Individual vector features (e.g. polygons) and groups of grid cells exhibit a range of distinctive spatial attributes — for example the length of a polyline or polygon boundary and the area of a polygon or grid patch. In this section we examine a number of basic geometric attributes that apply to such features. These basic attributes may be included in the attribute table record associated with each feature, or may be treated as an intrinsic attribute that may be computed on demand and optionally added as a field in the associated table. If it is necessary to perform operations on these attributes, for example sorting areas by size, or calculating the ratio of areas to perimeters, it is helpful and often more efficient to add the data as an explicit field if it is not already available.