Introduction to GIS
Lecture 2:
Part 1. Understanding Spatial Data
Structures
Part 2. Legend editing, choropleth
mapping and layouts
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Part 1. Understanding Spatial Data
Structures
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Perception, Semantics, and Space
• How do we deal with representing semantic
constructions of spatial objects, like “mountain,”
“river,” “street,” “city,”
• How about representing more conceptual semantic
constructions like “temperature,” “migration
pattern,” “traditional homeland,” “habitat,”
“geographic range,” etc?
• Answer: we have various data models which use
different abstractions of reality
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Introduction to GIS
Entities and Fields
• There are two general approaches for
representing things in space:
– Entities/ Objects: precise location and
dimensions and discrete boundaries
(remember, points are abstractions).
– Fields, or phenomena: a Cartesian
coordinate system where values vary
continuously and smoothly; these values
exist everywhere but change over space
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Introduction to GIS
Entities and Boundaries
• There are two general types of boundaries, bona fide
and fiat (D. Mark, B. Smith, A. Varzi)
• Pure bona fide boundaries represent real
discontinuities in the world, like roads, faults,
coastlines, power lines, rivers, islands, etc.
• Pure Fiat boundaries are a human cognitive or legal
construction, based on a categorization, such as
administrative unit, nation state, hemisphere
• Some have elements of both, like soil type areas
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Introduction to GIS
Two major data models
• Entity approach roughly corresponds with the
vector model
• Field approach roughly corresponds with
raster model
• Any geographic phenomenon can be
represented with both, but one approach is
usually better for a particular circumstance
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Introduction to GIS
Raster
•
•
•
•
•
•
Spatial features modeled with grids, or pixels
Cartesian grid whose cell size is constant
Grids identified by row and column number
Grid cells are usually square in shape
Area of each cell defines the resolution
Raster files store only one attribute, in the form of a
“z” value, or grid code.
• Consider the contrary….
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Introduction to GIS
Vector
• Vector layers either represent:
– Points (no dimensions)
– Lines, or “arcs” (1 dimension) or
– Areas, or “polygons” (2 or 3 dimensions)
• Points are used to define lines and lines are
used to scribe polygons
• Each point line or polygon is a “feature,” with
its own record and its own attributes
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Introduction to GIS
Raster and Vector representations of the same terrain
Raster: great for surfaces
Vector:
limited
with surfaces
All lecture material by Austin Troy (c)
2003 except where
noted
Introduction to GIS
Raster and Vector
representations of the same
land use
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Introduction to GIS
Raster and Vector
representations of the same
land use: closer in
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Introduction to GIS
Vector vs. Raster: bounding
Raster: bad with bounding
Vector:
boundary
precision
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(c) 2003 except where
noted
Introduction to GIS
Vector vs. Raster: Sample points
Cancer rates across space
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Introduction to GIS
Moving between vector and raster
• In Arc View and Arc GIS, we can covert vector layers
to grids, based on an attribute, or grids to vector layers
• The disadvantage of vector to raster is that boundaries
can be imprecise because of cell shape
• Each time you convert, you introduce more error too
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Introduction to GIS
WHEN TO USE RASTER OR
VECTOR???
