Thematic Mapping: Dots & Cartograms

Question 1

What is a situation where a Dot map would be appropriate?

Answer 1

When data are collected for an enumeration area and exact location of data is unknown, mapped at the collection level and the data are often not equally distributed within the enumeration area

Question 2

What does a Dot represent in a Dot map?

Answer 2

A certain amount of a phenomenon (People, trees, etc.)

• One to One
• One to Many

Question 3

What is the Projection for a Dot map?

Answer 3

Equivalent: Equal Areas

Because perception of density/area is key to the message with a dot map not the shape (conformal/angles)

Question 4

What are 3 elements of creating a dot map?

Answer 4

• Dot Value
• Dot Size (all same size)
• Dot Placement

Question 5

Guidelines for Dot Value

Answer 5

From Dent:

• Easily understood
• Balance value and size to get a coalescence in highest density area
• Value and size harmonize with map scale

Question 6

Why do we want easily understood dot values?

Answer 6

So that there is instant recognition for the audience

- Simple and Straightforward (ex. 10’s, 201/s, not 7’s)

Question 7

Why do we harmonize dot value/size with map scale?

Answer 7

So that the map appears neither too accurate or too general

Question 8

Guidelines for Dot Placement

Answer 8

• Locate the dot as near to phenomena as possible (Centre of gravity Principle)
• Ancillary information
• Don’t use geometric spacing
• Be careful not to create boundaries or impressions of lines

Question 9

Ancillary Vs. Random placement of dots

Answer 9

• Ancillary can help locate dot value close to where it belongs (Often not accessible)
• Random uses uniform density (no artificial patterns or geometric spacing)

Question 10

3 notes on Dot size

Answer 10

• Don’t make too small
• Don’t make too large
• Don’t make too many (overkill, can lead to impressions of boundaries)

Question 11

Dot map Legend

Answer 11

1) State the Value

2) Set of at least 3 squares (or other areas) illustrating 3 different densities

Question 12

Ancillary Vs. Random placement of dots Example

Answer 12

Ancillary data can help place population dots in a county where most of them cluster around a city and gives message of urban info but if ancillary is not available then random placement throughout county is also acceptable

Question 13

Limitations of Dot maps

Answer 13

• Balance between dot size and value
• Extracting exact data from dot map is next to impossible
• Numerousness and Density (perception)

Question 14

Numerousness and Density

Answer 14

Subjective reaction to physical number of objects in visual field without counting all the objects (Dent)

• Perceptual
• Audience judges relative densities but cannot recover original data (count all the dots!)
• Most often dots are underestimated
• Large densities give impression that area is full of phenomena and sparse gives opposite

Question 15

What is the breakdown of the word Cartogram

Answer 15

Map - gram

Question 16

What is a Cartogram?

Answer 16

Value by area map

• Spatial unit area is proportional to data represented
• Geography becomes part of map, scale disappears
• Unit itself reflects the data without the use of symbols

Question 17

What kind of data can be mapped with a cartogram?

Answer 17

Raw, derived, ratio, interval

Question 18

Advantages of Contiguous Cartogram

Answer 18

• Preserves common boundaries and geographic relationships
• No Jig-saw effect
• Study area extent is retained

Question 19

Advantages of Non-Contiguous Cartogram

Answer 19

• Straightforward to scale and construct
• Preservation of areal shapes
• Don’t need to worry about shared boundary lines

Question 20

Contiguous Cartogram

Answer 20

Borders are connected and geographic relationship is preserved between pieces
- Lots of geometric math involved

Question 21

Non-Contiguous Cartogram

Answer 21

Geographies are broken into floating pieces where shared borders are not of concern

Question 22

Disadvantages of Contiguous Cartogram

Answer 22

• Complex to construct (math, but some scripts are available)
• Shapes/areas become unrecognizable due to scale distortions
• Lines must be both big and small at the same time

Question 23

Disadvantages of Non-Contiguous Cartogram

Answer 23

• Geographic space is not illustrated (gaps considered geospace)
• Map extent is not retained (larger due to spaces between units)

Question 24

GIL

Answer 24

Geographic Information Literacy

Question 25

Limitations of Cartograms

Answer 25

• Best suited to Areal data
• Can’t use directly proportional data (unit size will not change enough)
• Don’t use a cartogram if the message isn’t Dramatic!
• Best made by hand

Question 26

Areal data for Cartograms

Answer 26

polygon data

- Don’t use points or lines

Question 27

Advantages of Cartograms

Answer 27

• Shock Value! Dramatic message!
• Clarity where unnecessary detail may be present
• Illustrate spatial patterns concealed by traditional map techniques

Question 28

Disadvantages of Cartograms

Answer 28

• inaccurate geographies
• Confusing for audiences with limited GIL
• Difficulty orienting due to location distortions

Question 29

Stepped Statistical Surface

Answer 29

• Not a true cartogram
• Extrusion of polygon to 3D to represent value (Height is value, step is proportional to value)
• Manipulation of the geographic space