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!