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Raster data analysis is better for
representing phenomena:
• where boundaries are not precise
• that occur everywhere within a frame and can
be expressed as continuous numeric values
• where change is gradual across space
• where the attribute of a cell is a function of the
attributes of surrounding cells
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Introduction to GIS
Raster technical advantages :
• Simple file structure
• Simple overlay operations
• Small, uniform unit of analysis
Raster technical disadvantages :
• Big file size, especially for fine-grained data
• Difficult and error-prone reprojections
• Square pixels are unrealistic
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Introduction to GIS
Vector analysis is better :
•
•
•
•
Where there are definable regions
Where the relative position of objects is important
Where precise boundary definition is needed
Where multiple attributes are being analyzed for a
given spatial object
• For modeling of routes and networks
• For modeling regions where multiple overlapping
attributes are involved
• EG: units with man-made boundaries (cities, zip
codes, blocks), roads, rivers
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Introduction to GIS
Vector technical advantages :
• Smaller file size (in general)
• More graphically interpretable
• Allows for topology (see further on)
Vector technical disadvantages :
• Complicated file structure
• Minimum mapping units are inconsistent
between overlapping layers
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Introduction to GIS
Specific Vector Usages
• All legal and administrative boundaries (zip
codes, states, property lines, land ownership)
• Building footprints and 3-D models
• Roads
• Bedrock geology
• Pipelines, power lines, sewer lines
• Flight paths and transportation routes
• Coastlines
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Introduction to GIS
Specific Raster Usages
• Terrain modeling where micro-locational variability
is present and matters
• Groundwater modeling, where surface flow outside
of channels is important
• Representation of slope and aspect
• Representations of distance and proximity to
features
• Spatial representation of probabilities (logit)
• Modeling phenomena in nature with continuous
spatial variability and numeric attributes, like soil
moisture, depth to bedrock, percent canopy cover,
vegetative greenness index, species richness index
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Introduction to GIS
Tossups
• In many cases, though, the choice between
raster and vector may not be so clear.
• Often it depends on the application
• The following are some examples where you
could go either way:
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Terrain
• Vector-based models used for terrain,
including contours and TIN
– Problem: creates distinct terrain entities that
distort reality: terraces and triangular facets
• Raster based grids are more commonly used
– They are optimal for showing spatial microvariation in elevation although still have the
problem of being like miniature “steps”
– Lattices deal with this through interpolation
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Introduction to GIS
Soil
• Soil type: Vector
– Soil types are meant to represent discrete and
homogeneous areas and are qualitative. There is no
“slight gradation” between soil types like with pH
• Soil pH: raster
– pH is numeric, not categorical, and that number may vary
slightly within a single soil type polygon
– If pH were turned into categories, like High, Medium and
Low, vector might be better
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Introduction to GIS
Weather
• Weather station data: Vector, coded with points
• Average precipitation surface: Raster
interpolation of points
• Average precipitation contours: vector lines
• Both are interpolations, but one may be more
accurate in a given situation
• Downside of contours: terrace effect, fewer
intervals, more categorical
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Introduction to GIS
Rivers
• Most people think of a river as a discretely bounded
entity, hence vector
• What about where the river size fluctuates
seasonally, e.g. desert rivers?
• Or where the location of the river bed changes
slowly and gradually over the years
• Or where the river becomes delta, and the distinction
between “river” and “swamp” becomes fuzzy?
• Or where the river has a certain probability of
flowing or being dry at any given location and time
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Introduction to GIS
Rivers
• Depends on the type of analysis being done
• With vector can do network modeling of stream and
river system, but only in the arcs
– Vector stream model can take advantage of topologically
enabled analysis tools
• With raster, can do surface flow modeling
– More realistic, because when it rains water flows
everywhere, not just in channels, shows accumulation
– Think of every piece of land as mini stream channel
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Introduction to GIS
Metropolitan Areas
• No official administrative boundary for this
• Where does one metro area begin and another
end? Look at the New York New Jersey area.
• For a precise bounding, say for administrative
purposes, use vector
• Can also include “fuzzy boundaries”
• To represent a gradual change from one urban
area to another, use raster
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Introduction to GIS
Vegetation Mapping
• Vector works well for modeling vegetation stand type
where categories are broad, e.g. mixed conifer,
deciduous hardwood
• Raster works better where there is micro-locational
heterogeneity in species distribution
• Raster also works better for representing ecotones, or
edges between two stands
• The more specific and variable the classification, the
more likely the raster approach will be needed
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Part 2. Legend editing, choropleth
mapping, and layouts
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Visual Analysis
• The most intuitive form of vector analysis is
visual analysis, where we code features with
colors or symbols to deliver information
• Frequently, we code features by an attribute
value and let the color or symbol express the
attribute value
• Understanding legend editing and map
classification is critical to making maps that
effectively deliver information
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Mapping of Attribute Data
In GIS, each feature can have a number of
attributes attached to it (e.g. land parcel>>
property ID, assessed value, square footage)
We can map out these attribute values by their
corresponding geography
Two basic approaches for classifying the data:
1. Quantities approach
2. Category approach
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Introduction to GIS
Mapping of Attribute Data
Quantity approach: applies to numeric attributes
that are ordinal (have order to them); this
means one values is greater than or less than
another; good for continuous data.
