Transcript Cartograms
Mapping People Cartograms of Ireland Martin Charlton [email protected] http://ncg.nuim.ie Outline • Cartograms • Population Cartogram of Ireland • Population Change Places not people • People tend not to spread themselves uniformly across land areas • They tend to live where it’s more convenient to do so (for example: lowland areas, near rivers, near raw materials) • They’re also gregarious – live in settlements • They don’t usually live in the middle of deserts or tundra Showing people • We’re so used to thinking in terms of the physical or political earth that we forget about the social earth. • Our maps represent physical or administrative features (roads, trees, rivers, buildings) but not people Showing people… • Showing the results of an election or incidence of a disease presents a problem • In areas of high population density the physical size of the zones to be mapped is often small • Large rural areas with low populations dominate the visual effect and can give us a misleading impression of the underlying spatial pattern People based maps • Can we, therefore, come up with a map projection in which the sizes of the zones are in proportion to the number of people than live in them? • Yes… they’re known as – Value-by-area maps – Density-equalising maps – Cartograms Creating cartograms • In the late 1950s the US geographer Waldo Tobler became interested in the possibilities of using computers to carry out the calculations for cartograms • His PhD ‘Map Transformations of Geographic Space’ appeared in 1961 Gastner & Newman • Recently Michael Gastner and Michael Newman, both physicists, proposed another solution based on diffusion • Like Dorling’s method it allows regions to ‘trade their area until a fair distribution is reached’ • However it is not tied to an underlying lattice – results don’t look “blocky” Software • Gastner and Newman’s C code is available for download from their website http://www-personal.umich.edu/~mejn/election/ • It can be compiled and run on a desktop/laptop PC… • … or something more powerful Cartogram of Ireland • We used Gastner and Newman’s method to produce a densityequalized map of Irish counties • The starting point is a list of coordinates for each county boundary in the Irish National Grid system… • … and the populations of each county Ireland as we (think we) know it County Boundaries … applying the cartogram projection gives us something different… Changing Population • We can use the county populations from previous Censuses to examine the effects of population change • 1841 • 1926 • 1961 - 2002 1841 1926 1961 1971 1981 1991 2002 Population Scaling • The previous cartograms show how the segments of the Irish ‘cake’ are redistributed according to the changes in population • We can also scale the cartograms so that the total land area is in proportion to the total population in each year Population 1841-2002 7000000 6000000 Population 5000000 4000000 3000000 2000000 1000000 0 1841 1851 1861 1871 1881 1891 1901 1911 1921 Date 1931 1941 1951 1961 1971 1981 1991 Population by County 1841-2002 Carlow Cavan 1200000 Clare Cork Donegal Dublin 1000000 Galw ay Kerry Kildare 800000 Kilkenny Population Laois Leitrim Limerick 600000 Longford Louth Mayo Meath 400000 Monaghan Offaly Roscommon 200000 Sligo Tipperary N.R. Tipperary S.R. Waterford 0 1841 1851 1861 1871 1881 1891 1901 1911 1926 1936 1946 1951 1956 1961 1966 1971 1979 1981 1986 1991 1996 2002 Westmeath Wexford Date Wicklow 1841 1851 1861 1871 1881 1891 1901 1911 1926 1936 1946 1951 1961 1971 1981 1991 2002 Comparison • (a) 1926 – after Independence • (b) 1961 – population starts increasing • (c) 2002 – present day 1926 1951 2002 Cartograms • Cartograms provide another way of communicating data about people • They make us think about people space and not physical space • They make us think about the underlying social processes