Lecture 7 (Georeferencing) Flashcards
Georeferencing
- Assigning location on the Earth’s surface.
- > Essential in GIS, b/c all information must be linked to a specified geographic location
Types of Georeferencing
By place name By zip codes By IP addresses By street address – linear reference By grid system – Cadasters and PLSS By XY coordinates -> Longitude and Latitude -> Universal Transverse Mercator (UTM) -> State Plane Coordinates (SPCs)
Placenames
Assign Names to Places
- The simplest, earliest and the most commonly used form of georeferencing
- Many names of geographic features are universally recognized
-> Others may be understood only by locals
- Names work at many different scales
– Spatial resolution is varied and coarse
-> From continents to small villages and neighborhoods
- Names may pass out of use in time
-> Where was Camelot?
-> In the English legends of King Arthur’s location
Which Springfield?
Zip Code (Area or Point)
Postal addresses and Postcodes
- Improve the effectiveness of mail delivering
- Assumptions
-> Every dwelling and office is a potential destination for mail
-> Dwellings and offices are arrayed along streets,
and numbered accordingly
-> Streets have names that are unique within local
areas
-> Local areas have names that are unique within
larger regions
IF THESE ASSUMPTIONS ARE TRUE, THEN A POSTAL ADDRESS IS A USEFUL GEOREFERENCING
Where Postal Addresses failed as georeferences?
- In rural areas
- > Urban-style addresses have been extended recently to many rural areas
- For natural features
- > Lakes, mountains, and rivers cannot be located using postal addresses (PLSS)
- When numbering on streets is not sequential
- > i.e., in Japan, the street numbering reflects the date of construction – temporal georeferencing
Postal Codes
- Used and defined in many countries in order
to simplify the sorting for mail delivering.
-> E.g. ZIP codes in the US - Hierarchical structure
-> The first few characters define large areas
Subsequent characters designate smaller areas
Coarser spatial resolution than postal address
Useful for mapping
OR: 97***
97001: Wasco County
97002: Marion County
97006: Beaverton, Aloha, Hill
OUTWARD CODES for the Southend-on-Sea, UK, Local Delivery
Offices (LDOs). Outward codes form the first two, three, or four
characters of the UK postal code and are separated from the
three characters that identify the Inward Codes
IP Address (Internet Protocol)
- Every device (computer, cell-phone, etc.)
connected to the Internet has a unique IP - Determine the user’s location when user’s
computer is used to access a web site. - Ex: 172.200.255.164 (Binary numbers)
- Many Web services offer conversion between
IP address and geographic coordinates.
Street Address
Linear Referencing - A system for georeferencing positions on a street, highway, railroads, or river network. - Identifies locations on a network by measuring distance from a defined point (ex: intersection of roads) along a defined path (ex: road) in the network - Simple Linear Interpolation –US Street Style • Odd number: Right side of road • Even number: Left side of road (When driving from the beginning to the end of the road)
Users of Linear Referencing
Linear referencing systems are widely used in managing transportation infrastructure and in dealing with emergencies.
- Transportation authorities
-> To keep track of pavement quality, signs,
bridge, and traffic conditions on roads
• i.e., Exit 149 (mile) of the highway I85 - Police or Ambulance
-> To record/report the locations of accidents
Grid System (Cadasters and the U.S. Public Land Survey System)
Cadasters
- Maps of land ownership, showing the boundaries of property for taxing lands.
- The Public Land Survey System (PLSS) in the US and similar systems in other countries (i.e., Canada) provide a method of georeferencing linked to the Cadasters
- In the Western US, the PLSS is often used to
record locations of natural resources, e.g. oil and gas wells. SEE SLIDE
PLSS
Public Land Survey System
- Regulated by the U.S. Department of the Interior, Bureau of Land Management (BLM).
• Lands in the public domain are subject to subdivision by this rectangular system of surveys.
Professional Land Surveyors
SEE SLIDE
PLSS - Township and Range System
The offset shown between townships 16N and 17N is needed to accommodate the Earth’s curvature (shown much exaggerated)
See SLIDE
Within A PLSS Section
- The section is 1x1 square mile = 640 acre
- Locate the land by reading the subdivision information backwards.
¼ square mil or 160 acres
SEE SLIDE (PLSS EXAMPLE IN W.I.)
XY Coordinates
Approximations of the Earth’s Shape
Geodesy - the science of earth measurement.
Geoid—earth shape, an equipotential surface of the earth’s gravity field, which closely approximates mean sea level and is by definition perpendicular to the direction of gravity at
all points.
Representations of the Earth (Mean Sea Level is a surface of constant gravitational potential called the GEoid)
SEE SLIDEs
Datums
- Datum –defines the size and shape of the earth
and the origin and orientation of the coordinate
systems used to map the earth. - Ellipsoidal Datum; Orthometric Datum; Tidal
Datum
SEE SLIDE
Ellisoidal Datums
Most Commonly Used Earth Datum in the U.S.