Category approach: applies to categorical data,
where the categories can have, but don’t need
to have, order. If they do have order, the
category approach ignore that order
The same layer can have some quantitative and
some categorical attributes
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Introduction to GIS
Mapping of Attribute Data
Category approach, example: vegetation type
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Introduction to GIS
Mapping of Attribute Data
Quantity approach, example: population
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Introduction to GIS
Mapping Categories
This is the simplest type of mapping: we are
simply assigning a different color or symbol to
each feature with a given category value
Examples: vegetation types, land use, soil types,
geology types, forest types, party voting maps,
land management agency, recategorizations of
numeric data (“bad, good, best” or “low,
medium, high’). Can you think of any others?
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Mapping Categories
To map categories in ArcGIS, we simply double click on
the layer in the TOC and, in “layer properties,” click
on the “symbology” tab
Generally,we will choose “Categories>> Unique values”
The we choose our
values field that
contains the attribute
and then click the
“Add all values”
button
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Introduction to GIS
Mapping Categories
The symbology in the last slide gives us conservation
lands, categorized by type of ownership
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Introduction to GIS
Mapping Categories
Often categories must be aggregated and redefined: this land use
map had over 110 categories that were condensed to 12
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Mapping Categories
Do do this, we must group the “group values” function in the
symbology properties window
We can then give that
grouping a label
In this case 1262, 1263, 1264, 1265, etc. refers to different
subcategories of commercial land use
This classificationAllislecturesaved
when I save my ArcMap Document
material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Quantity Mapping
This is more complex, because there are so many
ways to map out quantities
Mapping options depends on the feature type:
• For points, lines and polygons, we can darken or
lighten the color to express magnitude: this is
called graduated color, or color ramping
• For lines and points we can increase symbol size
to express greater magnitude: this is called
graduated symbol; we can do this because
points and lines have fewer than 2 dimensions
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Choropleth Mapping
a thematic mapping technique that displays a quantitative
attribute using ordinal classes applied as uniform
symbolism over a whole areal feature. Sometimes
extended to include any thematic map based on
symbolism applied to areal objects.
-Nick Chrisman
A map that shows numerical data (but not simply
"counts") for a group of regions by (i) classifying the
data into classes and (ii) shading each class on the map.
-Keith Clarke
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
In Arc GIS layer
properties>>symbology, we choose
Quantities>>graduated color
We then choose a value to represent
In this case we choose
median house value
It automatically chooses
five classes for the data
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
The resulting map shows high housing value
areas with dark colors and low with light
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
In that case we used 5 classes. Changing the number of
classes changes the information delivered; more
classes: more info, but harder to see differences
3 classes
for median
value
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
In that case we used 5 classes. Changing the number of
classes changes the information delivered; more
classes: more info, but harder to see differences
15 classes
for median
value
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
The Classification
Method also
affects how the
mapped
attributes will
look. Arc GIS
normally
defaults to the
Jenks, or
natural breaks,
method
These are the breaks it makes, based on the
distribution of the data
small
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large
Introduction to GIS
Graduated Color
Now, here’s an
equal interval
approach. Notice
how all the breaks
are evenly spaced.
With a fairly
normal
distribution of
data, this is
usually OK
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
Here’s what the same
distribution looks
like with only 5
equal intervals.
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Introduction to GIS
Graduated Color
However, when the
distribution is skewed,
or there are significant
outliers, then equal
interval is problematic
because most intervals
have no data in them.
