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Importance of DatumsJacques Cloutier
Navajo Geospatial SummitApril 3rd 2013
Example from Google Earth Poorly defined datums used in data fusion
General Shape of the Earth: Ellipsoid
Ellipsoid Model: Mathematical representation of the general shape of the Earth
Flattened sphere
Defines map projections
Vertical basis (in some applications)
GPS likes ellipsoids for the basis of positioning
a = Semi major axis b = Semi minor axis f = a-b = Flattening a
b
a
Horizontal Datums
NAD27: Used as a basis for older mapping in North America
NAD83: Updated horizontal datum from NAD27
WGS84: Used primarily by US Military
ITRF: International reference frame, which includes tectonic plate velocities
North American Datum 1983 (NAD83)
Established in early 1980s More accurate than NAD27
Better ellipsoid (GRS80) Better defined origin
North American Plate is held fixed Be aware of which version is being used
CORS96, NSRS2007, 2011, etc.
Horizontal Projections
Round Earth to Flat Map
Each projection causes some sort of distortion
State Plane Coordinate Systems
Universal Transverse Mercator (UTM)
NHA Stereographic Double Projection System
NN Coordinate System (NNCS)
ELLIPSOIDS works well with GPS
A specific and set distance from geographic center of
earth
Distance not affected by terrain elevation or gravity
Data gathered is used in navigation systems
GEOIDS works well with
NATURE
Distances undulate based on gravity/locality
Always perpendicular to force of gravity
Used for terrain /contour mapping
Vertical Datums
H hELLIPSOID
NGEOID
Topographic Surface
If N is negative, GEOID is below ellipsoidIf N is positive, GEOID is above ellipsoid
H = h – N(H) Orthometric Height(h) Ellipsoid Height(N) Geoid Height
Vertical Datums - Heights Relative to a Defined Surface
Vertical Datum Examples
National Geodetic Vertical Datum of 1929 (NGVD29)
North American Vertical Datum of 1988 (NAVD88) – Most Common
Locally Established Datums
Final Thoughts on Datums
Technology Is Only as Accurate…
…as the Data It Uses!
Sources
Doyle, D. (2009)
Datums and Projections: Demystifying the Reference Frame
Retrieved March 13, 2013 from: www.ngs.noaa.gov/corbin/class_description/Datums_Projections.shtml
Doyle, D. (2001)
State Plane Coordinates and Datum Transformations
Retrieved from National Geodetic Survey website: www.ngs.noaa.gov
AeroMetric
AeroMetric
Aero-Metric, Inc.
www.aerometric.co
m
Thank You!
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