NAD 27 (North American Datum of 1927) uses the Clarke (1866)
ellipsoid on a non geocentric axis of rotation and has its center at
Meades Ranch, Kansas. (based on field survey)
NAD 83 (North American Datum of 1983) uses the GRS80 (Geodetic
Reference System of 1980) ellipsoid on a geocentric axis of rotation
(based on satellite measurement)
Conversion between two datums (NAD27 and 83) are needed and the
differences can be as much as 100 m
WGS 84 (World Geodetic System of 1984) uses WGS 84 ellipsoid and
reference GRS 80 ellipsoid, almost the same as NAD83 (LiDAR units use
WGS 84)
The International Terrestrial Reference Frame (ITRF) of 1994, 1996,
1997, 2000, 2005, and 2008.
NAD27
(a very common horizontal datum – “old” data)
Meades
Ranch,
Kansas
Ellipsoid
Center
Mass Center of Earth
Uses the Clarke 1866 Ellipsoid which minimizes error between the
ellipsoid and the geoid at Meades Ranch, Kansas. (The center of the
U.S.; unfortunately, not the world.) SEE SLIDe
NAD83
(a very common horizontal datum – “newer” data)
Uses the GRS80 Ellipsoid which minimizes error between the ellipsoidand the geoid on average around the world. (Resulting in a ellipsoid center
much closer to the mass center of the Earth.)
SEE SLIDES
Ex. NAD27 and NAD83 Shift Selected Sartell, MN Streets
Latitude and Longitude
- Geographic coordinates system
- The most comprehensive and powerful method of georeferencing
- > Metric, standard, stable, unique
- > Calculation of distance between points on the curved surface of the Earth
- Use a well-defined and fixed reference frame
- > Based on the Earth’s rotation and center of mass, and the Greenwich Meridian
Def of Latitude
SEE SLIDE
(1) Take a point S on the surface of the ellipsoid and define there the tangent plane, mn
(2) Define the line pq through S and normal to the
tangent plane
(3) Angle pqr which this line makes with the equatorialplane is the latitude , of point S
Def of Long
SEE SLIDE
λ = the angle between a cutting plane on the prime meridianand the cutting plane on the meridian through the point, P
Cutting a Plane of a Meridian
Meridian is imaginary circle on the earth’s surface from the north pole to the south pole.
Greenwich
Latitude and Longitude on a Sphere
SEE SLIDE
Map Projection Process
Terrestrial Surface -> Geoid -> Ellipsoid -> Nominal or generating globe -> 2D Map
Projections
Cylindrical Projections
Azimuthal/ Planar Projection
Conic Projections
Cartesian Coordinate System
Define the location of the blue cross -> Two coordinates -> Two measured distances -> One origin -> Two axes (x, y) (easting, northing) (Longitude, Latitude) (λ, φ)
SEE SLIDE
UTM
Universal Transverse Mercator Projection
- A type of cylindrical projection
- Transverse Mercator because the cylinder
is wrapped around the Poles, not the Equator
- Implemented as an international standard coordinate system
-> Initially devised as a military standard
- Uses a system of 60 zones
- Unit: Meters
There are 60 zones. Each zone is six degrees of longitude in width and numbered as shown
at the top, from W to E.
A UTM georeference consists of a zone number, a six-digit easting and a seven-digit
northing E.g., 14N, (468324E, 5362789N)
C~X, excepting for “I” and ”O”
SEE SLIDE (EX. UTM ZONES Of US)
UPS
Universal Polar Stereographic (UPS) coordinate system SEE SLIDE
SPCs
State Plane Coordinates
- In 1930s, each U.S. state adopts its own projection and coordinate systems
—-> Although the distortion of the UTM system is small, the UTM zone boundaries are a problem in many applications (arbitrary lines of longitude can’t be used for boundaries between jurisdictions)
—-> Provides less distortion than UTM support high accuracy applications (i.e., survey)
- Defined in the US by each state
-> Some states use multiple zones
-> Several different types of projections are used by the system
• Transverse Mercator, Lambert Conformal Conic and Oblique Mercator
SEE SLIDES
SPCs
- Projection are chosen to minimize distortion over area of the state.
-> Shape of state
East-West States (e.g. Texas) use Lambert Conformal Conic,
North-South States (e.g. Georgia) use
Transverse Mercator, portions of Alaska use
oblique Mercator.
-> Size of state - Large and elongated states have internal
zones within their SPCs
-> Texas has five zones based on Lambert
Conformal Conic Projection
-> Hawaii has five zones on the Transverse
Mercator Projection
SEE SLIDEs
Oregon Coordinate System and Districts
NORTH and SOUTH (See Slide)
Converting Georeferences
- GIS applications often require conversion of
projections and ellipsoids/datums
-> There are standard functions in popular GIS packages - Placenames can be converted to associated
coordinates (latitude and longitude)
-> using gazetteers - Street addresses must be converted to coordinates for mapping and analysis
-> Using geocoding functions (in ArcGIS Desktop)
Converting Georeferences
-> Requiring a street centric database and address range on each side of a street
-> Addresses are geocoded by finding the appropriate street segment record, and estimating a location based on linear interpolation with the address range.
• Google Maps
• MapInfo
• MapQuest
• …
