Intro to GIS | Spring 2011

Georeferencing

SHAPE OF THE EARTH

http://earthobservatory.nasa.gov/Features/BlueMarble/

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