Transcript GIS definition

GIS definition
GIS is an information system that allows for capture, storage, retrieval,
analysis and display of spatial data.
Note that GIS is closely tied to the infrastructure that exists for creating
spatial data – satellites, census protocols, weather stations, land survey
techniques, and more.
Characteristics of GIS?
GIS
Computer
Hardware
Computer
Software
Data
People
Geographic Data
• Features must be
referenced to some
real world location
• Known as
georeferencing
What is Geodesy?
More simply, geodesy is the study of the
Earth’s size and shape.
The Earth is:
A Spheroid
Geoid -- The surface on which
gravity is the same as its strength
at mean sea level
An Ellipsoid
Types of projections
(a) Azimuthal (b) Cylindrical (c) Conic. Light source
positions, also called perspective positions, play an
especially important role in planar projections. (Different
aspects, such as polar, equatorial, or oblique, will also affect the map
projection)
Tangent vs. Secant Projections
Standard line
Standard line
Standard line
standard point/lines: on a projected map, the
location(s) free of all distortion at the exact point
or lines where the surface (cylinder, cone, plane)
touches the globe.
Geographic Coordinate System
• This is a Global Coordinate System
• Based on angles on the ellipsoidal Earth
latitude
positive in n. hemisphere
negative in s. hemisphere
longitude
positive east of Prime Meridian
negative west of Prime Meridian
Cartesian Coordinates
Computationally, it is much simpler to work
with Cartesian coordinates than with
spherical coordinates
x,y coordinates
referred to as “eastings” & “northings”
defined units, e.g. meters, feet
UTM Zone Projection
Transverse Mercator Projection
applied to each 6o zone to
minimize distortion
Universal Transverse
Mercator
• Each half has a different
y-origin:
• For the Northern half of
the zone, the y-origin is at
the Equator
– Northing at equator is
0m
• For the Southern half of
zone the y-origin is
10,000,000 meters south
of the Equator
– Northing at equator is
10,000,000m
UTM
• The central meridian, which runs
down the middle of the zone, is used
to define the position of the origin
• Distance units in UTM are defined
to be in meters, and distance from
the origin is measured as an
Easting (in the x-direction) and a
Northing (in the y-direction)
• The x-origin is west of the zone (a
false easting), and is placed such
that the central meridian has an
Easting of 500,000 meters
UTM vs SPCS
• SPCS
– More accurate than UTM
– used primarily for engineering applications, e.g. utility
companies, local governments to do accurate
surveying of facilities network (sewers, power lines)
– Used for small areas
– Difficult to use over larger areas (when multiple zones
are necessary)
• UTM
– allows overlap between zones for mapping purposes
– The UTM system is global
Terminology
• Data Object -- digital representation of an
entity
Pasture
House
Road
Review
Topology
• The relationships between data objects in
space.
2 GIS Data Models
• Entities in the real world are represented
as one of the following in a GIS:
– Vector data:
• Points
• Lines
• Areas (or polygons)
– Raster data
• Pixels in an array
Key concept!
Raster data model (details later)
rows
The raster data model represents the Earth’s surface as a
two-dimensional array of grid cells, with each cell
having an associated value:
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columns
Cell (x,y)
Cell value
Cell size = resolution
Vector Data Objects
Geographic building blocks
• Points
– 0 dimensional
• Lines
– 1 dimensional
• Polygons
– 2 dimensional
Thought question:
How are you going to represent the
California OAK tree in digital format?
A point? A polygon? Or a pixel?
It will depend on:
- Scale of observation
- Purpose of your research
- The type of data you have access to in the GIS
Topology: Connectivity
Connected arcs are determined by searching through the list for common
node numbers.
10
①
11
②
Arc-node list
12
③
⑤
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④
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15
Arc
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From-Node
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To-Node
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15
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Because of the common node 11, arcs 1, 2, and 3 all intersect. The computer
can determine that it is possible to travel along arc 1 and turn onto arc 3. But it
is not possible to turn directly from arc 1 to arc 5.
Topology: Contiguity
Two geographic features which share a boundary are called
adjacent. Contiguity is the topological concept which allows
the vector data model to determine adjacency.
