Integrating multi-criteria evaluation with geographical information systems

Abstract

Geographical information systems (GIS) provide the decision-maker with a powerful set of tools for the manipulation and analysis of spatial information. The functionality of GIS is, however, limited to certain deterministic analyses in key application areas such as spatial search. The integration of multi-criteria evaluation (MCE) techniques with GIS is forwarded as providing the user with the means to evaluate various alternatives on the basis of multiple and conflicting criteria and objectives. An example application based on the search for suitable sites for the disposal of radioactive waste in the UK using the Arc/Info GIS is included. The potential use of a combined GIS-MCE approach in the development of spatial decision support systems is considered.     

Integrating GIS and spatial data analysis: problems and possibilities

Abstract

This article is an agreed summary of a workshop held in Sheffield between 18-20 March 1991. The focus here is on three of the themes of the workshop: the mutual benefits of closer links between geographical information systems (GIS) and the methods of spatial data analysis (SDA); the specific areas of SDA that should be linked with GIS; how the linkage should be made in practice. Directions for future research are also reviewed. The emphasis throughout is on statistical SDA and principally from the perspective of human rather than physical geography.     

Æolian dust deposition rates, particle-sizes and contributions to soils along a transect in semi-arid New South Wales, Australia

The entrainment, transport and deposition of æolian dust are important processes affecting soil development at the margins of deserts. To assess accurately the impact of deposited æolian dust on soil development, it is important not only to measure total dust deposition rates, but to distinguish additions of remotely and regionally sourced dust from locally derived material as well (which should not be viewed as new soil material as it is derived from æolian re‐mobilization of an existing soil). Because of the well‐established relationship between dust particle‐size and distance travelled from source, the particle‐size distribution of deposited dusts can be used to identify the distance to probable source regions, in addition to identifying matching topsoil particle populations. Three dust traps were located along a transect of semi‐arid south‐eastern Australia, to measure æolian dust deposition rates between late 2000 and late 2001. The particle‐size distributions of selected dust deposits were measured and, with the aid of meteorological data, probable dust source regions determined. Particle‐size distributions of the topsoils at each dust trap location were also measured to determine if any dust and soil particle populations matched. Although the sampling period was relatively short, and there was relatively little dust storm activity in this part of Australia during this time, a clear pattern of diminishing total dust deposition was measured downwind along the south‐east dust path. Dust deposition rates were also moderate to low by global standards. Source regions of deposited dust were interpreted as being: (i) a combination of south‐western and local sources; (ii) a combination of north‐western and local sources; and (iii) a mixed source where dust was rained‐out. The relative importance of these sources was correlated strongly with seasonal weather conditions, although the proportion of local dust in each deposit was greater at the arid (western) end of the transect. The effects of deposited dust on soil profile development are greatest at the western and central transect locations. At the western location, a fine‐grained particle population distinctive of the south‐western and north‐western regional dusts and a coarse silt‐sized particle population characteristic of local dusts, are also present in the topsoil, altering the texture of this Arenosol derived from dune sand. At the central location, where a Calcisol has formed in lacustrine lunette sediment, local dust of the same size as the dominant silt population of the topsoil continues to be deposited, while minor topsoil populations of very fine‐grained silt match regional and long‐distance dusts deposited at the site. Deposited dust appears to have been a less important soil development factor at the more humid eastern site because of the much smaller rates of dust deposition there. Nevertheless, a minor population of very fine silt particles in the Vertisol topsoil matches long‐distance dusts deposited at the site, suggesting a small input of dust to this alluvial soil. The particle‐size methodology used here has applications in other studies of dust contributions to peri‐desert soils. In particular, particle‐size distribution ‘smoothness’ may provide an indication of whether soils have simple or complex origins.     

Burrows and trails produced by Quinqueloculina impressa Reuss, a benthic foraminifer, in fine-grained sediment

The benthic foraminifer Quinqueloculina impressa Reuss, was buried in various types of sediment in order to assess its capability for producing sediment disturbances and thus, ichnofossils. Silts and silty-clays showed distinct burrows; fine sands did not. Two types of burrows were produced: fairly straight, vertical burrows from 4 cm below the water-sediment interface to within 1 cm of the sediment surface, and a horizontal and vertical maze-like burrow system in the top centimetre of the sediment. Individuals moving on the sediment surface also produced visible trails. When the sediments were dried the burrows were always destroyed; in some cases the surface trails were preserved. We propose that the vertical burrows are escape structures and that the horizontal and vertical maze-like structures are living burrows. Ichnofossils similar to the escape structures and surface structures have been described. Presence of these ichnofossils indicates a low energy sedimentary environment and a lack of macrofaunal bioturbation. The presence of escape structures may indicate a pulsatory pattern of sedimentation.     

