2. Choropleth Maps (greek CHORO = region. Note this is not 'chloro-' with a letter L)
- divide a region into sub-regions in which data were collected:
--- countries, provinces, census tracts...
- data may be actual measured values:
--- tonnes of wheat, dollar value produced, population...
- or may be derived from the raw data:
--- mean income, population / square kilometre....
- regions are quantitatively different (e.g. 1 to 100, 101 to 200)
--- depicted with a progression of tones, colours, patterns.
--- progression of shading must be logical:
------ darker shades of grey represent higher values.
------ warm colours represent higher values than cool colours.
--- a clear legend is essential.
3. Dasymetric Maps
- like a choropleth map, but simplified by combining adjacent areas with the same shading.
- example - population density:
--- map Ontario population at township level.
--- combine adjacent townships in the same population class.
--- Fewer regions, easier to draw and interpret.
--- Gives abetter idea of distribution than a county level choropleth map with the same number of regions.
- needs a legend like a choropleth map
4. Prism Maps
- derived from choropleth, dasymetric or sometimes isoline maps.
- regions with higher values raised above regions with lower values.
--- height proportional to value.
--- needs a 'vertical scale' to explain heights.
--- Difficult to draw but visually striking.
5. Isoline Maps
- continuous data only.
- divide region into zones with different numerical values of some characteristic. e.g. elevation - a contour map.
--- Dividing lines connect points of equal value, 'isolines'.
- Each zone between two isolines has a specified range of values (e.g. above 200 m, below 300 m).
- heights indicated by labelling the isolines.
--- place labels in chains down slopes (easier to find).
--- label every second or third (fifth etc. ...) contour if lines are crowded.
- may use logical sequence of colours or patterns to draw attention to high or low zones (e.g. dark = low, light = high).
--- Shading between elevation zones = 'hypsometric shading'.
- Construction:
--- usually drawn from known values at random or selected points (selected data much more reliable).
--- between each pair of points, locations of places at intended isoline values are interpolated.
--- those places are connected to form isolines.
--- The interpolation only makes sense if data are continuous!!!
- Computer programs including GIS can draw isolines. None are as effective as an experienced person.
--- you need to understand the contouring process to understand contour maps.