Geodetic Control and
Datums
Where is it?
How Accurately can we
map it?
Mission Statement for the
South Carolina Geodetic Survey
• The primary mission of the Geodetic Survey is to establish
horizontal and vertical geodetic control throughout the
state to allow land and land-related items to be
referenced to the national horizontal and vertical
coordinate system, to ensure the integrity of new
geodetic data, and to maintain geodetic files.
• Mapping Coordination supports the development of an
accurate, uniform statewide mapping system on a
county-by-county base. These mapping systems will
provide a large-scale map base for county and state
land information systems that will enhance land records,
engineering, land planning and economic
development.
Spatial Data
• Data that identifies the geographic location of features
• Data is generally stored as coordinates and topology
• Data is often overlaid onto some form of a map
• Data is often accessed, manipulated or analyzed
through Land/Geographic Information Systems
Coordinate System
• A reference system for defining points in space or on a
particular surface by means of distances or angles or
both, with relation to designated axes, planes or
surfaces.
• Generally refer to two common types of systems
• Spherical
• Rectangular
Coordinates Defined
Datum, Geodetic Datum, Geocentric Datum
A set of 8 constants specifying the coordinate
system used for calculating coordinates of points on
the Earth
3 Constants to Define the Coordinate Origin
3 Constants to Define the Coordinate Orientation
2 Constants to Define the Ellipsoid Shape and Size
Datums
set of constants specifying the coordinate system
used for geodetic control, i.e., for calculating the
coordinates of points on the Earth."
• "The datum, as defined in (1), together with the
coordinate system and the set of all points and lines
whose coordinates, lengths, and directions have
been determined by measurement or calculation."
•
"A
• When the first definition is used, the published
coordinates of control points can change when
better measurements allow better determinations.
With the second definition, a change in coordinates
should result in a new datum. NGS has used the first
definition for NAD 1983.
Vertical Datums
• NGVD 29
o A vertical datum renamed in 1973 from the Sea
Level Datum of 1929. Held 21 tide gauges in the
US fixed. 19 along the East Coast. The datum did
not represent mean sea levels on East and West
coasts of the U.S.
• NAVD 88
o Established in 1991. A refined datum, much like
NAD 83. Factors addressed with NAVD 88
included crustal motion, post glacial rebound,
and subsidence due to withdrawal of
underground water.
Gotta Love Science?
Why Should I change Datums?
• More players than just the United States
• Improved technology and Surveying techniques
• EDMs, VLBI Sites, Introduction of Satellite
Navigation
• Improved understanding of the shape and mass of
mother Earth
Why Survey with Satellites?
Okay – Lets look at the Geodesy
OMG
Another Datum
WGS 84
World Geodetic System of 1984
• Reference frame used by DoD
• Defined in 1987 Using Doppler Satellites
• Was used as the reference frame for broadcast GPS
Ephemerides
• Differed very little from NAD 83 (86) in its infancy
• Has been revised 3 times since inception . The latest was
using the International Earth Rotation Service (IERS)
Terrestrial Reference Frame (ITRF)
• Differs from NAD 83 by ~2 meters
What do I do now?
• Corrections need to be applied to data collected
with GPS receivers. What is available?
•
•
•
•
DGPS
WAAS
Differential Corrections
SC RTN
Reference/Base Stations
IERS and ITRF
The Differences between
Datums
Coordinate Shift
2 ft
0.61 m
1.2 ft
0.36 m
0.8 ft
0.24 m
Improving Positional Accuracy
NETWORK
TIME
SPAN
NETWORK
ACCURACY
LOCAL
ACCURACY
NAD 27
1927-1986
10 Meters
First-Order
NAD 83
1986-1991
1 Meter
First-Order
NAD 83 (1995)
(CORS96)
1991-1997
0.1 Meter
B-Order
A-Order
NAD 83 (2001)
(HARN)
2001
0.01 Meter
0.01 Meter
NAD 83 (2007)
(SC RTN)
2007
0.01 Meter
0.01 Meter
SC Height Modernization
Projects
Explanations of Datums from
the NGS
• OK-your situation is a little more complicated than most
because of the desire of South Carolina and North Carolina
not to publish the results of any HARN readjustments until there
was a full national adjustment (which happened in 2007).
• Passive control was never tagged as NAD83 (CORS)-only
CORS coordinates had this tag. This I believe would have the
projection tag of (CORS96)
• No coordinates were published with the tag of NAD83 (HARN).
This was the 2001 adjustment. This was adjustment was
adopted in SC.
• When the 2007 national adjustment was completed, the 2007
coordinates for both states were published and any previous
values in the database showed up as superseded.
Questions?
Thank-you!
Matt Wellslager
803-896-7715 or 803 528 4857
[email protected]
Considerations for the Future
Light Squared
New York Times
More Light Squared
Washington Post