Ellipsoids and Datums

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Lecture 07 Coordinate Transformation I
Overview
Coordinate system for Earth Ellipsoids and
Datums Map projections (3D to 2D) Planar
coordinate transformation (2D to 2D) Geocoding
(discrete to continuous)
Topics
Ellipsoids and Datums Map projections (3D to 2D)
References
Chapter 3 in Jan Van Sickles Basic GIS
Coordinates (2004) Chapter 5 in Robinson et al
(1995) pp. 61-88
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Lecture 06 Digital Terrain Analysis II
Outlines
1. Ellipsoids and Datums A coordinate system
for Earth needs Ellipsoid and Datum 1.1
Ellipsoids A shape used to approximation
the shape of earth Parameters of a
biaxial ellipsoid (The biaxial
ellipsoid figure) Common Ellipsoids
Clarke1866 WGS (world geodetic
system) 72 and 84 Airy 1830 -
Britain GRS80 U.S.
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1.2 Datums 1) A datum is an actual
coordinate system 2) Creation of a datum
(definition of a datum) (1) Steps
definition of an ellipsoid (a, b)
attaching the ellipsoid to earth
Initial point and orientation
(five parameters)
realization (2) Methods
(a) the old approach
definition of an ellipsoid
attaching the ellipsoid to Earth
through an initial point
with an azimuth to another reference point
Survey of monuments
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1.2 Datums (continued) Drawbacks
regional and nongeocentric
(The nongeocentric Ellipsoids Figure)
Common datums NAD27 (Clarke
1866) (b) The new approach
Definition of an ellipsoid (Geocentric)
Attaching the ellipsoid to Earth
Mass center of the earth Three
dimensional (The Three
Dimensional Figure) GPS positions
of monuments Advantage global,
geocentric, 3-dementional (with Z)
Common datums NAD83 (GRS80),
NAD83/91 WGS84 (WGS84),
WGS84(G730) WGS84(G873)
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1.3 Coordinate transformation between datums
1) Molodenski transformation (The
Molodenski Transformation Figure) 2)
Seven-parameter transformation (three
translation, three rotation, one scale)
(The Seven-parameter Transformation Figure)
3) Surface fitting NAD27 to NAD83
(The Surface Fitting Figure)
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2. Map Projection (3D to 2D) 1) Purpose
2) Difficulty Shape of the earth 3)
Distortion Size (Equivalence) v.s.
shape (Conformality) 4) Type of projections
(1) The UTM Projection (The
Transverse Mercator Figure) (2) Conformal
angular relationships are
maintained (The Lambert Conformal
Figure) (3) Equal area (The
Albers Equal Area Figure) (4) The State
Plane Coordinate System varies
between states.