Intro to GIS | Spring 2011
Georeferencing
SHAPE OF THE EARTH
Earth's Shape
• Geoid: shape of earth minus topographic features
(irregular due to local variations in gravity)
• Ellipsoid: elongated sphere used to represent geiod for
mapping purposes (+/- 110m variation from geoid)
• Geodesy: study of the Earth's shape & representation
How do we measure points on the Earth's Ellipsoid?
Geographic Coordinate System
GEOGRAPHIC COORDINATE SYSTEM (GCS)
GCS • Latitude/ Longitude coordinate system based on a
geodetic datum, which is based on a reference
ellipsoid
Latitude 45° 45' 33'' or 45.759167 Degrees, Minutes, Second or Decimal Degrees
Longitude 122° 43' 59'' or 122.733056 Degrees, Minutes, Second or Decimal Degrees
Geographic Coordinates
• meridian: line of constant longitude
• parallel: line of constant latitude
• prime meridian: 0° longitude; passes through the
Royal Observatory in Greenwich, England
• international date line: 180° (approx) longitude; date
changes when the line is crossed
• equator: 0° latitude; separates northern & southern
hemispheres
Geographic Coordinate System (GCS)
• Lat/long is not a projection and therefore cannot be
shown on a flat surface (map, screen)
• Shown in ArcGIS as the “Platte Carre” cylindrical
projection (the “unprojected” projection)
GEODETIC DATUMS
Geodetic Datums
• Define the size and shape of the Earth for a particular
location based on a set of precisely surveyed points
and an ellipsoid
• Nations and agencies use different datums, as a basis
for coordinate systems
• Referencing geodetic coordinates to the wrong datum
can result in position errors of hundreds of meters
North American Datum 1927 (NAD27)
• North American datum based on the Clarke 1866
ellipsoid
• Uses a single survey point (in Kansas) as reference
• Not as accurate as newer datums (which use more
precise ellipsoids)
• Still used on most USGS topographic quads (DRGs,
DLGs)
North American Datum 1983 (NAD83)
• North American datum based on the Geodetic
Reference System of 1980 (GRS80) ellipsoid
• Compatible with modern survey techniques
• Official datum of the Federal government and most
states
• Current version is NAD83 HARN (High Accuracy
Reference Network)
World Geodetic Survey 1984 (WGS84)
• Based on the WGS84 ellipsoid
• Compatible with modern survey techniques
• World reference system first developed by Department
of Defense in the 1950’s
• Official datum of bombs (and Google Earth)
• Used by the GPS system
48.7438798534299 -122.46818353793 WGS 1984
48.7438798543649 -122.46818353793 NAD 1983
48.7440490722656 -122.46690368652 NAD 1927
Latitude Longitude Datum
Bellingham, WA, coordinates in decimal degrees
How do we represent the Earth's ellipsoid on a flat surface?
Projected Coordinate Systems
Georeferencing Demo in ArcGIS