Datums

1
Geographic Datums
2
Lesson Outline

• Defining Location
• Horizontal Datums
• Gravity
• Vertical Datums

3
What is Location?
Consider
Location only makes sense when compared to some
frame of reference. Relative - Bldg 215 is
to the left of the post
office Absolute - 38? 41 20 N, 77 ? 08 09
W
4
What is Location?
The Relative Approach
Post Office
Building 215
5
What is Location?
The Absolute Approach
N
Greenwich, UK
Equator
Prime Meridian
6
Horizontal Datum Defined

• Horizontal Datum - A base reference for a
  coordinate system. It includes the position of
  an initial point of origin and the orientation of
  an ellipsoid that models the surface of the earth
  in the region of interest.

Z
Prime Meridian
Y
X
7
Why do we need an ellipsoid?
European Remote Sensing satellite, ERS-1 from
780Km

This image depicts the earths shape without
water and clouds. It looks like a sloppily peeled
potato, not a smoothly shaped ellipsoid.
Calculation of geographic position on this
irregular surface is very complex. A simpler
model is needed.
This simplified mathematical surface is an
ellipsoid.
8
Ellipsoid
Rotate Ellipse in 3 Dimensions
b
a
Semi-major Axis a 6371837 m Semi-minor Axis
b 6356752.3142 Flattening Ratio f(a-b)/a
1/298.257223563
9
Traditional Horizontal Datums
The Traditional Approach

• Many nations established their own regional datum
• Used various national standards and procedures
• Different time frames
• Calculated ellipsoids that fit well locally
• Established initial point location and
  orientation with astronomic observations
• Result
• Inconsistent Datums

10
Traditional Approach
Initial Point

• Astronomic positions are measured by observing
  stars
• Variations in the Earth's gravity field
  complicate this procedure

local level plane
11
Traditional Approach
Control Network
A
C
B
D
Purpose of the control network is the orientation
of the coordinate system
12
NAD27 Example
Traditional Approach in North America circa 1927
Meades Ranch - Kansas
Clarke Ellipsoid (1866)
13
Traditional Horizontal Datums
Limitations to the Traditional Approach
NAD 27
ED 50
(Clarke Ellipsoid )
(International Ellipsoid)
14
Traditional Horizontal Datums

• Example (circa 1960) How well can we hit
  Minsk, USSR with a missile from Kansas?
• Minsk (Pulkovo, 1942)
• N 53 52' 57.78" E 028 01' 58.00"
• Minsk (NAD-27)
• N 53 53' 02.76" E 028 01' 43.06
• ? Latitude 5, ? Longitude 15
• Around 313 meters of error

15
Horizontal Datums
Regional vs. Global Approach

• Global replaces regional datums with a common,
  accurate standard
• One system for maps of the entire planet

16
DoDs Satellite Derived Horizontal Datum
NIMAs World Geodetic System 1984
WGS - 84 is an Earth Centered Earth
FixedCartesian coordinatesystem with its
originfixed to /- 10 cm. An ellipsoid is
placed on top of the axis tocreate a
geodeticfoundation for the variouscoordinate
systems.

• WGS 84

17
What is the Problem for DoD?
World Geodetic System 1984--The DoD Standard

• WGS 84 is used on almost all new NIMA-produced
  GIS products (all digital)
• There are still hundreds of legacy local datums
  in use throughout the world on existing products
• gt 30 Datums with gt 5 Ellipsoids used by NIMA
• gt 150 Datums with gt 25 Ellipsoids used by others
• It would take NIMA years and lots of DoD money to
  update the entire existing product line
• You can transform Datums and convert coordinates
  between WGS 84 and over 200 local datums

18
Where is the Datum Problem?
Most widely used local Datum/Ellipsoid pairs
19
Impact of Datum Mismatch
Map dated 1989 WGS-84
Map dated 1957NAD-27
15S WC4330
15S WN4430
20
Datum Shifts
21
WGS-84 vs. Traditional Datums
Coordinate Shifts From Local Datums to WGS 84
Location
Datum
Shift
(US)
18Q VT 81170149
NAD27
201meters
(Japan)
52S BS 84457638
TOKYO
754 meters

(Europe)
29
1812.7N
176 meters

ED-50
4657.9E
47

(Africa)
1818.4S
01
ARC-50
296 meters

15
4656.6E
22
WGS-20XX?

• WGS-84 will not change to WGS-20XX
• Estimates of Geoid Separation (deviation of the
  vertical) will improve
• Earth Gravitational Model 180-180 Separation
  accurate to /- 2-3 meters (globally) Local
  deviations can be up to 20 meters
• Earth Gravitational Model 1996 (EGM 96)
  Separation accurate to /- 50 cm-1 meter
  (globally) Local deviations can be up to
  several meters

23
Vertical Datums
Like horizontal measurements, elevation only has
meaning when referenced to some start point.
MSL Elevation
High Tide
Mean Sea Level
Low Tide
Mean sea level is the most common vertical datum.
24
Chart Datum
Marine Navigation charts use variations of low
tide instead of mean sea level for referencing
depth.
Q Why?
25
Geoid
A Gravitational Surface of Equal Potential, which
Approximates Mean Sea Level.
26
Shape of the Geoid
The Geoid is an undulating surface, not smooth
and regular, due to density variations inside the
earth.
Density Variations
27
Shape of the Geoid
28
Instrument Measurement
Problem Measuring from Geoid Calculating on
Ellipsoid
Ellipsoidal Surface
Geopotential Surface
Instrument centered on plumb line
Error
Topographic Surface
29
Geoid - Ellipsoid Separation
Defining the Vertical Position
-
h
Geodetic Height (Height above Ellipsoid)
-
Orthometric Height (Height above Mean Sea Level)
H
-
Geoid Separation
N


H
h

N
Geoid
Topo Surface
Ellipsoid
H is measured traditionally h is approximately
NHN is modeled using Earth Geoid Model 96 or
180
30
World Geoid Separation
31
Model Accuracy

• Earth Gravitational Model 180-180 Separation
  accurate to /- 2-3 meters (globally) Local
  deviations can be up to 20 meters
• Earth Gravitational Model 1996 (EGM 96)
  Separation accurate to /- 50 cm-1 meter
  (globally) Local deviations can be up to
  several meters
• Many NIMA DoD systems still use EGM 180
  (e.g. Production Equipment, GPS receivers)

32
Review

• Horizontal and Vertical Datums provide a frame
  of reference in which to calculate locations
• A datum definition requires an initial point
  location, orientation, and an ellipsoid
• WGS-84 is the datum of choice for NIMA products
• WGS-84 is an earth centered Cartesian system
  using a globally fitted ellipsoid
• Gravity affects horizontal and vertical
  instrument measurements
• Datum mismatches can cause serious operational
  errors

33
Datums, Projections, Coordinates Review

• Know What Datums Exist in AOR
• Always Pass Datum w/Coordinate
• Understand Map Projection Used for Your Products
• Understand Coordinate System in Use
• Know Resources to Transform Datums and Convert
  Coordinates

Questions?