This case is a simplified system; practical earthing systems are more complex than a single rod, and the soil will have varying resistivity. It can, however, reliably be said that the resistance of a ground grid is inversely proportional to the area it covers; this rule can be used to quickly assess the degree of difficulty for a particular site. Programs running on desktop personal computers can model ground resistance effects and produce detailed calculations of ground potential rise, using various techniques including the finite element method.