database of green spaces (and associated attributes) and a detailed network dataset for the city of Cardiff, Wales, we examine the sensitivity of findings to the ways in which different metrics are calculated. This is illustrated by examining the variations in association between such metrics and a census-based deprivation index widely used in health studies to measure socioeconomic conditions. Our findings demonstrate that not only will the distances to green spaces vary according to the methodologies adopted but that any study that aims to investigate relationships with attributes of the nearest green space should acknowledge that matches may vary widely according to the techniques used. We conclude by warning against the use of inappropriate methodologies in examining access to green space which may directly influence directions (and levels) of association and hence may limit their relevance in wider geographical contexts.

Computerized identification of pollen grains by texture analysis

Abstract Automation of palynology could lead to many advances: rapid results, larger data sets, objectivity, fine resolution sampling and possibly finer determinations. To test the feasibility of automation, SEM photographs of six modern pollen taxa were used. The images were digitised and samples of exine texture covering approximately 1 10 of the total pollen area were extracted from the digital images. Texture analysis was applied to 192 samples obtained in this way. First, a co-occurrence matrix of grey levels was established for each sample. Then texture measures were calculated and used as input to a classification programme. With a leave-one-out strategy and a variable selection procedure, the proportion of pollen grains correctly identified rose to 94.3%. The procedure required c.10 seconds of processing on a VAX computer for each grain. With faster computers and programs, this could be cut to 1 second.