Gradient Estimation from Irregularly Spaced Data Sets

Topographic data sets are often composed of terrain samples arranged in irregular meshes. Many gradient estimation methods require the data to be arranged in a regular mesh; irregular meshes present a difficulty for them. This paper presents a novel method of estimating surface gradients from irregularly spaced inputs. The method is derived using directional derivatives and is shown to be a generalization of traditional finite difference methods. Analytical tests were used to evaluate the method, which was shown to be accurate and robust.     

Sky view factors in forest canopies calculated with IDRISI

Summary  The sky view factor (SVF) is used in forest, road and urban climatology to characterise radiative properties. We now propose a method suitable for forest canopies using the raster based and commercially available software IDRISI. It uses quadratic pixels in rows and columns in a scanned equiangular fish-eye image. A threshold value is chosen to divide the image into sky and non-sky areas. The resulting image is then multiplied with a sky view weight image, where the weights of the pixels depend on the angular distance from zenith. The sum of pixel products gives the SVF. Quality analysis of the method is also performed. The choice of threshold value gives some uncertainties due to leaves reflecting sunlight. This error will be reduced by observing details (branches etc.) in the image and by choosing an overcast day for capturing the image. The precision range for SVF calculations will be better than 0.1. Received November 22, 1999 Revised July 17, 2000     

Evidence of tidal processes from the lower part of the Witwatersrand Supergroup,South Africa

A 1600-m succession of quartz arenites and associated shaley deposits comprising the Hospital Hill Subgroup at the base of the Witwatersrand Supergroup is considered to have been deposited largely under the influence of tidal processes. Facies analysis indicates that deposition occurred in the following environments: (1) marine shalf; (2) shallow subtidal to intertidal; (3) intertidal flat; and (4) tidal inlet. The presence of strong tidal currents implies that the Witwatersrand Basin was open to an ocean basin, at least during the early stages of its evolution. Palaeocurrent trends and isopach data suggest that this probably lay to the southwest, an area now occupied by the high grade Natal—Namaqua metamorphic belt. The contrast between the supermature quartz arenites of the Hospital Hill Subgroup and the overlying gold-bearing immature subgreywackes, feldspathic quartzites and conglomerates of fluvial origin is believed to be a function of tidal reworking of sediments.     

Three-dimensional computer-assisted basin modelling to generate exploration target areas: an example from the late Archaean-early Proterozoic Transvaal Supergroup, South Africa

Second- and third-order fault-bounded Precambrian basins frequently host deposits of the sedimentary massive sulphide group. Three-dimensional geometric modelling of the thickness of preserved basin-fill successions of the Transvaal Supergroup, using DATAMINE software, and residual gravity modelling of the contemporary basement floor, help delineate areas of exploration potential in this unit. Two main depositional axes are tentatively identified for the basal volcano-sedimentary protobasinal Transvaal successions. A sheet-like geometry was indicated for the succeeding Black Reef sandstones and Chuniespoort Group chemical sedimentary rocks. The uppermost Pretoria Group thickness model delineates eastern and western second-order basins separated by a central submerged palaeohigh. A similar isopach pattern is noted for the thick shales of the Silverton Formation in this group, with, in addition, a well-defined third-order basin in the northwest of the western second-order basin. The residual gravity model indicates two linear palaeovalleys adjacent to this western basin, one coincident with one of the axes inferred for the protobasinal rocks. The fault-bounded second- and third-order basins and depositional axes postulated here are consistent with known geological data and suggested sedimentation models. Cumulative distortions implicit in the DATAMINE computer modelling technique are reduced when the method is applied on the basin-wide scale, enabling identification of regional exploration target areas rather than immediate prospecting targets. Received: 14 August 1996 / Accepted: 13 March 1997     

Pleistocene sediment-hosted uranium deposits at Henkries, South Africa: using settling tubes to delineate buried ore bodies

Sediment-hosted uranium ores at Henkries in northwest South Africa occur in fine-grained sands, carbonaceous muds and diatomaceous earth within late Pleistocene lake deposits. The lakes are linked by short fluvial channel reaches and these aqueous beds are encompassed in predominant aeolian dune deposits. The late Pleistocene fluvial-lacustrine-aeolian succession is succeeded by a Holocene dune cover. Textural characterisation of lacustrine, fluvial and aeolian sands was based on volume percentages observed in sediment settling tubes. Vortex action during Holocene dune migration contaminated these aeolian cover sands with small amounts of substrate material, whose presence could be detected in settling tube patterns of surface aeolian sediment samples. It was thus possible to map buried lacustrine ore bodies, which were shown, by a successful drilling programme, to be displaced downwind. Received: 28 August 1996 / Accepted: 3 September 1996     

Comparison of versioning methods to improve the information flow in the planning and building processes

Version management is a prerequisite for digital information flow between phases in the planning and building processes. Information evolves over time and many parties retrieve information from the various phases. The aim of this article is to evaluate versioning methods, focusing on geodata buildings in the 3D cadastre process. The main attention in the evaluation is on the comprehensive ISO standard Product Lifecycle Support (PLCS). PLCS is evaluated against two simpler versioning methods, the versioning in CityGML 3.0 and a modified Git versioning method implemented in CityJSON. CityGML 3.0 fulfils all but one requirement and PLCS meets all. The methods vary in complexity; the Git proposal is a simple solution, easy to implement and maintain, while PLCS includes all functionalities and is complex to implement. There is a trade‐off between number of functionalities and complexity, it is therefore important that the intended purpose determines the choice of versioning method.     

One century sedimentary record of Hg and Pb pollution in the Sagua estuary (Cuba) derived from Pb and Cs chronology

The vertical distribution of Hg and Pb were determined in a sediment core collected from the Sagua estuary (North Cuba) that receives input from the Sagua river, one of the most polluted rivers discharging into the Cuban coastal environment. Depth profiles of metal concentrations were converted to time-based profiles using the ²¹⁰Pb dating method and confirmed with the ¹³⁷Cs fallout peak. The mean mass accumulation rate was estimated to be 0.17 ± 0.04 g cm⁻² y⁻¹ (mean sediment accumulation rate 0.52 ± 0.13 cm y⁻¹) and the core bottom was estimated to date back about 130 years. The historical sedimentary record showed a strong enrichment of mercury concentrations in the past decades, caused by the incomplete treatment of industrial wastes from a chlor-alkali plant with mercury-cell technology in the Sagua river basin. Lead fluxes to sediments showed a gradual increase from the 1920s to present, which agrees with a population increase in Sagua la Grande City. Fluxes of both metals have increased the past 25 years, with values reaching a maximum of 0.5 and 3.9 μg cm⁻² y⁻¹ for Hg and Pb, respectively.