The terms centre and centroid have a variety of different meanings and formulas, depending on the writer and/or software package in which they are implemented. There are also specific measures used for polygons, point sets and lines so we treat each separately below. Centres or centroids are provided in many GIS packages for multiple polygons and/or lines, and in such cases the combined location is typically computed in the same manner as for point sets, using the centres or centroids of the individual objects as the input points. In almost all cases the metric utilised is d_E or d_S, although other metrics may be more appropriate for some problems. Centroids are often defined as the nominal centre of gravity for an object or collection of objects, but many other measures of centrality are commonly employed within GIS and related packages. In these latter instances the term centre (qualified where necessary by how this centre has been determined) is more appropriate than centroid, the latter being a term that should be reserved for the centre of gravity.