A depositional model for stratigraphic complexes and facies superimposition in lodgement tills

A typical stratigraphy below a streamlined till plain in Northumberland, England, consists of cross-cutting lodgement till units, within and between which occur repeated shoestring interbeds of ‘cut and fill’ channels. Till units have erosional lower contacts; in certain cases marked changes in erratic content and local ice flow direction are evident from one till unit to another. These lodgement till complexes have hitherto been described by ‘tripartite’ schemes of lower grey till (s) and upper reddened till (s) identified with respect to ‘middle’ fluvial horizons; regional correlation proceeding on the basis of matching ‘middle’ horizons, with the whole sequence commonly interpreted as evidence for multiple glaciation. Data indicates, by way of contrast, that these lodgement till complexes were deposited during a single phase of subglacial deposition. Till deposition was not continuous but was interrupted by erosional episodes. Changes in the mix of bedrock lithologies transported by the glacier down a single flow line or by lateral displacement of basal ice flow units within the glacier result in till units of different facies to be emplaced when deposition recommences, a process referred to as ‘unconformable facies superimposition’. Subglacial meltwater flow was also a characteristic of the glacier bed; channeled glaciofluvial sediment bodies are found as ribbon-like inclusions in the till and appear to have been deposited rapidly. These so-called ‘middle’ fluvial horizons occur repeatedly in section, their lateral extent at any given exposure being dependent upon the orientation of the exposure with respect to former ice flow direction. These lenses act as internal drainage blankets and have accelerated postglacial soil formation in the drier climate of eastern Britain accounting for the reddened colour of upper till(s). It is suggested that this model of subglacial deposition can be employed in other areas of northern England characterized by subglacial (lodgement till plain) terrains.     

Anatomy of an avulsion

The Cumberland Marshes in east-central Saskatchewan, Canada, occupy over 5000 km² and contain a variety of active and abandoned fluvial features, including straight to sinuous isolated channels, anastomosed channel systems, levees, and crevasse splays in addition to marshes, lakes and bogs. In 1873, an avulsion of the Saskatchewan River diverted most of its flow into a portion of the Cumberland Marshes (locally termed the breakout area), and altered the alluvial terrain as the invaded wetlands adjusted to the influx of sediment and water. These adjustments continue today, and so far over 500 km² of wetlands have been affected by the avulsion. Avulsion-controlled modification of the wetlands involves the initiation and evolution of crevasse splays and splay complexes. Three intergradational forms are recognized, each associated with characteristic sand-body geometries. Stage I splays are small, lobate in plan, crossed by unstable distributary channels, and form wedge-shaped sheets which depositionally overlie fine-grained, organic-rich wetland sediments. Stage II splays and splay complexes evolve both spatially and temporally from Stage I splays. They are larger, contain dense networks of anastomosed channels, and form disconnected tabular sand bodies or continuous sand sheets, some of which incise underlying wetland sediments. Stage III splays develop from either Stage I or II splays and contain few but stable anastomosed channels that deposit isolated stringer sands encased in fine-grained floodplain sediments. Although sand bodies deposited by splays comprise important components of the evolving floodplain, various fine-grained facies occurring in levees, shallow lakes, abandoned splay channels, and interchannel floodplains dominate the avulsion deposits. The post-1873 record of deposition and terrain modification in the breakout area suggests four stages of floodplain evolution following avulsion. In the initial avulsion stage, new channels and splay complexes increase in numbers rapidly as diverted discharge of water and sediment overwhelm the adjacent floodbasin. The anastomosed stage is characterized by an approximate balance between rates of new channel and splay development, and abandonment of old ones. This stage continues for as long as new floodplain areas are invaded. The rate of new splay development eventually decreases as accessible floodplain becomes aggraded, forcing a higher rate of channel abandonment and concentration of remaining flow into fewer but larger channels (reversion stage). The result of reversion is eventual return to a single channel stage, completing the avulsive sequence and initiating a new alluvial ridge. For the Saskatchewan River, this final-stage single channel will likely produce a meander belt which occupies only a portion of the more extensive avulsion belt which preceded it.     

