Computers & Geosciences 31 (2005) 877–881 Analysing potential field data using visibility $ G.R.J. Cooper School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa Received 19 October 2004; received in revised form 4 March 2005; accepted 4 March 2005 Abstract The viewshed of a topographic dataset is the region around a given observation point that is visible from that location. Most GIS packages allow its computation. It has many uses, such as in urban environment planning, or in analysing archaeological site data. The viewshed is used here as a data enhancement tool to aid in the interpretation of geophysical potential field data. The fraction of the area of a moving window visible from each point on the dataset (considered as if it were topography) is computed. Since this area changes if the observer is raised above the surface, its vertical derivative can also be computed. This derivative was found to be a useful method of enhancing subtle features, and is demonstrated on aeromagnetic and gravity datasets from South Africa. r 2005 Elsevier Ltd. All rights reserved. Keywords: Feature enhancement; Gradients; Viewshed; DTM 1. Introduction The determination of the visible region around a given observation point has long been computable in Geo- graphical Information System (GIS) software packages. Its many uses include enabling the optimum siting of radio and television transmitters (De Floriani et al., 1994), and the estimation of the visible impact upon the environment of the change of land use. In some applications the curvature of the Earth needs to be taken into account. Clearly, the visibility from a chosen point will be a function of the direction in which the observer is looking. Some applications require only the line of sight visibility between two points, while others require the computation of the entire visible region around a given point. Geophysical potential field data is used to determine the subsurface structure of the Earth, commonly with the aim of aiding mineral or oil exploration projects. The measured values of the Earth’s gravity or magnetic field are interpolated onto a regular grid, then processed using a wide variety of techniques to remove noise and enhance detail. Commonly applied filters include hor- izontal and vertical derivatives (Gunn, 1975), and textural analysis (Dentith, 1995; Cooper, 2004). Wavelet processing is becoming increasingly common (Fedi, 1998; Fedi et al., 2000). This paper discusses the use of aspects of the viewshed as a processing technique for potential field data, and compares it with related commonly used filters. 2. Computing the viewshed Determining the viewshed of a chosen point can be very computer intensive, and there are several ARTICLE IN PRESS www.elsevier.com/locate/cageo 0098-3004/$-see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cageo.2005.03.002 $ Code on server at http://www.iamg.org/CGEitor/index.htm. E-mail address: cooperg@geosciences.wits.ac.za. URL: http://www.wits.ac.za/science/geophysics/gc.htm.