Level of Data Measurement Thematic Mapping Data...
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EnvSci 360 Computer and
Analytical Cartography
Lecture 4
Level of Data Measurement
Thematic Mapping
Data Classification
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EnvSci 360 - Lecture 4 2
Types of Maps
Data always plays a role
Maps – no matter what type -- are used to visualize measurements of:
– Spatial Data
• Entities
– AspatialData
• AttributesLink together
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EnvSci 360 - Lecture 4 3
Qualitative
Quantitative
Level of Measurement
Nominal
Ordinal
Interval
Ratio
Ways to
organize data
-- Affects
analysis and
symbolization
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EnvSci 360 - Lecture 4 4
Level of Measurement: Nominal
Distinguish features based on qualitative considerations, i.e. differences in kind– Establish identity of features
– Assign items to groups or categories based on type (not on ordered value – no ranking)
– Examples: hospital ER status, land use
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EnvSci 360 - Lecture 4 5
Level of Measurement: Ordinal
Implies value by sequence or rank(good, better, best; low, medium, high) – order by some quantitative measure– Intervals between
the numbers are not necessarily equal
– No magnitudeof difference;not measurable
– Symbology must reflect the quantitative hierarchy
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EnvSci 360 - Lecture 4 6
Level of Measurement: Interval
Differences between values is measurable but
no absolute zero (may be based on a
standard “starting point”)
– Can’t say how many
times higher one
value is than another
– Example: F or C
temperature scale
• 30 degrees F is
15 degrees higher
than 15 degrees F,
but not twice as
warm
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EnvSci 360 - Lecture 4 7
Level of Measurement: Ratio
Differences between values is measurable(based on weight, length and area)
Has a true zero (no features or sea level, e,g.)
– Examples: bathymetry, population (totals or density)
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EnvSci 360 - Lecture 4 8
Level of Measurement
Comparison using same symbols, but different levels of measurement
Ordinal
Interval(Range-graded)
Ratio
MegaLargeMediumSmall
Legend of Population of Cities
250k - 500k100k - 250k70k - 100k50k - 70k
500k250k100k70k
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EnvSci 360 - Lecture 4 9
Level of Measurement
NOTE: Nested in one direction only
– data at higher levels can be reduced or generalized to lower levels, but not vice versa
– on maps, data can be displayed below, but not above, their measurement level
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EnvSci 360 - Lecture 4 10
Nature of Geographic Phenomena
Discrete
– Individual items at particular locations
– Empty intervening areas or zero value
– Examples - houses, monitoring stations, roads, calderas
Continuous
– No location is empty
– Surfaces may be smoothed or stepped
– Examples - land use, temperature, elevation
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EnvSci 360 - Lecture 4 11
Absolute vs. Derived Data
Absolute - “raw” data values
– Single class of features
– Uses appropriate measurement scale
– Examples:
• land use
• temperature
• soil types
• elevation
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EnvSci 360 - Lecture 4 12
Absolute vs. Derived Data
Derived– Obtained from raw data
– Summarization or relationship between features (averages, ratios, densities, and potentials).
• population density, average temperature, per capita income, etc.
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Thematic Maps
Let’s take a look at how levels of measurement, discrete and continuous data, and absolute and derived data come in to play when designing thematic maps
EnvSci 360 - Lecture 4 13
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EnvSci 360 - Lecture 4 14
Thematic Maps
Known as “special purpose” maps
Display aspatial data spatially (by linking them together)
Focus on a
feature and
display its
spatial pattern
There are a
number of
mapping
techniques for
displaying
thematic
information
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EnvSci 360 - Lecture 4 15
Thematic Maps
Consider whether data are Qualitative or Quantitative
Qualitative Quantitative
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EnvSci 360 - Lecture 4 16
Qualitative vs. Quantitative Data
The graphic language used to
represent information about
the world, whether reference or
thematic, qualitative or quantitative,
consists of point, line, and area
symbols.
