Map Accuracy and Uncertainty
Map accuracy – Closeness of the map representation to the geographic phenomena.
Basic Characteristics of Maps
Maps are graphical representation of a geographical setting; a collection of symbols used
to represent a portion of the earth.
- All maps are concerned with two primary elements – locations and attributes.
- All maps are reductions of reality and therefore require scale conversions.
- All maps are transformations of space involving applications of map
projections and coordinate systems.
- All are also abstractions of reality and therefore require generalization of the
- All maps use signs and symbolism; this is called cartographic symbolization
Maps must maintain their connection to—and present a useful representation of—reality.
This has to be balanced against scale, projection, generalization, and symbolism
requirements. Some degree of representational quality has to be sacrificed.
Scale and Accuracy
- The ability to show detail on a map is determined by its scale because that dictates
the amount of space available.
- When features are displayed in smaller spaces, distances. between the symbols
and lengths of features are reduced.
- When features become too small to see, everything else becomes small and
- Visual chaos can result.
- Cartographers have to take decisions carefully in making maps.
- A large scale map of 1:1,000 will show less area with more detail than a smaller-
scale map of 1:250,000
- All maps are abstractions of reality, they require reduction of information content
in order for features to be legible.
- Cartographic Abstraction is the process of transforming data that have ben
collected about our environment into a graphical representation of features and
attributes relevant to the purpose of the map.
- To transform geographic data to a map, cartographers rely on selection,
generalization, classification, and symbolism.
o Selection: how much information to portray
o Generalization and classification: they eliminate or deemphasize unwanted
or unneeded detail
o Symbolization: how the information will be shown graphically
- Cartographic selection refers to deciding which classes of features to show on the
map. (Based on purpose of map)
o Example: on large-scale topographic map. You will usually find perennial
streams (those that flow a large portion of the time but cease to flow
occasionally or seasonally) and ephemeral streams (those that flow only
during and immediately after periods of rainfall or snowmelt).
o On smaller scale maps, the above will be eliminated.
- Cartographic generalization refers to reducing the amount of information on a
map through change to the geometric representation of features.
- The degree of generalization is proportional to a features spatial detail, and
inversely proportional to map scale.
- Loss of detail line generalization for features like boundaries, river, and roads.
o This also causes displacement of features as smoothing cuts off natural
irregularities or causes features to overlap.
o Causes Positional error
o Think of Generalization effects not errors
Vector Generalization operations
- Variety of vector generalization methods
- Source scale – the scale at which the data originally derived from their sources.
- Common vector generalization methods:
o Simplification – selectively reducing the number of points required to
represent an object
o Smoothing – Reducing angularity of angles between lines
o Aggregation – grouping point locations and representing them as areal
o Amalgamation -- grouping of individual areal features into a larger
o Collapse – replacing an objects physical deatails with a ymbol
representing the object
o Merging – grouping of line features
o Refinement – selecting specific portions of an object to represent the
o Exaggeration – to amplify a specific portion of an object
o Enhancement – to elevate the message imparted by the object
o Displacement – separating object
Map accuracy closeness of the map representation to the geographic phenomena. Maps are graphical representation of a geographical setting; a collection of symbols used to represent a portion of the earth. All maps are concerned with two primary elements locations and attributes. All maps are reductions of reality and therefore require scale conversions. All maps are transformations of space involving applications of map projections and coordinate systems. All are also abstractions of reality and therefore require generalization of the geographic information. All maps use signs and symbolism; this is called cartographic symbolization. Maps must maintain their connection to and present a useful representation of reality. This has to be balanced against scale, projection, generalization, and symbolism requirements. Some degree of representational quality has to be sacrificed. The ability to show detail on a map is determined by its scale because that dictates the amount of space available.