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Geodetic Reference Standards

Taken from "Nebraska Geographic Information Systems Steering Committee - Gis Technical Standards" December 15, 1993, version.

Chapter 6. Geodetic Reference Standards

Referencing systems

(Obj 5 Task 1 - Identify the implement GIS-referencing system)

Coordinate systems are the means of referencing geographic information to locations on the earth's surface and are the reference to standard models of the earth's surface represented as an oblate spheroid of revolution (a flattened sphere). Until recently the dimensions defined by the Clarke's 1866 spheroid were used to define the datum for the United States. It is known as the North American Datum of 1927 (NAD27). The dimensions of the Clarke's 1866 spheroid fit the United States quite well but not the entire earth.

Coordinate values are expressed on the surface of the spheroid and for NAD27 this surface was at, or very near, the geoidal surface ( the geoid). This geoid may be defined as the equipotential surface of the earth's gravity field. The origin of NAD27 was a single triangulation station named "Meades Ranch" and an azimuth to Station Waldo, which are located in North Central Kansas.

Over time, as the National Network grew, distortions were introduced and advances in surveying and computing technologies presented an opportunity to more precisely measure the earth. Additionally the world has become a more global oriented society.

This led to the development of a new datum known as the North American Datum of 1983 (NAD83). This datum is based upon an ellipsoid referred to as the Geodetic Reference System of 1980 (GRS80). This ellipsoid fits the world quite well, but does not coincide with the geoid in the United States. Also, it is an earth-centered system, not confined to a single survey station for an origin nor is it at the geoidal surface.

The United State Military developed an ellipsoid referred to as the World Geodetic System of 1984 (WGS84). This ellipsoid is used in conjunction with GPS surveys. The tow ellipsoids, GRS80 and WGS84, are only slightly different in dimensions and for practical purposes can be considered the same.

The use of NAD83 has resulted in the shift of the calculated latitude and longitude for each point in the United States. Most of the maps in use today in the United States and many sets of geographic data bases, are based upon the NAD27 datum. This requires that data sets that are on NAD27 be transformed to NAD83 values. The National Geodetic Survey (NGS), the United State Geological Survey (USGS) and the Army Corps of Engineers (COE) have developed computer programs for transforming data from one datum to another and from one coordinate system to another. These programs are widely available and have been embedded in a variety of commercial software packages. These programs compute approximate values that are not necessarily exact. Care should be taken that the latest versions of these programs are utilized.

Latitude and longitude may also be expressed as plane-rectangular coordinates on a grid. There are primarily two projections in civil use in the United States. One being a transverse macerator projection and the being the Lambert conformal projection. The transverse mercator projection works well in states that extend in a north-south direction because the greater distortion of the projection is in an east-west direction. Similarly for states extending in an east-west direction, the Lambert conformal projection is used because the greater distortion is in a north- south direction.

Nebraska uses the Lambert projection. On the NAD27 datum the state is divided into two zones, a north zone and south zone and the coordinates are expressed in the U.S. survey foot. On the NAD83 datum, there is one zone for the entire state and the NGS publishes the values in meters. One reason for this is that Nebraska does not have legislation specifying the use of the U.S, survey foot or the International survey foot. There is but one accepted value for a meter. The difference between the foot values is that there are 3.280833333 feet per meter for the U.S. survey foot and 3.280839895 feet per meter for the International survey foot. Care should be taken when using feet as measurements in order to prevent the intermixing of the two foot values. A source for additional information about NAD83 state plane coordinates is the "NOAA Manual NOS NGS 5", titled, "State Plane Coordinate System of 1983", which is available from the NGS. The Military uses the Universal Transverse Mercator (UTM) grid which is not discussed here.

Until very recently elevations were referenced to the National Geodetic Vertical Datum of 1929 (NGVD29) which, prior to 1973 was known as the Sea Level Datum of 1929. It's origin is the observed heights of mean sea level at 26 tide gauges, 21 in the United States and 5 in Canada. Since mean sea level is not constant over time the name for the datum was not correct. The datum is not mean sea level, the geoid or any other equipotential surface.

As with the Horizontal Network, the Vertical Network developed distortions over time and crustal motion occurrences affected much of the networks. This led to the decision to re-adjust the network and the North American Vertical Datum of 1988 (NAVD88) came into being. For this datum, the height of the primary tidal bench mark at Father Point/Rimouski, Quebec, Canada was held as the fixed constraint. The mark is located at the mouth of the St. Lawrence River. The National Geodetic Survey will abandon support for the NGVD29 over the next 5 to 7 years and the NAVD88 is now the new National Vertical Datum. In Nebraska the NAVD88 heights are greater than the NGVD29 heights. The NGS has developed a program called VERTCON to do transformations between the 2 datums. The transformation yields an approximation and is not necessarily exact. This USGS bench marks are not on the NAVD88 datum. Most maps in use today in the United States, and many sets of geographic data based on the NGVD29 datum and will need to be transformed to NAVD88.

The recognized standard for conventional (Terrestrial) high accuracy data for coordinate and vertical control is the "Standards and Specifications for Geodetic Control" (Federal Geodetic Control Committee 1984). The standard and the resultant geodetic monumentation is suitable for the development of high- accuracy geographic information. A guideline for high accuracy GPS survey is "Geometric Geodetic Accuracy Standards and Specifications for using Relative Positioning Techniques" (Federal Geodetic Control Committee, Version 5.0, May 11, 1988, reprinted August 1, 1989). This is a preliminary document and has a disclaimer so stating.

Coordinate systems must have coordinate control so that geographically derived information can be referenced to the earth model, and hence, to other geographic information. There are two issue related with coordinate control; (1) The level of coordinate control use for any given geographic information should be commensurate with its intended use and, (2) all geographic information should have the basis for the coordinated system and control referenced in its metadata.

Generally there three methods to obtain accurate coordinates. From conventional surveying, using theodolites and electronic distance measures instruments (EDMI), or total station instruments which combine the features of these two instruments. From photogrammetric methods and from the newer technology of GPS surveying.

Accurate elevations are obtained from spirit leveling, Less accurate elevations can be obtained from GPS surveys, vertical angle surveys and photogrammetric survey. Hopefully, in the future, accurate elevations can be determined by geodetic GPS observations.

GPS surveying has proven to be a faster, economical, and accurate method of surveying. It is fully compatible with the NAD83 and NAVD88 datums. It is not readily compatible with NAD27 and NGVD29 datums. Geodetic GPS surveys yield a height above the reference ellipsoid. When the height is combined with a geoid height (geoid height is the difference between the geoid and the ellipsoid). The result is a height nearly equal to elevation. With further refinements, in the future, to models of the atmosphere and the geoid the NGS expects to achieve accurate elevations.

Because the new datums and adjustments reflect the most accurate values, both datums are more compatible to GPS surveys, and NGS will not support the older datums. These datums should be the basis for geographic information.

Recommendations:

  1. The North American Datum of 1983 (NAD83) should be the basis for coordinates for geographic information.
  2. The North American Vertical Datum (NAVD88) should be the basis for elevations for geographic information.
  3. The Nebraska State Plane Coordinates System of 1983 should be used for coordinated system and control reference in the metadata.
  4. All original data for geodetic control should be retained to provide a basis for future datum adjustment.

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