UNIT 6 - SAMPLING THE WORLD

UNIT 6 - SAMPLING THE WORLD

Compiled with assistance from Charles Parson, Bemidji State University and Timothy Nyerges, University of Washington


For Information that Supplements the Contents of this Unit:

Errors in Maps (Chrisman/U of Washington) -- US data quality standards.
Error, Accuracy and Precision (Geographer's Craft) -- (A few graphics); types of errors; sources of inaccuracy and imprecision; problems of propagation and cascading; beware of false precision and false accuracy; dangers of undocumented data; principles of managing error.
Measurement Basics (Chrisman/U of Washington) -- Graphics and description for: levels of measurement (nominal, ordinal, interval, ratio); other forms of measurement (absolute, counts, cyclical, multi-dimensional).
Managing Error (Foote and Huebner/Geographer's Craft) -- Managing problems of error, accuracy and precision; setting standards for procedures and products (establishing criteria, training and testing); documenting, measuring and testing; calibrating a data set to ascertain how error influences solutions; etc.


 

This unit begins the section on data acquisition by looking at how the infinite complexity of the real world can be discretized and sampled.

UNIT 6 - SAMPLING THE WORLD

Compiled with assistance from Charles Parson, Bemidji State

A. INTRODUCTION

 

B. REPRESENTING REALITY

 

Continuous variation

 

C. SPATIAL DATA

 

Location

 

Attributes

 

Time

 

D. SAMPLING REALITY

Scales of measurement

 

1. Nominal

 

2. Ordinal

 

3. Interval

 

4. Ratio

 

Multiple representations

 

E. DATA SOURCES

Primary data collection

 

Secondary data sources

 

F. STANDARDS

 

Sharing data

 

Agency standards

 

G. ERRORS AND ACCURACY

 

Original Sin - errors in sources

 

Boundaries

 

Classification errors

 

Data capture errors

 

Accuracy standards

 

REFERENCES

Berry, B.J.L and A.M. Baker, 1968. "Geographic sampling. In B.J.L. Berry and D.F. Marble, editors, Spatial Analysis. Prentice Hall, Englewood Cliffs NJ, 91-100. A classic paper on sampling geographical distributions.

Hopkins, Lewis D., 1977, "Methods for generating land suitability maps: A comparative evaluation,"AIP Journal October 1977:386-400. An excellent discussion of the different measurement scales is given in an appendix.

Sinton, D., 1978. "The inherent structure of information as a constraint to analysis: mapped thematic data as a case study, Harvard Papers on Geographic Information Systems, Vol. 7, G. Dutton (ed.), Addison Wesley, Reading, MA. A classic paper on the relationships between the database and reality.

Standard sampling theory is covered in many texts on scientific measurement.

EXAM AND DISCUSSION QUESTIONS

1. Take an example map showing the observed occurrences of some rare event, and discuss the factors influencing the sampling process. Good examples are maps of tornado sightings, herbarium records of rare plants.

2. Using a topographic map, discuss the ways in which the contents and design of the map influence the user's view of the real world.

3. Review the accuracy information available for several different scales and types of maps, and spatial databases if available.

4. The Global Positioning System (GPS) will soon be capable of providing latitude and longitude positions to the nearest meter using portable receivers weighing on the order of 1 kg, in no more than one minute. This is significantly more accurate than the best base mapping generally available in the US (1:24,000). Discuss what effect this system might have on map makers and map users.


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