Regional Horizontal Datums (1930s to 1950s)

Following World War Two, all the basic observations of most of the countries of western Europe were re-examined and extended, with the use of early electronic computers, to form European Datum (1950), one example of a regional datum.

The next force for change was the launch of the first artificial Earth satellites which required an observation and control network on a unified datum and, eventually, provided the means of obtaining such a unified datum.

World-wide Horizontal Datums

In the early 1960s the USA launched a series of Transit satellites to improve the navigation of submarines. Methods were soon developed to exploit a series of observations of the signals to establish an accurate position for a fixed object, referred to the same datum as that used by the satellites. This datum was initially known as World Geodetic System (1960) which was refined to WGS 66, WGS 72 and, following many additional observations and calculations, WGS 84 Datum.

The difference between the latter two datums is a maximum of 17 metres at the Equator reducing to zero at the poles (since the difference is predominantly in longitude). For all practical purposes WGS 84 Datum is likely to be the definitive datum for many years to come. WGS 84 refers to both datum and spheroid; on average, the spheroid provides the closest match to the Earth's surface, although in specific areas it may not match as well as a previously calculated spheroid.

WGS 84 Datum is that used by the GPS (Global Positioning System) satellites and thus the datum to which positions from those satellites are normally referred. Although navigation positions from the Standard Positioning Service (SPS) of GPS are only accurate to ±22-5 metres (for 95% of the time), it is possible to establish positions to geodetic (better than 1 metre) accuracy. This makes it possible to compare positions on different datums and compute a transformation.

Modern computers enable simultaneous calculation of (almost) unlimited numbers of observations, so removing another of the difficulties faced by previous surveyors. Unfortunately many countries have a large core of maps, charts and legal documents containing positions which are referred to existing, sometimes less accurate, datums. Difficult decisions need to be made to decide whether to retain the existing series or convert it to the new datum or keep two series. Investigation has shown that mariners do not want isolated charts converted from one datum to another which would result in incompatibilities when moving between charts of an area. The better solution would be to convert groups of charts (all those in an area such as the approaches to a port) at the same time. This is the route being applied at present to transfer all the charts of Great Britain from the local datum (Ordnance Survey of Great Britain 1936 Datum) to European Terrestrial Reference System 1989 Datum, which is compatible with WGS 84 Datum. Unfortunately, synchronizing the publication of many such conversions is difficult to achieve and this program will take another couple of years to complete.

The situation for mariners is improving with recent surveys referred directly to WGS 84 Datum, increasing numbers of charts referred to WGS 84 Datum (or to regional equivalent datums which are the same for practical navigation purposes) and increased international co-operation with the exchange of parameters and information. Unfortunately it will be many years before all areas are resurveyed and all charts revised.

Until that happens, mariners should remain alert to danger. A GPS receiver may output a position to a precision of three decimal places of a minute, but that does not mean that all its positions are accurate to 2 metres. Nor does it mean that the resulting position is compatible with the positions of objects shown on modern charts (paper or digital) which may have been established 100 years ago. The chart title notes and cautions and the Source Diagram, which shows the ages of surveys, mustalways be consulted for indications of limitations.

Answer the following questions

What aspects affect charted positional accuracy?
What can you say about surveying accuracy and its major limits?
What is the intention of paper charts in the view of accuracy?
What forecast could be developed concerning accuracy?

Enumerate the major stages of the datums’ history and describe each of them shortly.

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Local Horizontal Datums (before -1930s)	\|	UNIT 15

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