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Rob Naylor BS: Map Projections (33) RE: BS: Map Projections 19 Mar 18

IainS:WGS 84. The World Geodetic System or WSG assumes a coordinate origin at the center of the mass of the earth. The last revision was made in 1984 and is referred to as WSG84. It is the reference system being used by the global positioning system or GPS which has now assumed universal usage for helping people to find their way to their destinations or to pinpoint their locations. As per this the ellipsoidal height of Mount Everest is 8823.51, whereas the Geoidal height is 8846.10 meters. The latitude and longitude positions remain the same.

Actually, WGS84 is a dynamic system itself, and under constant revision. WGS84 coordinates of a spot you measure today are likely to differ from WGS84 coordinates measured for the same spot a number of years ago. Revision is partly due to continental drift, and partly due to improvements that are constantly being made to the gravitational model of the earth.

Below is an extract of the Abstract of a paper I will shortly be giving to the Society of Exploration Geophysicists:

"ITRF (International Terrestrial Reference Frame) is the fundamental reference datum used in Geodesy and is realised by large networks of fiducial sites comprising GPS; Satellite Laser Ranging (SLR); Very Long Baseline Interferometry (VLBI) and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) station coordinates which are defined at a Reference Epoch and with associated site velocities (to account for tectonic movement). This means that ITRF is a dynamic reference system whose coordinates are changing over time to reflect tectonic movement on a global scale.

It is less well-known that the WGS 84 CRS is also dynamic in nature, as the ground stations used to define the system are part of the ITRF definition system. The datum is defined by the coordinates and velocities of 18 GPS tracking stations. Each realisation of the WGS 84 datum is designated by the GPS week number of its inception. WGS 84 is kept in alignment with ITRF at the sub-decimetre level to ensure that the GPS Broadcast Ephemeris is not degraded by holding coordinates of the GPS tracking stations fixed when in fact they are moving tectonically.

This means that the WGS 84 coordinates of a point measured using GPS in, say, 1998, are likely to differ from WGS 84 coordinates of the same point measured in, say, 2018."

This whole topic is quite complex. The huge number of significantly large errors we see in maps, databases and data management systems are largely as a result of people without a requisite detailed knowledge of Geodesy and Cartography using inappropriately the easily-available systems for both data acquisition (eg GNSS such as GPS or GLONASS) and data manipulation (eg GIS systems such as ArcGIS).

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