Brief History of North American Horizontal Datums New England Datum of 1879 United States Standard Datum of 1901 North American Datum of 1913 North American Datum of 1927 North American Datum of 1983
Ellipsoid – a slightly flattened sphere – an oblate spheroid of revolution – an ellipse that is rotated about it’s shorter axis.
The earth is not an exact ellipsoid – no single ellipsoid will match the earth’s surface.
Originally the method was to measure ‘arcs of meridians’ in various regions of the world, and create different reference ellipsoids, that ‘best fit’ the regional curvature of the earth, and use these for mapping datums. These are called Regional Datums .
GEOID – An abstract surface – the surface of the earth if it’s surface was equal to sea level everywhere . This is an undulating surface that varies no more than ~ 100 meters above or below a well fitting ellipsoid.
It is the equipotential surface of the earth’s gravity field which best fits global mean sea level .
It was first mathematically defined by Stokes in 1849, when he computed the “surface of the earth’s original fluidity” from surface gravity measurements.
The GEOID is constantly being revised – due to sea level changes, tectonics, and better measurements of the earth’s gravity field.
We now use GEOID 99 & GEOID 03 for our work in the U.S.
Ellipsoid – Geoid surfaces in comparison with local topography
Meades Ranch in Osborne County, Kansas was chosen because it is very close to the geographic center of the lower 48 states, and the station was also in the 1899 arc of triangulation.
New England Datum based on Clarke 1866 ellipsoid
a = 6378206.40 meters
b = 6356583.80 meters
f = 1 / 294.978698 meters
‘ f’ the degree to which the ellipsoid is flattened to match the earth’s shape.
Geodetic Latitude & Longitude are converted to Planar Coordinates for Triangulation and extension of the control network.
Originally – early planar coordinate systems were tangent plane coordinates, and the geodetic positions were projected onto various map projections which fit into one of 5 zones.
The National Geodetic Survey developed the State Plane Coordinate System in 1933. Every state is now covered – separate systems are assigned to zones. Each zone uses it’s own projection and coordinate center. 120 zones cover the U.S.
Preparing a station for observations – North Dakota - 1913
There are differences in the two datums ranging from ~100m in the western U.S., to ~1m – 3m in the central and eastern U.S. as show below.
Readjustment of the North American Datums NAD-27 to NAD-83
Geodetic positions of control points can change between 10m – 100m within the United States – more than 200m in Alaska – and more than 400m in Hawaii.
The shifts between the two datums is not uniform across the United States – and there is no single value that can be applied to convert Latitude & Longitude in NAD-27 to Latitude & Longitude in NAD-83.
NADCON was developed in order to help in conversions between the two datums.
The grids used by NADCON is based on more than 150,000 horizontal control points.