Commission for Geodesy in Africa Fourth Symposium on Geodesy in Africa

Future IAG-Meeting

Commission for Geodesy in Africa Fourth Symposium on Geodesy in Africa Tunis-Tunisia 21–27 May 1990     

Report on the International Symposium of the Commission on Recent Crustal Movements

Journal of Geodesy -     

Edgar Hynes Thompson: aerial triangulation and ISP Commission III

This author worked with E. H. Thompson in several different contexts in the course of his military and civilian career, notably in developing Thompson's ideas about analytical aerial triangulation into a production system at the Ordnance Survey and later as a lecturer under Thompson at University College London and as Secretary of ISP Commission III when Thompson was Commission President. This essay thus stands as a separate contribution from a close colleague.     

Catchment infiltration II: Contributing areas

This study considers two issues of interest to the hydrologic and geographical information systems community. One deals with identifying the spatial distribution of infiltration and runoff contributing areas. The other addresses process modelling within a GIS framework. The study operates on the premise that partitioning of precipitation into runoff or infiltration depends on rainfall intensity and on soil properties. The problem is that neither local rainfall intensity, nor soil properties such as infiltration capacity and macroporosity are known well enough for all points of a catchment and need to be estimated. We infer local intensity from the interpolated distribution of cumulated rain depths over the catchment and record duration at the official met site. Measured values of sorptivity and hydraulic conductivity define infiltration. Negative head infiltration describes macroporosity. To scale-up measured point values to larger areas and to model infiltration and macropore continuity at a catchment scale we use geostatistical kriging and conditional simulation together with standard GIS techniques of overlay manipulation. Results delineate areas contributing to runoff and infiltration and relate them to macroporosity. By intersecting overlays of precipitation with those of infiltration we create alternate GIS masks targeting specific portions of the watershed as either runoff or infiltration contributing zones. Choice of cell size and time interval define the scales of averaging for the application. Kriged surfaces illustrate the distribution of catchment infiltration, while conditional simulation provides a mechanism to define model uncertainty.     

Applying Cost/Benefit Analysis to Evaluate Investment in GIS: The Case of Ghana's Lands Commission Secretariat, Accra

One of the many reasons why GIS programmes in Africa and elsewhere in the developing world never get started is that top management are not fully persuaded to support such innovations. A favourable financial analysis of GIS investment will greatly enhance its chances of approval and continued funding if the computation is tied to the goals set by organisations. We argue that an approach provided by a score and weight process that is part of the Investment Evaluation Methodology (IEM) developed by Digital Equipment Corporation USA, for cost benefit analysis fulfils this objective as it is designed to assist top managers and policy‐makers in coming to an understanding of the nature and scope of the proposed GIS project. The methodology involves measuring benefits that are quantifiable (tangibles) and identifying those that are not (intangibles). A coherent and consistent framework for evaluating the GIS project is therefore made. The methodology is generic enough to be applied in any developing country context.