Smoothing

The spatial interpolation of the incidence rates using the uniform methodology invented and developed at the Finnish Cancer Registry (Pukkala et al, 2001). This method has also been used for cancer map production for several other regions, including Northern Europe (North-European Cancer Atlas), all Nordic countries (incidence,mortality), Stockholm and Gotland region in Sweden, The Netherlands, Belgium and Manila-Rizal region in Philippines. The illustration method also works with other type of phenomenom such as swine flu (THL).

Rates for 20 most populated cities in Finland are shown as circles with a diameter relative to the population size. A colour shading of the circles is indicating the incidence rate in that city. The remaining rates are shown as floating averages of several neighbouring municipalities. The rate for each grid (size 2 x 2 km) on the map was defined as a weighted average of the age-adjusted incidence rates in the municipalities, with population centres within 200 km from the centre of the grid. The weights were inversely-associated with distance; the weight being halved where the distance was 25 km (see weighting function). In addition, the weights were made directly proportional to the sizes of the populations within the 200 km circle. The northern parts of Finland are so sparsely populated (less than 1 inhabitant per km²) that even the interpolated rates are prone to a high degree of random variation. A white lining was superimposed on the colours of these areas to reduce emphasis on the underlying rates.