UNIT 14

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HORIZONTAL DATUMS ON CHARTS AND SATELLITE-DERIVED POSITIONS
(Part 2)

It must notbe assumed that all charts in a region are referred to the regional datum. For example, although most metric charts of mainland European waters are referred to European Datum (1950), many charts are also referred to local datums such as Swedish Datum, Finnish Datum, Lisboa Datum, Monte Mario Datum and Hermanskogel Datum. Mariners are advised to keep their GPS receiver referred to WGS 84 Datum and apply the datum shift note from the chart.

It must be remembered by all prudent mariners that, while modern systems such as GPS enable a ship's position to be known to an accuracy of 20 metres or better, this may not be reflected in the accuracy of the chart they are using. Apart from the differences in positions between different horizontal datums, two other aspects affect charted positional accuracy. These aspects are:

· The accuracy to which features are surveyed

· The accuracy with which they are compiled onto a chart

Surveying accuracy

Hydrographic surveys have inevitably been conducted using the best position-fixing technology available at the time. This was limited to visual fixing until the Second World War, but used terrestrially based electronic position fixing (such as Decca, Hifix, Hyperfix and Trisponder) until the 1980s. DGPS is the current standard for most hydrographic surveys.

Generally, position fixing for surveying was more accurate than that for navigation in the first two categories, but currently DGPS is being made more widely available for use by all mariners with the appropriate equipment. The result is that current navigation with DGPS is, commonly, more accurate than position-fixing used for surveys conducted longer ago than about 1985. The consequence is that, although a modern vessel may know its absolute position to an accuracy of better than 10 metres, the positions of objects on the seabed may only be known to an accuracy of 20 metres or much worse, depending on the age of the latest survey and/or its distance from the coast. Furthermore, it is only comparatively recently (since approximately 1980) that surveying systems have had the computer processing capacity to enable the observations to be analysed in order to generate an estimate of the accuracy of position fixing. The result being that, although the current accuracy standard of position fixing surveys conducted by, or on behalf of, the Royal Navy can be stated, it is impossible to provide anything other than general estimates for older surveys.

The current accuracy standard for positioning is ±13 metres for most surveys with the standard of ±5 metres (both 95% of the time) for certain special purpose surveys. It can be confidently stated that the former value is significantly improved upon in most cases. Further improvements will undoubtedly be made as a result of technological developments, but at present there has to be a balance between the cost of a survey and the quality and quantity of the results achieved.

Thus, although the positions of maritime objects derived from modern surveys will be accurate to better than 10 metres, this cannot be used as a general statement about all such objects around the coasts of the United Kingdom or elsewhere.

Charting accuracy

Most UKHO paper charts and their derived digital versions are assembled from a variety of sources such as maps, surveys, photogrammetric plots etc. The intention is to provide the mariner with the best available information for all parts of that chart and the usual procedure is to start with the most accurate sources, but it is often impossible to complete the whole chart without recourse to older, less accurate sources. The result is reflected in the Source Diagram shown on most modern charts. When sources are referred to different datums, transformations have to be calculated and applied to make the sources compatible. The intention is for such transformations to have an accuracy of 0-3 mm at chart scale, this being the effective limit of manual cartography, but depending on the information available, this may not always be possible.

When the positions of objects critical to navigation are accurately known, the intention is that they are located on a chart to an accuracy of 0-3 mm. The obvious consequence is that accuracy varies with chart scale: 0-3 mm at a scale of 1:10,000 is 3 metres 0-3 mm at a scale of 1:50,000 is 15 metres 0-3 mm at a scale of 1:150,000 is 45 metres This limitation must be heeded if positions of objects read from smaller scale charts are plotted on larger scale charts.

This situation will obviously change when chart data becomes available digitally, but much of the early digital data will be derived from these paper charts and the limitations will remain. Furthermore, a pixel on a computer display screen is approximately 0-2 mm square, roughly equivalent to the accuracy available on the paper chart.

History of datums

For safety of navigation it is important that all positions are accurately related to each other, but it has only been since the launching of artificial earth satellites that truly world-wide horizontal datums, such as World Geodetic System 1984 (WGS 84) Datum, have become a practical reality. In historical terms, datums were constructed from terrestrially observed triangulations carried out on the Earth's surface, but transferred to a theoretical surface known as the spheroid (or the ellipsoid). This is the simplest mathematical shape that most closely matches the Earth's surface and can be thought of either as a sphere compressed at the poles or as a rotated ellipse.

The construction of horizontal datums has passed through three distinct phases: local datums covering single countries, regional datums covering groups of countries and world-wide datums.

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UNIT 13	\|	Local Horizontal Datums (before -1930s)

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