is

Ellipsoid or Spheroid
Rotate an ellipse around an axis
O
X
Z
Y
a
a
b
Rotational axis

Standard Ellipsoids
Ellipsoid
Major
axis,
a
(m)
Minor
axis,
b
(m)
Flattening
ratio,
f
Clarke
(1866)
6,378,206
6,356,584
1/294.98
GRS80
6,378,137
6,356,752
1/298.57
Ref: Snyder, Map Projections, A working manual, USGS
Professional Paper 1395, p.12

Horizontal Earth Datums
•
An earth datum is defined by an
ellipse
and
an
axis of rotation
•
NAD27
(North American Datum of 1927)
uses the Clarke (1866) ellipsoid
•
NAD83
(NAD,1983) uses the GRS80
ellipsoid on a geocentric axis of rotation
•
WGS84
(World Geodetic System of 1984)
uses GRS80, almost the same as NAD83

Definition of Latitude,
f
(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 equatorial
plane is the latitude
f
, of point S
O
f
S
m
n
q
p
r

Cutting Plane of a Meridian
P
Meridian
Equator
Prime Meridian

l