Here’s an example,
with number of vacant
houses per tract—most
have near none, but a
very few have a lot
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
This map of vacant
properties tells us
almost nothing,
because almost all the
records fall into the
first class
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Introduction to GIS
Graduated Color
Notice how with natural
breaks there are now
more classes on the
left side, where most
of the data are
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Introduction to GIS
Graduated Color
This map, made
with Natural
Breaks, is more
intelligible
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Introduction to GIS
Graduated Color
There is a similar approach to
Natural Breaks called
Quantile. This method sets
class boundaries so each
class has equal numbers of
observations in it
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
This essentially sets the
class boundaries so as to
maximize the perceived
variation in the map, as
we see here
Natural Breaks is similar,
but does not necessarily
result in an equal number
of data points in each
class; rather it uses Jenks'
Goodness of Variance Fit
(GVF) statistic
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Graduated Color
Graduated color can also be applied to points.
Here are houses display by sales price
Natural breaks
Equal interval
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Introduction to GIS
Graduated Symbol
Since points and lines are not dimensionally realistic, the
symbols representing them can also be graduated. Here
the size of the dot represents the house price
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Introduction to GIS
Graduated Symbol
The same thing can also be done with lines—for
instance, the width of a line feature showing rivers can
be made to represent the flow of that river segment.
For many line features, like streets, ArcGIS comes
preloaded with symbol palettes that recognize the
attribute codes and put the appropriate symbol
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Introduction to GIS
Symbol Styles
We can also choose to “match to symbols in a palette” and then
apply the “transportation.style” palette to the CFCC, or road
category, attribute in
our roads layer
Choose your style palette here
Must click
here to
match
Results in this map
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Symbol Styles
One could also manually create symbol styles for each street type.
Clicking on each symbol in either the TOC or properties
windows brings up a manual symbol selector. You can assign a
separate one to each category.
Includes
many
more
classes of
symbols
that are
industry
standar
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Introduction to GIS
Symbol Styles
There are also a huge variety of industry-specific point symbols
that can be either assigned through matching symbols to a
predefined style or manually assigning those symbols
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Introduction to GIS
Charts displayed geographically
Attributes for point, line or polygon features can also be
displayed as charts on the map
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Introduction to GIS
Normalization
With graduated color or symbol, we can also show an
attribute normalized by another attribute or expressed
as a percentage of total. Here we have number of
vacancies per tract as a percentage of total households.
Otherwise we’re only tracking total number.
numerator
denominator
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Introduction to GIS
Layouts
•
•
You can very simply create a map for layout
in Arc GIS by simply clicking
View>>Layout view.
Layouts are designed to cartographically
acceptable, which means they must have the
key elements of a printed map, such as scale
bars, north arrows, legends and titles. These
can be added from the Insert menu
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Layouts
•
Example
layout (from
lab 6)
title
North arrow
Scale bar
legend
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Introduction to GIS
Layouts
•
Legends are edited in the Legends property window,
which can be accessed by double clicking the
legends. Best way to learn about it is try it out
Legends can show layer
name as well as intervals
for quantitative data and
category names for
categorical data
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Layouts
•
You can change names of the layers for the sake of
your layout legend (since most layers have pretty
unintuitive names) in the layer properties window
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Layouts
•
In layouts you can have detailed and highly formatted
labeling and annotation. You can use an attribute field
to label; this is specified in layer properties
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
MXD Files
•
•
You can save your layout, along with all other
preferences and settings by saving an Arc Map
Document (MXD) file. However, this is not saving
your data, only the settings, including the layout. If
you move the MXD, you must move the layers
with it. This is one reason why a geodatabase is
easier than multiple shapefiles
To save, just go to File>>save as
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Layer Files
•
•
Layer (.lyr) files save all your settings and
preferences for one single file. It is primarily for
saving legend settings. So, for instance, if I a layer
with 300 land use categories, and I create a legend
classification that regroups them into 30 categories,
each with a special color or hatching, I can save
that as a layer file.
Once created, opening a layer file will open the
data layer with all the preferences saved. You can
move the data around without moving the layer file
as long as both are on the same system.
All lecture material by Austin Troy (c) 2003 except where noted
Introduction to GIS
Layer Files
•
This is done in Arc Catalog, by right clicking and
clicking “create layer.” Then I can create the legend
preferences in Arc Catalog
Then, double clicking in Arc
Catalog will give me the layer
properties, which can be changed
All lecture material by Austin Troy (c) 2003 except where noted