Unit 13

1. Read and translate the following text

Horizontal datums on charts and satellite-derived positions (Part 2)

It must not be 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.

Local Horizontal Datums (before -1930s)

In the era before artificial earth satellites, it was only possible to observe all points within a given datum from at least two other points within that datum using angles and distances. Thus datums could only be developed over areas limited by geography or by territorial concerns.

The geographical limitation was any large area of water; for example, it was possible to observe between points on either side of the English Channel, but not possible to observe across the North Sea. Terrain such as forest, mountains or desert caused difficulties, but these could be overcome.

The territorial concerns limited the willingness of adjoining countries to divulge details of their surveys to their neighbours.

Because of irregularities in the Earth's shape and the limited area over which that shape could be observed, different countries calculated or adopted differently sized spheroids for their own triangulation. Thus the Airy spheroid matches the shape of the Earth well in the British Isles, the Everest spheroid in the Indian subcontinent and the Clarke spheroid in North America. Since they were differently sized and positioned, the theoretical centres of these spheroids are offset relative to each other in three dimensions. None of these was "wrong" for the areas in which they were used, but could not all be "right" for the whole Earth.

Irregularities in the Earth's shape and gravity field also cause anomalies in the direction of the vertical (particularly on isolated islands or near mountain ranges). Any uncertainty in the direction of the vertical will result in an error in any astronomical observations, traditionally used as the origin of a datum.

The final difficulty faced in the production of a coherent system of latitudes and longitudes was the calculation of the positions from a large number of angle and distance observations. Ideally such calculations should all be carried out simultaneously but, when they had to be carried out by hand with the use of tables, this was clearly impractical. The solution was to split the area into a series of manageable blocks, calculate one and then move on to an adjacent block, holding points on the border fixed. Unfortunately, this caused errors to build up throughout the calculations leading to local distortions in the datum.

Datums thus suffered from problems of origin (from the differing astronomical observations), calculation (from the large number of terrestrial observations) and surface (from the differing spheroids) When the different geographical positions of an object on different datums are known, it is possible to calculate a transformation between the datums and then convert all positions from one datum to another. However, if the only common points fall in a small region, the resulting transformation is likely to become inaccurate away from that region.

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, must always be consulted for indications of limitations.

2. Answer the following questions

1. What aspects affect charted positional accuracy?

2. What can you say about surveying accuracy and its major limits?

3. What is the intention of paper charts in the view of accuracy?

4. What forecast could be developed concerning accuracy?

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