Well-point techniques and the shallow water table in boulder clay

The reliability of water-table measurements in clay soil is currently under review (Twocock, 1971; Bonell, 1971; Visvalingam, 1972). This paper summarizes some of the experimental results from a boulder clay catchment in East Yorkshire. The experiments investigated the functioning characteristics of cased auger holes and piezometers in clay soil and compared the results with observations made with a neutron moisture probe. It appears that well-point technique, especially piezometers, are extremely unreliable in clay soil. The measured water level is demonstrated to be influenced by not only the position of the ‘water table’ but also the permeability of the soil; in which context the type, diameter, and length of tubing, as well as the time of installation, become important considerations

A computer science perspective on the bendsimplification algorithm

The primary aim of this study was to evaluate whether the use of bends provides a better basis than point elimination for research on line structuring. These investigations were undertaken using Arc/Info 7.1.1. Comparative experimental results suggest that the algorithm may not be as widely applicable as the much simpler geometric filters, such as the Douglas-Peucker or Visvalingam algorithms. The paper therefore provides a brief review of these three algorithms. A more detailed conceptual and empirical evaluation of the bendsimplification system follows, highlighting some problems with implementing the system in Arc/Info. The paper then questions the value of over-coupling model- and image-oriented generalization processes within the black-box bendsimplification system. It suggests the type of parameters which could enhance the utility and usability of the Bendsimplify option within the Arc/Info (and perhaps also within the ArcView) environment and provides some pointers for further research. With respect to the main aim of the research, the evidence suggests that bendsimplification is less useful for line segmentation than Visvalingam's algorithm. Further research is needed to assess the value of the iterative bend elimination operator within bendsimplification

Line generalisation by repeated elimination of points

This paper presents a new approach to line generalisation which uses the concept of ‘effective area’ for progressive simplification of a line by point elimination. Two coastlines are used to compare the performance of this, with that of the widely used Douglas-Peucker, algorithm. The results from the area-based algorithm compare favourably with manual generalisation of the same lines. It is capable of achieving both imperceptible minimal simplifications and caricatural generalisations. By careful selection of cutoff values, it is possible to use the same algorithm for scale-dependent and scale-independent generalisations. More importantly, it offers scope for modelling cartographic lines as consisting of features within features so that their geometric manipulation may be modified by application- and/or user-defined rules and weights. The paper examines the merits and limitations of the algorithm and the opportunities it offers for further research and progress in the field of line generalisation. © 1993 Maney Publishing

Indexing with coded deltas—a data compaction technique

The paper describes the coded delta scheme, which is one of the methods used by the Census Research Unit, University of Durham, for compacting the 1971 U.K. census data. It evaluates the merits and limitations of the technique in relation to the characteristics of the data set and other techniques available for compact encoding of numeric and string data

Cartographic Algorithms: Problems of Implementation and Evaluation and the Impact of Digitising Errors

Cartographic generalisation remains one of the outstanding challenges in digital cartography and Geographical Information Systems (GIS). It is generally assumed that computerisation will lead to the removal of spurious variability introduced by the subjective decisions of individual cartographers. This paper demonstrates through an in‐depth study of a line simplification algorithm that computerisation introduces its own sources of variability. The algorithm, referred to as the Douglas‐Peucker algorithm in cartographic literature, has been widely used in image processing, pattern recognition and GIS for some 20 years. An analysis of this algorithm and study of some implementations in wide use identify the presence of variability resulting from the subjective decisions of software implementors. Spurious variability in software complicates the processes of evaluation and comparison of alternative algorithms for cartographic tasks. No doubt, variability in implementation could be removed by rigorous study and specification of algorithms. Such future work must address the presence of digitising error in cartographic data. Our analysis suggests that it would be difficult to adapt the Douglas‐Peucker algorithm to cope with digitising error without altering the method. Copyright © 1991, Wiley Blackwell. All rights reserve

The Douglas-Peucker algorithm for line simplification: Re-evaluation through visualization

The primary aim of this paper is to illustrate the value of visualization in cartography and to indicate that tools for the generation and manipulation of realistic images are of limited value within this application. This paper demonstrates the value of visualization within one problem in cartography, namely the generalisation of lines. It reports on the evaluation of the Douglas-Peucker algorithm for line simplification. Visualization of the simplification process and of the results suggest that the mathematical measures of performance proposed by some other researchers are inappropriate, misleading and questionable

The neutron method for measuring soil moisture content - a review

THE various methods of measuring the soil moisture content and its variations in space and time have been reviewed by many authors (Taylor, 1955; Marshall, 1959; Todd, 1960; Ballard and Gardner, 1965; Cope and Trickett, 1965). The moisture content is either measured directly, as in the gravimetric method, or it is estimated by determining its relationship to some other property of the soil as in the electric resistance, tensiometer and neutron scattering methods. The neutron scattering method estimates the moisture content of the soil by measurement of its hydrogen content. This paper summarizes the theoretical and practical aspects of the method and provides a bibliography which includes references to papers published more recently than those provided by Sweeny (1962), Ballard and Gardner (1965), and the Commonwealth Bureau of Soils (1968)