An Arc
left
From-Node
Direction
right
To-Node
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B
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Arc
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Left
Polygon
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Right
Polygon
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?
Raster Data Model - Lines
A line = a series of connected cells that portray length
Problems with this representation?
• Line may be narrower than pixels show.
• e.g. Road layer: how much of a road needs to be
in a cell in order to be considered a ‘road’ cell?
Raster Data Model - Areas
Area = a group of connected cells that portray a shape
What problems could we have with this representation?
• What if a lake’s edge falls in a pixel?
Cell Size & Resolution
• The size of the cells in the raster data
model determines the resolution at
which features can be represented
• The resolution can have an effect on
which features are represented in what
locations:
10 m Resolution 5 m Resolution 1 m Resolution
Rules for Assigning Cell Values
•Cell values can be assigned to cells accorded to
some set of rules, and selecting those rules
differently can also effect the representation of
features:
Raster Data Storage –
Run Length Encoding
This approach takes advantage of patterns in the data,
taking advantage of the repetition of values in a row:
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row
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0, 2, 1, 6, 0,2
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0, 10
0, 10 45 values
There is a tendency towards spatial autocorrelation; for
nearby cells to have similar values - values often occur
in runs across several cells
Vector Data Model - Advantages
• It is a good representation of the world as we see it
(our visual systems automatically segments the
world we see by identifying objects)
• The topology of a layer can be fully described and
explicitly stored
• It is efficient in terms of data storage
– It only uses storage for objects of interest and
does not need to store values for the spaces in
between
• No jaggy edges (raster has these on any diagonal)
• Useful for network analysis and modeling flows of
linear features
Vector Data Model Disadvantages
• The data structure is more complex
– especially when you have fully encoded
topology (i.e. using the arc-node model)
• It is more difficult to write computer
programs to manipulate data
• Spatial analysis operations can be more
difficult
Raster Data Model - Advantages
• The data structure is much simpler
• It is easy to overlay and combine layers
• This model better represents continuous
data
• Raster data is easily integrated with
satellite (and other remotely-sensed) data
• Writing programs to manipulate raster is
easier
• It is easy to do simulation modeling due to
uniform size and shape of grids (i.e. it is easy
to define uniform modeling units)
Raster Data Model Disadvantages
• Because a value must be stored for each
and every cell in a grid, there is a great deal
of redundancy and large storage
requirements
• Location can be captured only as
accurately as the resolution allows, which
is determined by the cell size
• Spatial analyses that are based on
topological relationships are not well
supported by this model
Intended use
•
Scale of your data must be chosen
based on:
1. Your data needs
2. The intended use of the existing data
Vector setting: attribute data:
Column = property
Row = object
rows
Raster attributes: cell values
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Cell (x,y)
Cell value
Attribute data
Three types of data:
• Nominal scale
• Ordinal level
• Interval and ratio levels
Spectral Resolution of Landsat TM
These bands provide a coarse summary of spectral signatures.
TM Band: 1
2
3
4
5
7
Remote Sensing
• Remote sensors are devices that sense
energy from a remote location (i.e., a
device not in physical contact with what it
is sensing)
• Remote sensing is the science of
acquiring, processing and interpreting
information/data collected by remote
sensors.
Remote Sensing
• Active – emit energy and detect reflections
– Sonar
– Radar
– Lidar
• Passive - detect emitted/reflected energy from
other sources
–
–
–
–
Satellite sensors
Air photos
Cameras
Video recorders
Satellite Imagery
• Digital data is obtained by sensors on satellite
platforms.
Satellite Imagery
• Described by five resolutions
– Spatial resolution: area on ground represented by each pixel
•
•
•
•
Advanced Very High Resolution Radiometer (AVHRR) – 1 km
Landsat - 30m
SPOT – 2.5m - 20m / 2.5m - 10m
IKONOS - 1m/4m
– Temporal resolution: how often a satellite obtains imagery of a
particular area
– Spectral resolution: specific wavelength intervals in the
electromagnetic spectrum captured by each sensor
– Radiometric Resolution: number of possible data values reportable
by each sensor (how sensitive the sensor is to changes in
brightness of objects that it views)
– View angle resolution: the number of angles at which the ground
objects are recorded by the sensor.
