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...Review

III. Map Abstraction and Infrastructure

Map (or cartographic) Abstraction: "the process of transforming geographic data, which represents the actual human and physical environment, into a map

• Map Content, Map Scale, Map Generalization, Map Symbolization

Map Infrastructure: basic elements upon which all maps are constructed

• locational reference systems, map projections, accuracy

Summary: Map Projections

1. What are the basic characteristics of the map projection process?

• how does map projection work?
  
• lumpy earth, transformed into a sphere,then projected onto a flat surface

2. What kind of flat surface to we project the spherical earth on to?

• flat surface, a cone, and a cylinder: projection families

3. What are the basic kinds of map projection distortions?

• preserving area: equivalent projections

• preserving shape: conformal projections
  
• cannot preserve both area and shape

• preserving distance: equidistant projections
  
• cannot preserve both area and distance

• preserving direction: azimuthal projections
  
• can preserve direction
  
• and area OR shape OR distance

• compromise: don't preserve area, shape, distance, or direction
  
• but minimize distortion of all of these: good default for world maps

Next...Map Accuracy...

Accuracy: "The quality or character of what is precise"

Map accuracy: "the true-to-reality nature of maps"

• but map accuracy is complicated...

  
  

• Are mental maps inaccurate?
  

  
  

  Are cartograms inaccurate?

  

  
  

  Are Medieval European "Mappamundi" (world maps) inaccurate?

  

  
  

  Is the Peter's Map Projection inaccurate?

  

Accuracy is complicated: difficult to judge what accuracy means without understanding the particular map and its intended use

Portolan Chart

• There is no general standard of accuracy by which all maps can be judged

What Kinds of Questions about Map Accuracy can a User Ask?

1. Are the Factual Details on the Map Accurate?

• Monmonier's "cartographic blunders"
  WWW) USGS Accuracy Standards

Map series by governmental agencies have strict regulations and editing guidelines

• most maps don't have such stringent standards

Examples

• ex) Features are left off of the map by accident

  
  
  
  

• ex) The maps that exist are crappy

  
  
  

  PROP) Trewartha book

  
  
  

  ex) editing errors

  
  

  

  Oops.

In all these cases

• the errors are mistakes

• the errors would be corrected if they were known

• the errors could be eliminated by careful editing

Purposeful errors

Examples

• Copyright traps: what they are

  
  
  
  

• ex) Maps deliberately manipulated for political or military purposes:
  

  Propaganda Maps

  
  

  

  Cold War Soviet topographic maps

2. Are the Positions of the Mapped Features Accurate?

Positional accuracy

PROP) US Topo Map

USGS Topographic Maps have set standards of positional accuracy

• and all maps in the series must meet these standards

• such maps are created by the USGS and private contractors

• to check accuracy, selected maps are tested

Specific standards for a 1:24000 topo map:

• up to 10 percent of well defined points can be greater than 1/30th of an inch away from where they should actually be on the map

• up to 10 percent of elevations can be more than 1/2 the contour interval away from where they should actually be on the map

• if more than 10 percent of the points and elevations are off: remake the map

• not all points and elevations are checked: just a sample

• you cannot say that any given point meets the standards: but the map does in general

This is about as good as it gets: many maps have no set standards

• ex) WWW) WWW Mapping

Further interesting issue: we don't always want the most positionally accurate map!

Recall the idea of map generalization

how generalization adds purposeful locational inaccuracies

3. Is the Abstraction of the Mapped Features Accurate?

• Recall that reality - and the features out in the environment - are complicated
  • Feature: "Some thing inherent and distinctive"

• Features in the natural and human environment:
  • tangible, concrete: hills, rivers, trees

  • intangible, abstract: boundaries

Tangible and intangible features have dimensionality

• We can set some standards by which we can judge the abstraction behind this process

• USGS topo maps: have set abstraction standards
  ex) what is "woodland": green on map

  • "an area of normally dry land containing tree cover or brush that is potential tree cover. The growth must be at least 6 feet tall and dense enough to afford cover for troops"

  • guidelines which can be tested to see if the mapmaker has properly abstracted the complicated vegetation landscape into the category of "woodland."

In most cases there are no set standards

Cultural Regions of the US

4. What is the Temporal Accuracy of the Map?

• The data on maps has a time

Recall that maps kill!

News) "Ski Lift Cable is Not Shown on Map Used by U.S. Pilot"

News) Accidental Bombing of Chinese Embassy

More mundane problem, but still important:

WWW) Census Data: Average Income

• average income (divide population into total income)

• but uses 2000 population and 2002 income data

• population may have changed between 2000-02

How to judge temporal accuracy

• does the map have a date?

