Under this bleak scenario, one of the principal purposes of a WGS is to provide the

means whereby these numerous local geodetic datums can be referenced to a common

system (or to each other indirectly through extrapolation) and then, WGS can facilitate

simplification of the global MC&G; problem.

To achieve the referencing of a local datum to WGS 84, one major requirement is to have

well distributed control points common to both the systems.  DMA maintains a world wide

database of NNSS Doppler station.  A search of this database produced 1591 good quality

Doppler stations, which also had coordinates defined in the local datum of the area.

These 1591 Doppler stations cover 83 local geodetic datums spread out over all the six

continents [3]. From a high of 405 Doppler stations common with the North American

Datum (NAD) 1927 in the contiguous U.S., there are  29 datums with only one common

station.  This limitation of not having any check station thus affects about 35% (29 of 83)

of the datums.

As the local geodetic datums are generally defined only horizontally and provide mean

sea level (MSL) heights from separately defined vertical datums, the geodetic heights

in the local datum (H

LD

), required to compute datum transformation parameters, were

generated using the following equation:

                        H

LD

   h

msl

 + N

LD

(6)

In the above equation, the local datum geoidal heights (N

LD

) were obtained by appropriate

transformation from the WGS 84 geoidal heights.  These local geoids are absolute

(contrary to the relative astro geodetic that are available for a few of the local datums) and

consistent in definition with each other and also with the WGS 84 geoid worldwide.

Table B 3 provides a sample listing of the transformation parameters between the 83 local

datums and the WGS 84; a full listing is available elsewhere.

4

In addition to the 83 local datums related to the WGS 84 through Doppler ties to the local

control, transformation parameters (based on non Doppler information) are also available

for seven additional local datums.

4

B.8  ACCURACY

The accuracy of the WGS 84 coordinates of a site significantly influenced by the method

used to determine the coordinates.  Table B 4 lists the four methods generally available to

establish the coordinate of a WGS 84 site and the associated accuracies achievable

through each of the methods.

B 7