Distinguish differences in data with cartographic “visual variables”
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EnvSci 360 - Lecture 4 17
Qualitative Data Maps
Based on unique numeric or character fields in attribute table
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EnvSci 360 - Lecture 4 18
Qualitative Data Maps
Classification method using color hue and shape and texture to symbolize different categories of data
Areas
Surficial Geology Map Units
Lines
Linear Water Features
Points
- Can take the form of dots, circles, letters, icons
School Type
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EnvSci 360 - Lecture 4 19
Qualitative Data Maps
Types of symbols– Geometric Symbols
• Dot
• Line
• Solid- or line-filled area
– Mimetic Symbols• Pictorial, look like the feature
• Must be unambiguous
• Not too much detail if small
• Don’t be too whimsical – may lose detail
• You can create your own and combine existing symbols together
� � � � �
� �
� � �
Proposed
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EnvSci 360 - Lecture 4 20
Qualitative Data Maps
Qualitative thematic maps are also known as descriptive maps. Some examples:
– Region Maps
– Resource Maps
Boundaries based on pre-defined areas (e.g., states, Census Blocks) Boundaries defined by spatial distribution of data
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EnvSci 360 - Lecture 4 21
Qualitative Data Maps
More resource-based thematic maps
Two more examples where the boundaries between categories are defined by the spatial distribution of the data,
independent of political or other administrative areas.
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EnvSci 360 - Lecture 4 22
Qualitative Data Maps
You can even use picture-fill symbols
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EnvSci 360 - Lecture 4 23
Quantitative Data Maps
Based on numeric attribute data, grouped into statistical categories (classes)
– Not every unique value is mapped
World Infant Mortality Rates, 2008
Deaths/1000 Live Births
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EnvSci 360 - Lecture 4 24
Types of Quantitative Data Maps
Choropleth– Use a uniform color or pattern to fill an area based on pre-
defined zones (e.g., states, counties, Census boundaries)
Isopleth– Use color to fill areas defined by isolines (lines
representing equal values)
Graduated symbol, Proportional symbol– Use scaled symbols (points or lines) in order to indicate
the relative quantity of a particular data attribute
Chart/Graphs– Use pie charts or bar graphs to represent values of a pre-
defined zone• May also be proportionally sized
Dot (dot density)– Use a fixed size dot symbol to represent a fixed quantity of
data
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EnvSci 360 - Lecture 4 25
Choropleth Maps
Use of reporting zones (areal units) to symbolize volume
– Zones are independent of (not defined by) data
– Types of attributes:
• population density
• mortality rates
• average income
• Etc., etc., etc.
– Use normalized,not raw datavalues (divide one field by another), or group values
Per Capita Income for Connecticut Towns, 1998
Per Capita Income13,271 - 22,69522,696 - 29,23529,236 - 38,48138,482 - 53,84853,849 - 75,714
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EnvSci 360 - Lecture 4 26
Choropleth Maps
As long as you can link
data (aspatial numeric
attributes) to your
area of interest
(spatial component)
using common fields
(like State abbrev.,
Town ID, etc.) you can
group and map them
From Choros - place,
Pleth - value
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EnvSci 360 - Lecture 4 27
Choropleth Maps
Values usually follow the convention of darker meaning more and lighter meaning less
You would normally use actual values unless text is meaningful and understood by a map reader.
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EnvSci 360 - Lecture 4 28
Choropleth Maps
Considerations of classed choropleth mapping– Size and shape of unit areas
• Better if small
• Better if equal size
– Number of classes• Consider maximum
that can be easily read
• Depends on monochromatic or color
– Determining class limits
• Should highlight critical values
• Beware of biased views
Missouri
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EnvSci 360 - Lecture 4 29
Choropleth Maps
Comparison of raw vs. normalized data (ratio mapping, derived data)
Raw values
Normalized
Provides a truer picture of actual phenomena
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EnvSci 360 - Lecture 4 30
Classification Schemes
Ways to form the classes:Natural Breaks – based on natural groupings inherent in the data
Equal Interval – data divided into equal-sized ranges, based on set number of classes
Defined Interval – user defines the interval, which determines number of classes
Quantile – each class contains an equal number of features
Standard Deviation – shows how much an attribute value varies from the mean
Manual – user defines class breaks manually
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EnvSci 360 - Lecture 4 31
Classification Schemes
Base the scheme on the data you’re using
Look at the distribution of values (view the histogram in ArcMap) –unique to each dataset
It’s often difficult to distinguish more than7 classes
Histogram view in ArcMap
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EnvSci 360 - Lecture 4 32
Classification Schemes
Comparison of four choroplethclassification schemes to map the same data
The method you choose can greatly affect the appearance of the map.
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EnvSci 360 - Lecture 4 33
Classification Rules
Each class interval is unique
– No data value can be in more than 1 category
– Use 1-2, 3-4 or 1-2.9, 3-3.9 (not 1-2, 2-3, etc.)
