Choropleth Map
Choropleth map is a thematic map (designed to show a particular theme
connected with a specific geographic area) in which areas of the map are shaded
or patterned in proportion to the statistical variable that is being measured
and displayed on the map, such as population density or per-capita income.
These maps are usually attached to enumeration units (e.g. counties, provinces,
countries); standardized to show rates or ratios and could conceptually measure
the phenomena anywhere in space.
Example datasets appropriate for choropleth maps:
•
world map of income tax rates by country
•
map showing number of births per 100,000 in 2009, reported by U.S.
county
•
map showing the percentage change in skin cancer from 1990 to 2010 by
Australian state.
•
world map of percentage of population under 18 years old, reported by
country
map showing the
percentage increase in home value from 1980 to 1990 by Canadian province
Isarithmic Map
Isarithmic maps are maps that represent data sets that have a continuous
distribution and smooth change in value. The data must be continuous or contain
data for any possible point within the data set. The boundaries are based on
the data and the data type must be quantitative.
Two types of isarithmic maps:
1) Isometric map: constructed
from true point data
e.g. toxic level map is constructed from the point data at sampled
location
2) Isopleth map: constructed
from perceptual point data
e.g. demographic trends map is constructed from the point data at
centroid of enumeration unit
Dot and Density Map
Dot and density maps are an affective way to show density differences
in geographic distributions across a landscape. Dot density maps
have been popular for 150 years because they are easy to understand and, at a
glance, show us intuitively where things clump or cluster. There are two basic
types: one-to-one dot density maps (one dot represents one
object or count) and one-to-many dot density maps in which one
dot stands for a number of things or a value (e.g., 1 dot = 1,000 acres of
wheat production).
Proportional Symbol Map
Proportional symbol maps scale the size of simple symbols (usually a
circle or square) proportionally to the data value found at that location. They
are a simple concept to grasp: The larger the symbol, the "more" of
something exists at a location.
There are three methods for setting symbol size: absolute scaling,
apparent magnitude (perceptual) scaling, and range grading.
Absolute scaling involves scaling symbol area proportionately to the data
value. For example, a symbol for a point with a data value of 100 should be
twice the area of the symbol for a point with the value of 50. Apparent
magnitude scaling incorporates correction factors to compensate for map-reader
underestimation of symbol area. With range grading, a symbol represents a range
of data values. As in the choropleth technique, data are classed and then each
class is assigned a symbol of a distinctly different size.
Source: http://indiemapper.com