Multispectral Imagery
Display
Band Composite Output =
Color Guns =
Landsat TM Band =
2
4
7
Band Combination =
BLUE
GREEN
RED
1
2
3
NEAR IR
4
SHORT
WAVE IR
5
MIDWAVE IR
7
LONGWAVE IR
6
Color Composite Image
Color composite image
Band A
Blue color gun
Band B
Band C
Green color gun
Red color gun
Deforestation
Evidence of Tropical Forest Change
Rondonia
Brazil
MSS (80 m) 19 June 1975
MSS (80 m) 1 August 1986
TM (30 m) 22 June 1992
Change Detection
Mount St. Helen’s National Monument Park
Landsat TM 9/30/87
IRS LISS-2 9/6/94
The two left images (one from 1987 and the
other from 1994) were merged for Change
Detection, to show revegetation patterns since
the 1980 eruption of Mt. St. Helen. The
resulting image (above) shows places where
revegetation has taken place (in green) and
places where timber has been harvested (in
red). Gray areas indicate no change.
Flood Assessment
Landsat TM (SWIR, bands 7-4-2)
Damage
Assessments
Extent of damage is assessed from imagery…
tornado path in yellow
Buildings with structural damage
are identified and outlined
Contrast Enhancement
example:
• A linear stretch is one of the most common types of
contrast enhancement
• Minimum BV is remapped to 0
• Maximum BV is remapped to 255
0
0
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158
127
255
255
sun
Scattering
Scattering occurs when particles or large gas
molecules present in the atmosphere interact
with and cause the electromagnetic radiation
to be redirected from its original path.
The sky is blue because of Rayleigh Scattering. Shorter
wavelengths (i.e. blue) of the visible spectrum are scattered
more than longer visible wavelengths.
So why isn’t the sky indigo or violet?
Atmospheric Windows
Those areas of the spectrum which are not severely
influenced by atmospheric absorption and thus, are
useful to remote sensors, are called atmospheric
windows.
Geometric Correction
•
Four Basic Steps of Rectification
1.
2.
3.
4.
Collect ground control points (GCPs)
“Tie” points on the image to GCPs.
Transform all image pixel coordinates using mathematical
functions that allow “tied” points to stay correctly mapped to
GCPs.
Resample the pixel values (BVs) from the input image to put
values in the newly georeferenced image
Cartographic Communication
• Success or failure on the part of the
cartographer depends on whether or not
the map communicates the intended
information.
Uses of Maps
• To record and store information for reference.
• To analyze locational distributions and spatial
patterns.
• To present information and communicate
findings (often to help facilitate decisionmaking).
Thematic vs. Reference
• Thematic maps
– spatial distribution of particular variables
– Often created by GIS analysts
• Reference maps
– USGS Topo maps, navigation charts, tax
maps
– Generally created by large mapping
organizations
Neat line
Map Elements
Border
Title
Map/figure
Legend
Scale
Inset
North Arrow
Text:
Projection/source
Shape
• Differences in forms of symbols
• Options:
– abstract (e.g circle, square)
– iconographic (e.g. airplane, flag, schoolhouse)
• Caution: too many different shapes can get very
busy and difficult for the map reader to
discriminate
Utility
Ordinal
Ratio
Nominal
Bad 
Bad 
Good 
Hierarchical Organization
• Subdivisional
– Portrays the internal divisions of a
hierarchy
– Example: Regions of North Carolina
Western
Mountains
Piedmont
Coastal Plain
Chloropleth
Source: http://www.gis.psu.edu/geog121/pop.html
U.S. Census Data
The U.S. Census acquires data from households about:
• Demographic info
•Race
•Population
•Disabilities
•Migration
• Economic info
•Living quarters
•Occupation
•Income
• Social info
•Languages spoken
•School enrollment
•Family structure in the home
•Marital status
•Ancestry
•Foreign-born population
U.S. Census Data & TIGER/Line Files
• TIGER/Line files:
– The “geography” of the census
• TIGER designed to:
• support pre-census functions in preparation for Census
of Population and Housing
• support census-taking efforts
• evaluate success of the Census
• provide geographic framework for analysis of Census
data
TIGER line and address data
• Roads
– attributes
include basic
road type,
address ranges