Railways and local population growth: Northamptonshire and Rutland, 1801–91 MARK CASSON , A . E . M . SATCHELL , LEIGH SHAW - TAYLOR AND
The quantity and quality of land, and the endowment of infrastructure, are summarised by a set of key parish characteristics. Some of these characteristics are constant (i.e. time-invariant) and others are variable (i.e. time-dependent). Locational factors are mainly constant, especially those that reflect natural features. Soil fertility can change, however, as a result of changes in farming techniques (e.g. application of fertiliser and investment in drainage schemes). Rivers may be rendered navigable by cuts, and the value of non-navigable rivers may be influenced by the construction of mill streams or fish weirs. Such changes are accounted for, not by changing the location factors, but by allowing for constant location factors to have changing impacts, as explained below.