Post on 29-Dec-2015
Geography 370
The size and shape of the Earth
(Ellipsoid, Geoid, Latitude/longitude) Datum Map projection
Locating Positions on the Locating Positions on the EarthEarth
So the Earth is Round, Right?
Eratosthenes
(276-196 B.C.)
Measured the circumference of the earth.
His measurement: 40,250 km
Modern Measurement: 38,762 km
Eratosthenes
Size and shape of the Earth
17th and 18th centuries
The Earth’s shape
A Spheroid An Ellipsoid
r2=1
r1=1 r1=2r2=1
f = (r1-r2)/r1
Flatten factor?
The Earth as an Ellipsoid
Why not use the same ellipsoid?
Earth’s true shape diverges from the shape of an ellipsoid
Ellipsoidal parameters were fit for each continent or large survey area
Different sets of measurements in each region or continent
Globally applicable ellipsoids
WGS72 WGS84 GRS80
Continuing measurements from satellite and GPS may lead to further ellipsoid refinements.
The Earth as Geoid
The Earth as a Geoid Geoid -- The surface on which gravity is the
same as its strength at mean sea level
The surface that would exist if the sea was admitted under the land portion of the earth by small frictionless channels.
Earth Surfaces The topographical surface of the earth is the
actual surface of the land and sea at some moment in time.
Gravity models attempt to describe in detail the variations in the gravity field. Local variations in gravity, caused by variations in the earth's core and surface materials, cause this gravity surface to be irregular.
http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html
Reference for elevation:
• Ellipsoid
• Geoid
• Local sea level
Spatial Location and Reference: Latitude / Longitude Most commonly-used coordinate system Lines of latitude are called parallels Lines of longitude are called meridians
Latitude / Longitude
Prime Meridian & Equator are the reference points used to define latitude and longitude
Determine distance between any pair of points
Great circle
Geodetic Datum
Datum -- n. (dat - m) \ any numerical or geometric quantity which serves as a reference or base for other quantities
ee•Horizontal datum •Vertical datum
Geodetic Control Networks
Surveyors use these known points in the control networks for surveys and mapping.
Geodetic Control Network
Three main horizontal datums
North American Datum of 1927 (NAD27) 26,000 survey stations in United States and Canada.
North American Datum of 1983 (NAD83) 250,000 stations and 2,000,000 distance measures
World Geodetic System of 1984 (WGS84) (updated based on satellite measurements)
Ellipsoids & Datums: Importance
Differences exist between different ellipsoids & datums– Coordinates different in each can be
significant distance
– Elevation can be major differences at large scales
Note: Be aware of the ellipsoid & datum for datasets you are working with