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Copyright © 2009 by Maribeth H. Price
2-1
Chapter 2
Mapping GIS Data
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Copyright © 2009 by Maribeth H. Price
2-2
Outline• GIS Concepts
– Map scale– Ways to map data– Classifying numeric data– Displaying rasters
• About ArcGIS– Map documents and data frames– Using ArcMap– Data frame coordinate systems– Symbolizing features– Symbolizing rasters
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Copyright © 2009 by Maribeth H. Price
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Map scale
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Copyright © 2009 by Maribeth H. Price
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How maps portray the world
Point features
Line features
Polygon features
Annotation features
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Copyright © 2009 by Maribeth H. Price
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Map scale
• Ratio of distance on the map to distance on the ground
• Dimensionless: cm or inches or mm…
1 cm on map = 100,000 cm on ground
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Copyright © 2009 by Maribeth H. Price
2-6
Talking about map scale
• A large denominator gives a small fraction a small scale map. It shows a large area.
• A small denominator gives a larger fraction a large scale map. It shows a small area.
1--------
50,000,000
1--------
500,000
1--------5,000
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Copyright © 2009 by Maribeth H. Price
2-7
Generalization and Scalehttp://encarta.msn.com/map_701515760/portsmouth.html
Polygons at one scale may be points or lines at a different scale
Large scale map
Intermediate scale map
Small scale map
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Copyright © 2009 by Maribeth H. Price
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Scale and precision
1:100,000 scale map3-pt highway symbol1-pt local road symbol
1 inch = 72 points
3 pts = 1 S 100000S = 300,000 pts S = 4166 inches
S= 347 feet
Let S be the size of the highway represented by a 3-point line symbol
So the highway location has an uncertainty of nearly 350 ft due solely to the symbol used to portray it. The local roads have an uncertainty of about 115 feet.
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Copyright © 2009 by Maribeth H. Price
2-9
Estimating precision from scale
• A 1-pt line is about 0.001 feet
• Map scale / 1000 gives approximate precision in feet for a 1-pt thick line
1:5,000 5 ft
1:24,000 24 ft
1:100,000 100 ft
1: 1 million 1000 ft
Larger symbols would have lower precision.
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Copyright © 2009 by Maribeth H. Price
2-10
Source scale and display scale
• Most GIS data have an intrinsic scale inherited from the source
• Display scale varies
1:24,000 USGS Topo Map (source scale)
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Copyright © 2009 by Maribeth H. Price
2-11
Display vs source scale
• Once in GIS data may be displayed at any scale, BUT
• Original scale of the map does impact the precision and accuracy of the data.
Original scale1:25 million
Original scale1:5 million
You should not display or analyze data at scales very different from the original source data.
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Copyright © 2009 by Maribeth H. Price
2-12
Scale and resolution
• Resolution is the sampling distance of the stored x-y values.
1:5M scale source 1:25M scale source
A larger scale map generally has a finer sampling distance and better spatial resolution. It can represent features with better precision.
Display scale approximately 1:500,000
Too fine a resolution wastes storage space and slows drawing—stores more points than needed at a particular display scale
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Copyright © 2009 by Maribeth H. Price
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Finding source scale
• Usually documented in the metadata– Scale of original paper map source– Scale or precision at which data were
gathered
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Ways to map data
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Copyright © 2009 by Maribeth H. Price
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Map Types and Data Types
• Single symbol maps• Unique values maps• Quantities maps
– Graduated color– Graduated symbol– Dot density
• Nominal data• Categorical data• Ordinal data• Interval and Ratio
data
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Copyright © 2009 by Maribeth H. Price
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Nominal data
• Names or uniquely identifies objects– State names– Owner of parcel– Tax ID number– Parcel ID Number
• Each feature likely to have its own value
• Usually portrayed on a map as labels
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Single symbol maps
• Display all features with the same symbol• Combine with labels to portray nominal data
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Categorical data
• Places features into defined number of distinct categories
• Category names may be text or numeric• Portrayed by different symbol for each category
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Unique values maps
• Different symbol for each category or value
Geologic unitsVolcano types Road types
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Types of unique values
Nominal dataUse to show different features
Categorical dataState subregionUse to show patterns
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Copyright © 2009 by Maribeth H. Price
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Ordinal data
• Places features into ranked categories or along an arbitrary scale– Low, Medium, High
slope– Village, Town, City– Assistant, Associate,
Full professor– Grade A, B, C, D, F
A 0-40%B 40-70%C 70-100%
Portrayed as categories but choosing variations in symbol size or color to indicate increase
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2-22
Interval or Ratio data
• Interval data places values along a regular numeric scale– Supports addition/subtraction– Temperature, pH, elevation
• Ratio data places values along a regular scale with a meaningful zero point– Supports addition, subtraction, multiplication,
division– Population, rainfall, median rent
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Copyright © 2009 by Maribeth H. Price
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Mapping numeric data
• Interval and ratio data must be divided into classes before mapping
• Mapped using variations in symbol size, thickness, or hue
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2-24
Classed mapsGraduated color map(choropleth map) Graduated symbol map
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Colors for choropleth maps
• Generally use change in saturation or close hues to indicate increase
• Avoid using too many colors which tend to mask increase
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Copyright © 2009 by Maribeth H. Price
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Normalizing classed maps
• If the size of the sample impacts the measured value, data should be normalized– By percent of total
• Percent of farms in each state• Percent of mobile homes in
each state
– By another field• Farms divided by area• Mobile homes divided by total
housing units
Number of farms
Number of farms per sq. mile
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Copyright © 2009 by Maribeth H. Price
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Unclassed mapsProportional symbol map Dot density map
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Chart Maps
Proportional chart map
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Symbol psychology
Where is the water?
Where is there less rain?Which towns have more people? What’s there?
Where’s the danger?
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Choosing symbols
Which one looks more aesthetic?Which one is easier to understand?Which one shows the roads better?
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Classifying numeric data
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Ways to classify data
• Choose number of classes• Variety of different classification methods
– Jenks Natural Breaks– Equal Interval– Defined Interval– Quantile– Standard Deviation– Manual (set your own)
• Best methods vary depending on data distribution
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Common data distributions
Value
Num
ber
of
sam
ples
Normal
Uniform
Skewed
Bimodal
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Jenks Natural Breaks
•Exploits natural gaps in the data•Good for unevenly distributed or skewed data•Default method, works well for most data sets
Class breaks
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Equal Interval
Specify number of classesDivides into equally spaced classesWorks best for uniformly distributed data
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Defined interval
User chooses the class sizeData determines number of classesWorks best for uniformly distributed data
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Quantile
Same number of features in each classMay get very unevenly spaced class rangesResults depend on data distribution
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Geometrical Interval
Multiplies each succeeding class boundary by a constantWorks well for normal and skewed distributions
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Standard Deviation
Shows deviation from meanUser chooses units e.g. 0.5 standard deviationsAssumes data are normally distributed