The Munni Munni Complex, Western Australia: Stratigraphy, Structure and Petrogenesis

The late Archaean Munni Munni Complex is a layered mafic-ultramaficintrusion emplaced into granitic rocks of the west Pilbara Block.It consists of a lower Ultramafic Zone with a maximum thicknessof 1850 m and an overlying Gabbroic Zone at least 3600 m thick.There are strong geometrical and stratigraphic similaritiesto the Great Dyke of Zimbabwe. The Ultramafic Zone comprises multiple macrorhythmic cyclesof olivine-clinopyroxene adcumulates and mesocumulates. Layeringdips towards the centre of the intrusion and trends laterallyinto a narrow and variably contaminated chilled margin. Higherlayers extend progressively further up the sloping floor ofthe intrusion. Cryptic layering is defined by rapid fluctuationsin Cr content of cumulus clinopyroxene, accompanied by relativelysmall variation in Fe/Mg ratio. The base of the Gabbroic Zone is marked by the first appearanceof cumulus plagioclase and the simultaneous appearance of pigeoniteas a persistent cumulus phase. Magnetite appears as a cumulusphase 400–600 m above this. Gabbroic Zone cumulates showa gradual linear upward increase in Fe/Mg and an absence ofcyclic layering, suggesting crystallization in a closed chamber. Chilled margin samples show evidence of in situ contamination,but indicate that the parent magma to the ultramafic portionof the intrusion was a high-Mg, low-Ti basalt with similaritiesto typical Archaean siliceous high-Mg basalts. Partial meltingof granitic wall rocks occurred along steep side walls but wasless extensive along the shallow-dipping floor. A pyroxenitedyke, the Cadgerina Dyke, intersects the floor of the intrusionat a level close to the top of the Ultramafic Zone, and appearsto have acted as a feeder conduit to the Gabbroic Zone and theuppermost layers of the Ultramafic Zone. The contact zone between the Ultramafic Zone and the GabbroicZone is a distinctive 30–50 m thick pyroxenite layer,the Porphyritic Websterite Layer, which also exlends laterallyup the side walls of the intrusion to form a 200 m thick marginalborder zone separating Gabbroic Zone cumulates from countryrock granites. A distinctive suite of bronzite-rich xenoliths,some containing Al-rich, Cr-poor spinel seams, occurs withinand just above the Porphyritic Websterite Layer in the centralpart of the intrusion. There is a steep gradient of decreasing Cr and increasing Fe/Mgin cumulus clinopyroxenes across the upper 100 m of the UltramaficZone. A sharp downward step in Cr occurs a few metres belowthe base of the Gabbroic Zone, immediately beneath a stronglyorthocumulate layer of augite cumulate containing disseminatedplatinum-group element (PGE)-rich sulphides. Lateral pyroxenecomposition trends within the Porphyritic Websterite Layer canbe accounted for by an increase in cumulus porosity as thislayer approaches the floor of the intrusion. Quantitative modelling of pyroxene composition trends indicatesthat Ultramafic Zone cumulates crystallized from relativelysmall volumes of magma, an order of magnitude less than thesize of the magma body inferred from trends in the GabbroicZone. This conclusion, together with the geometry of the PorphyriticWebsterite Layer, implies that the Porphyritic Websterite Layermarks a level at which the chamber expanded as a result of amajor new influx of magma. Pyroxene composition trends indicatethat this influx was of a distinetly different and more fractionatedcomposition than that parental to the Ultramafic Zone. Injection of fractionated tholeiitic magma into more primitivehigh-Mg basalt resident magma formed a turbulent fountain, whichentrained the resident magma and formed a cool, dense basalhybrid layer. Crystallization of the Porphyritic WebsteriteLayer occurred where the top of this hybrid layer impinged onthe sloping floor. Continuing injection of tholeiitic magmaexpanded the thickness of the hybrid layer, causing the PorphyriticWebsterite Layer to accrete progressively up the sloping floorand the walls. After the conclusion of the influx phase, thehybrid layer became homogenized to a final tholeiite-rich composition,which eventually crystallized to form the Gabbroic Zone. Thexenolithic rocks within and above the Porphyritic WebsteriteLayer were probably derived initially by crystallization ofa contaminated silica-enriched melt layer at the roof of theintrusion, followed by detachment and sinking or slumping tothe floor. Orthopyroxene phenocrysts within the PorphyriticWebsterite Layer may also have originated within this roof zone.