• are the sources the map is based on noted; what is their date?

• be aware that most maps include data collected at different times

5. Is the Cartographic Symbolization Accurate?

Poor or bad map symbolization choices

• at best: slow down ability of people to interpret patterns:
  

  Poor and Good use of Color on Choropleth Map

  
  
  

  at worst: misinterpretation of map and data

  

  Reverse of dark means more convention

6. Does the Detail of the Data vary over the Map?

• the detail of the data varies over the map

  
  

  

  Wisconsin sand and gravel map

  
  

  may look coherent but many maps have been compiled from data collected at very different scales

  implications

7. Have Printing Problems Muddled the Look of the Map?

In the past, this was usually of a problem with the quality of the map printing

• example

Today: problems caused by computers

• what gets printed is not always what you see on the computer screen

• color maps printed in black and white on laser printers

Issues which affect map accuracy can start with the data (before mapping) through to the end (when you try and print the map)

• accuracy is complicated!

Effective map users and makers comprehend the myriad of ways inaccuracies can occur on maps

Communicating Map Accuracy

Given that all maps have some kind of inaccuracy, cartographers have worked to come up with ways to communicate to map users the fact that inaccuracies exist on a map

Strategies for Communicating Map Accuracy

Common problem with maps: some information is highly detailed and accurate, and other information is not

People tend to assume that all the mapped data is as accurate as the most accurate data

• Solution: generalize all the data to the level of the least detailed data
  

  Delta Airline map

Communicating data accuracy with symbols, notations, disclaimers

Communicating Accuracy


broken line to indicate lower level of accuracy than solid line
verbal notation on map warning of map accuracy problems


reliability diagrams




Wisconsin sand and gravel map

Digital Data and Map Accuracy

• The capacity to collect, process, and map data with computers is accentuating the importance of map accuracy issues

• In some cases new technologies are allowing us to more quickly and cheaply collect data directly from the environment

• However, much digital data is based on existing, non-digital sources

• The creation of "cartographic databases" - digital databases of information taken from pre-existing paper maps - raises some additional accuracy concerns

Geographic Information Systems

GIS Layers

From a technical perspective

• methods for data input: digitizing, scanning

• methods for data integration: make different data sets work together

Useful because this allows us to do some cool analytical things

WWW) GIS overlay example

But what does this assume?

• assumes that all the different "layers" of information are equal

• data is collected at the same time

• data is collected at the same level of detail

• data meets some basic accuracy standards

• data has been edited for errors

• and rarely is this the case

All sorts of odd stuff happens when we are digitizing and combining and integrating data from a diversity of sources, times, and collected for different purposes by different agencies and individuals

Edge matching in digital maps

Data of differing levels of detail

• examples

What happens when we impose these and use them together?

• anyone can easily create new digital maps by combining layers together

• and they can pass this new digital cartographic database along to someone over the internet; and this person can combine that database with different cartographic databases...and so on

• and often information about the differing levels of accuracy and how the data has been manipulated is lost

• errors are propagated: "extended over a considerable area or space"

• Error propagation in GIS and digital mapping: errors spread around digitally as digital data sources are used and reused in the process of creating new digital maps

Attempts to deal with this issue: metadata: data about data

• Dependable digital geographic data will include detailed metadata including:
  • identification information: general description of the data

  • data quality information: in terms of data quality standards

  • spatial data organization information: how spatial information in the data is represented

  • spatial reference information: coordinate and projection information

  • entity and attribute information: map data and associated attributes

  • distribution information: data creator, distributor, and use policy

  • metadata reference information: metadata creator

  • citation information: how to cite information when used

  • temporal information: when data was collected, updated

  • contact information: how to contact data creator

  • Source: Longley et al., Geographic Information Systems and Science, 2001

  
  

• represent the quality on the map

• include reliability diagrams

New mapping technologies can extend our map analysis capabilities, but they can also extend and amplify errors and accuracy problems that we have always had to deal with in maps

As you engage in making maps keep in mind the complexities of map accuracy...

1. Are the Factual Details on the Map Accurate?

2. Are the Positions of the Mapped Features Accurate?

3. Is the Abstraction of the Mapped Features Accurate?

4. What is the Temporal Accuracy of the Map?

5. Is the Cartographic Symbolization Accurate?

6. Does the Detail of the Data vary over the Map?

7. Have Printing or Reproduction Problems Muddled the Look of the Map?

Pay attention to the ways in which map makers tell map users about accuracy

• symbolizing accuracy
  
• reliability diagrams
  
• metadata

Consider the consequences of technological change in mapping and the propagation and amplification of accuracy problems

E-mail: jbkrygie@owu.edu

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