No gaps should exist between class intervals
– Even if values don’t exist (but may be overridden with ArcMap’s “Use data values” checkbox)
The number 0 (zero) is often a valid value
– The absence of something can be as meaningful as its presence
The classification scheme should make sense
– Should be consistent and use understandable categories and intervals
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EnvSci 360 - Lecture 4 34
Choropleth Maps
Visual effects
Choropleth maps with gray and color shading
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EnvSci 360 - Lecture 4 35
Choropleth Maps
Visual effects
Choropleth maps with and without boundaries
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EnvSci 360 - Lecture 4 36
Choropleth Maps
Visual effects
Choropleth maps at different scales
Small scale Large scale - easier to see shading differences when using outlines
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EnvSci 360 - Lecture 4 37
Choropleth Maps
Visual effects
Classless - show gradual changes
Simple - stick to the boundary
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EnvSci 360 - Lecture 4 38
Choropleth Maps
Dealing with small polygonsNice use of labels –with representative colors – for small, crowded states
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EnvSci 360 - Lecture 4 39
Choropleth Maps
Legend that does not show actual values
– Just shows general pattern
– Nor does the map give any indication as to what “normal” is
August 2012
OK for a quick graphic
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EnvSci 360 - Lecture 4 40
Choropleth Maps
No legend at all!
– User must read text under the title
– Also, no units on scale bar
Don’t do this!
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EnvSci 360 - Lecture 4 41
Choropleth vs Dasymetric Maps
Dasymetric - mapping unit boundaries are different from collection units– “Density measuring”, still with homogeneous areas
– Use other information, sources to realistically place symbols
– Very useful in showing spatial trends
Choropleth map Dasymetric mapSame population data,different mapping methods
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EnvSci 360 - Lecture 4 42
Isopleth Maps
Ideal when your data values vary continuously and smoothly over space
– Temperature, Rainfall, Elevation, etc.
Boundaries not based on pre-defined zones
Aka “isarithmicmapping”
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EnvSci 360 - Lecture 4 43
Isopleth Maps
Based on interpolated values of known point data locations with X,Y,Z values
Results in a generalized view of the data (the more known points the better)
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EnvSci 360 - Lecture 4 44
Isopleth Maps
Example shown with isolines displayed
Same area shown in 3D
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EnvSci 360 - Lecture 4 45
Isopleth Maps
Kinds of isarithmic mapping
– Values that can occur at points - isometric lines
• Actual - temperatures, spot heights
• Derived - averages, medians, ratios => referenced to actual values at points
– Values that cannot occur at points - isopleths
• Actual - “grouped-together”values (people in a city)
• Derived - only for an area as a whole (people per sq. mi.)
• A point is assigned to a location (random, or specific, like centroid)
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EnvSci 360 - Lecture 4 46
Graduated Symbol Maps
Symbols on map match fixed sizes in legend
The sizes of the circles on this map are the same as the five in the legend
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EnvSci 360 - Lecture 4 47
Proportional Symbol Maps
Symbols in legend are representative of sizes of symbols on the map. Shown here are point symbols.
The sizes of the circles on this map can be any size, based on the range in the legend
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EnvSci 360 - Lecture 4 48
Proportional Symbol Maps
Symbols in legend are representative of sizes of symbols on the map. Show here are line and point symbols.
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EnvSci 360 - Lecture 4 49
Chart/Graph Maps
Pie Charts
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EnvSci 360 - Lecture 4 50
Chart/Graph Maps
Bar Graphs
Can be tricky to display if the charts are big and the polygons are small
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EnvSci 360 - Lecture 4 51
Dot Density Maps
Concerned with the relative geographical crowding/clustering or sparseness of discrete phenomena
– People, animal or plant species, crimes, business locations, etc...
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EnvSci 360 - Lecture 4 52
Dot Density Maps
Success of a dot map
– Size of dots - not too small, not too big
– Unit value of a dot
• not too low, not too high
• usually > 1
– Location of the dots
• center of gravity?
• equal distribution?
• use other references
– You can mask areas where phenomena don’t occur (e.g. water bodies)
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EnvSci 360 - Lecture 4 53
Multi-variable Mapping
Different quantitative variables are shown on one map, with different symbolization
This map uses choropleth and dot density mapping techniques to portray the H3N2 swine epidemic situation in Ontario in 2005.
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EnvSci 360 - Lecture 4 54
Multi-variable Mapping
This map uses shaded polygons using two color gradients, one for each candidate, plus symbols indicating percent of voter turnout.
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EnvSci 360 - Lecture 4 55
Multi-variable Mapping
This map uses pie chart colors to represent increases and decreases in violent crime.
Further, the pies are broken into pieces, one for each crime category -murder, forcible rape, armed robbery, and aggravated assault.