Page 2 - Thematic Mapping Excerpt

De-emphasising land mass
Value-by-alpha choropleth
Mapping share of the vote or margin of victory treats every
area the same and shows a percentage measurement
Mn f oy (%) s n regardless of the number of votes. No account is taken of
n e -e oh mp whether a county is densely populated or sparsely populated.
A county with 10 people in which 7 voted Republican has the
25
same share of the vote as a county of 10,000 in which 7,000
25 voted Republican. Both are 70 percent. And numbers matter
22
21 because counties add up to totals at the state level and play a
part in the distribution of Electoral College votes. More people
(and votes) do not necessarily show up when you look at
Wh ln y h
the share of the vote or margin of victory maps. Many more
le (e e s i)
people live in small, densely populated places than large,
sparsely populated places.
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The value-by-alpha technique adds a secondary layer of data
73 2
to the map, laid over the main map. Because it is symbolised by
varying the alpha channel (transparency), it works by bringing
into focus some areas while visually muting others. The alpha
layer represents population density
to modify how
you see
the
Te
s n U
h
hd ey underlying blended-hue choropleth. Areas with hi
gh po
pulation
lr ms f oy
o e density are shown close to their original colour because the alpha
Rcn Pa, t he my layer tends toward fully transparent. Where there exists a low
et ln s. population density, the alpha layer tends toward opaque. Because
the base colour is white, the increasingly opaque overlaid layer
makes that area recede toward the same colour as the paper itself,
making it less prominent.
This technique introduces a visual correction to the map to accommodate widely varying population (and hence, voting) densities that are a function of administrative boundaries. It gives visual prominence to the distribution of the results of the voting process by population, and not just the distribution by land surface area.
Te mn mp (t) s e d u e o e eg mn f oy mp. Tee’s l a t f ue. Vuy l e tg Dcc aes mn e.
Wn e e e eg u e (t) e mp l s e tg Dcc aes t a t f e dk d hs n d e o a pe ln. Tht s, ee’s l oe d n e mp ce my f e mnl he f e e aes ht ae ue n e mn mp et o a d e n e eg mp.
30 Thematic mapping