Mapping “what?” Instead of “where?”

Two types of geographic data:

Horizontal location Vertical location

Vegetation types Soil types Land cover Number of people

Location Attributes

Attributes: characteristics of a place

Back to a previous topic …

Reference mapsReference maps Emphasize Emphasize locationlocation

Thematic mapsThematic maps Emphasize Emphasize patternspatterns

Leading candidate, by county

Place names are Place names are attributes, butattributes, but Location Location data is keydata is key

Need location data, but Need location data, but attributeattribute data is key data is key

Mapping attributes

Sources of attribute data Focus on census data

Examples of attributes:

Vegetation types

Soil types

Land cover (forested, grass, asphalt, etc)

Land use (wilderness, recreational, residential, commercial, etc.)

Number of people

People’s typical ages, incomes, etc!

Temperature

Precipitation

How would you collect

data for maps of these

phenomena?

Sources of attribute data:

Remote sensing:

Land cover

Vegetation types

Soil types

Precipitation

Change over time, for any of these topics

Aerial photography

Satellite-based scanners

Ground surveys

Count-based surveys

More sources of attribute data:

Vegetation, land cover Road type, land use

Population (human) Population (animal)

Count-based Surveys:some definitions

Census

All members of a population Sampling

Inferences about whole population based on some

Example: Decennial U.S. Census

Why count? It’s in the Constitution

Mandate of Government Agencies US Census Bureau: “to be the preeminent

collector and provider of timely, relevant, and quality data about the people and economy of the United States.”

2000 Reapportionment

US Census … Issue 1

Challenges to getting a complete count:

Time-consuming, laborious, costly

Dishonest or uncompleted responses

Homeless, transient populations, illegal immigrants

Challenges of a complete count

Non-response follow-up enumerations (NRFU) = expensive

Visit every house without a response

Non-respondents usually urban, poor, minority

Solution?

sample site

Stratified sampling

More efficient and accurate

Widely used

Example: survey of tree species in a forest

Traditional sampling approach: “Random sample”

Stratified sampling

The technique:

Divide a region into homogeneous regions

Assign sample sites to each stratum in proportion to what each area is thought to contribute

low

medium

high

biodiversity

sample site

“Stratified sample”

Complete Count v. Sampling Supreme Court ruling on use of sampling for

census NRFU’s:

WASHINGTON (CNN) – The U.S. Supreme Court on Monday ruled out the use of statistical sampling to adjust the 2000 census to make up for an expected undercount.The 5-4 ruling was a defeat for the Clinton administration, which had hoped statistical sampling would add population -- and subsequently House members -- to areas that traditionally vote Democratic.

-AP, Jan 25, 1999

Issue 2: Reporting census data

Aggregation: combining counts into spatial units Rather than recording

precise location of individuals

Less costly

Preserves confidentiality

10 8

6

12 2

4

3

US census aggregation units States

Counties Census Tracts

– Census Block Groups

Census Blocks

Challenge: changes Before 1990, census blocks

and tracts only in some areas

As population increases, units are divided

Challenge: Lost Detail Can aggregation lead to misrepresentation?

8 8

8 8

8 8

8 8

Accurate representation difficult to achieve

Usually convenient regions are used

Conclusions on Count-based Surveys:

Not “totally accurate” Reporting and map representation

challenges Next Lecture…