A standard Ordnance Survey or A-to-Z map shows areas of land in proportion to their physical area. Sometimes it may be useful to transform the map so that the areas, such as wards, are shown in proportion to, say their total population. Such a map is called a Cartogram.
A cartogram is a map on which actual land area is substituted for another thematic mapping variable (such as time, cost, AIDS rates, or Gross National Product). It is sometimes referred to as an isodemographic map or a value-by-area map.