Dot Maps: a first kind of thematic mapping

Objectives of lecture

1. Parts of a thematic map
   1. EVENT 04: Map Titles
2. Dot maps

Parts of a thematic map

[Dent calls them 'elements' page 241, Table 13.1 p. 242 (not in sense of Robinson...) ]

• Title
• Legend (key)
• Scale (somehow: text, bar scale, ...)
• Credits (sources)
• the map! (region covered: "base map" and symbols "thematic overlay")

Optional parts (depend on circumstance)

Title: (THIS IS ON THE EXAM! HINT - HINT)

should mention the three components:
time, space and attribute (when, where, what)

Many times, there are two kinds of "space" referenced:

• the total area of the map
• the "units" (granularity)

For example: Population density of the United States by counties 2000

The Legend usually needs a more specific title with more detail than provided by the Title: persons per square mile by county (Hint: give units of measure for ratio data)

Credits: Give your sources: example - "Source: US Census 2000"

Examples of titles from the walls, and the audience...

EVENT 04: Work in groups, write some titles

Dot Maps

You can tell a dot map from its legend:

1 dot = 1000 cows, or something like that.

A Dot Map uses a (simple!) form of a point symbol, see future lecture for more issues about point symbols.

Purpose is to show a continuous density through a discrete set of point symbols.

How to manipulate the dot map:

• number of objects to group into one dot
• size of the dot

Where does the dot go?

Ideally, you would have a lot of detail information and select a location that characterizes the distribution (keeping the dots along rivers or roads, for example...). This means that the dot map would require some hidden map layers. Nobody has written software to do what the old-style cartographer did by hand and local knowledge.

ArcMap just uses dots to fill an area, locates them randomly (or more uniformly, actually). This makes it a form of area symbol, in that the points are attached to areas.

Limitations:

dot can coalesce (get so tightly clustered that they cannot be distingusihed, the symbol gets totally dense, saturated). Depends on size of dot and choice of unit

Examples:

• Population of China 1960 (a student map from the manual cartography era)
• cartoon joke

Version of 4 April 2003