Page 8 - Thematic Mapping Excerpt

Stars and stripes
Exposing cross-sections across the map
A common technique in cartographic representation is the use
of transects, otherwise known as profiles or cross sections. They are constructed by drawing a line across the planimetric map and using the values of data that intersect the line to construct a vertical graph. This then is plotted as an elevation, or a side view of the data as if the terrain has been sliced open to reveal
its vertical structure. The most common form of vertical section uses a straight line, as in the example shown to the right. Several horizontal lines are drawn across the map of the United States, which provide a series of horizontal planes and vertical graphs at equal intervals.
The vertical graphs that rise above each line give an impression of the volume of data underneath—in this case, the share of the vote won by the winning candidate. Vertical sections could be shaded in many ways. A single colour might be used for a single variable being explored, but here, the profile lines cross counties in which the winner is one of two candidates, so the vertical section is shaded categorically according to the winner. Republican share of the vote is shown above a centre line and the Democratic share of the vote below. This gives a sense of the quantitative aspect of the results across the line. The more red, the greater the Republican share; the more blue, the greater the Democratic share.
Vertical sections provide an interesting look at the data and offer a different visual aesthetic from many maps but, of course, they have one major drawback in that they lose a lot of data from the final display. The lines drawn across the map miss many counties (and cities), and so the final vertical sections show only a small proportion of the data. Is it enough though? Sometimes these sorts of carto
graphic approaches are enough to give an overall sense of the pattern, particularly, as in this case, in which there are clear, albeit generalised, patterns from east to west that go blue, red, blue.
To orient map readers to the graphs, the legend includes a vertical scale so readers can interpret the vertical magnitudes. This is critical because the vertical dimension includes some vertical exaggeration to ensure a good variation in elevation across each profile. The choice of precisely how many lines, and where
to draw them, is predominantly governed by the amount of vertical space at the chosen scale, while also attempting to get a good representation of the data. Outliers can be chosen to be included or excluded. Lines also might be drawn to pass directly through key places. Labels represent key towns and cities across the US and are coloured red or blue to denote the results of the counties nearest the place itself. On a map like this, you cannot represent every place equally, and avoiding overlapping the lines is also important to avoid congestion.
Olympia
Portland
Salem
Seattle
Helena
Boise
Sacramento San Francisco
I’s t n e mp ee o n ht e mp t t e g s h s ht
t s g.
Tee’s a t f d n e c-s n s mp ce t’s g he f e , gs f e tl e f s. Tht’s n ott n e a mp ht dw e ecl s y tl s d k ey et d.
An, e u f e y s s e n f e y te d e s n ht ae. I t t t e e ttn t.
Carson City
Salt Lake City
Fresno
Bakersfield
Los Angeles
Las Vegas
Pheonix
San Diego
Tucson
Fairbanks
104 Thematic mapping
Anchorage