UNIT 13

1. Read and translate the following text

HORIZONTAL DATUMS ON CHARTS AND SATELLITE-DERIVED POSITIONS
(Part 1)

In recent years the accuracy of navigation of vessels has been improved by the introduction of satellite navigation systems such as the Global Positioning System (GPS). Since the removal of Selective Availability (SA) in May 2000, the Standard Positioning Service (SPS) is capable of accuracies better than 225 metres (for 95% of the time) and Differential GPS (DGPS), a refinement of GPS, allows a typical accuracy of ±5-10 metres. Because of these improved accuracies, prudent mariners must be aware of the relevance of geodetic datums and the practical limitations of the depiction of charted detail.

Previous sections described the characteristics of satellite navigation systems and the accuracies of which they are capable. Unfortunately most United Kingdom Hydrographic Office (UKHO) charts are not yet referred to WGS 84 datum, the same horizontal datum as that to which GPS is referred. This means that, in those cases, positions obtained from satellite navigation receivers will not be directly compatible with the chart and mustnot be used without adjustment because the differences may be significant to navigation. Hydrographic offices are attempting to refer as many new charts as possible to WGS 84 Datum or the regional equivalent, but there remain many areas of the world where information does not exist to enable such conversions to be carried out.

Some countries are adopting their own regional or continental equivalent of WGS 84 Datum to take account of tectonic plate movements. Two such equivalents are North American Datum 1983 (NAD 83) and European Terrestrial Reference System 1989 (ETRS 89) Datum. For practical navigation purposes, other than precise operations such as automated docking, the differences between these datums will remain insignificant for many years and charts referred to these datums will also carry the magenta marginal note that "WGS 84 POSITIONS can be plotted directly on this chart".

Many different definitions of a horizontal datum (also known as geodetic datum) exist, but a practical working definition is one based on that in the Mariner's Handbook (NP 100):

"A horizontal datum is a reference system for specifying positions on the Earth's surface. Each datum is associated with a particular reference spheroid that can be different in size, orientation and relative position from the spheroids associated with other horizontal datums. Positions referred to different datums can differ by several hundred metres."

The reasons for the differences between datums are described below, but the practical result is that a given geographical position, not associated with a specific datum, could refer to different physical objects. In other words, a physical object can have as many geographical positions as there are datums and these positions can differ by hundreds, and sometimes thousands, of metres.

The differences between datums shown above are not constant, but vary around the coastline and the United Kingdom Hydrographic Office uses 11 different sets of transformation values for charts and surveys of Great Britain to obtain the best accuracies. The following diagram shows the differences in metres between a WGS 84 Datum position and the equivalent charted position referred to Ordnance Survey of Great Britain (1936) Datum (OSGB(36) Datum) at 13 points along the coasts of Great Britain.

However, information about the datum name on its own is of limited benefit to the mariner. Since 1982 the United Kingdom Hydrographic Office has been adding "Satellite-Derived Positions" notes (usually situated close to the title) to indicate the shift that needs to be applied to WGS 84 Datum positions (as obtained from GPS receivers) before plotting them on the chart. This note provides a latitude and longitude shift (in minutes) to be applied to positions obtained directly from satellite navigation systems (such as GPS) to make them compatible with the horizontal datum of the chart.

These adjustments are an average value for the whole area covered by the chart and are quoted to 2 decimal places of a minute in both latitude and longitude. The result is that the maximum uncertainty is about 10 metres in both latitude and longitude (0'-005 and 0'-014 will both be rounded to 0'-01). This uncertainty can be plotted at scales larger than 1:30,000 (where it is represented by 0-3 mm on the chart). Inevitably, cases exist where overlapping charts show different latitude or longitude shift values. For example, one chart might show 0'-06 and its neighbour 0'-07; for each individual chart the value will be an average, but in the area common to both charts the value will range from 0'-064 to 0'-066.

Over 2200 chart panels (approximately 34%) cannot be related to any known datum. Although UKHO has gathered some information to establish these relationships, both the quality and quantity of data has varied depending on the region and there are many regions where the information is unavailable. Attempts continue to improve this situation, but in the short term all that can be done is to insert a caution on charts of those regions rather than mislead mariners with optimistic statements. Those chart panels which cannot be related to WGS 84 Datum have a note which states that "the differences between satellite-derived positions and positions on this chart cannot be determined" and that "differences MAY BE SIGNIFICANT TO NAVIGATION".

The largest difference found so far is 7 miles in the Pacific Ocean, but even larger shifts may exist. The notes referred to in paragraph 10 are usually added when charts are revised and some charts that have not been subject to New Edition action since 1981 do not contain any note about positions, but it must not be assumed that any shift is negligible. The United Kingdom Hydrographic Office is attempting to identify those additional shifts that we can calculate and these are included in Annual Notice to Mariners 19, but there will remain charts without printed notes for many years.

Most manufacturers of GPS receivers are now incorporating datum transformations into their software which enables users to (apparently) receive positions referred to datums other than WGS 84 Datum. Unfortunately many cases exist where a single transformation will not be accurate for a large regional datum. For example, the relationship between WGS 84 Datum and European Datum (1950) in Norway is very different from that in Egypt, despite the name being the same. Therefore, the position transformed to European Datum (1950) in the receiver by means of a Europe-wide average may differ from the WGS 84 Datum position output by the receiver, amended to European Datum (1950) by the shift note on an individual chart. This approximation may not be significant, but it is an additional source of error and is of major significance if DGPS is being used for precise navigation.

2. Answer the following questions

How has the accuracy of navigation of vessels been improved recently?
What does mean that most United Kingdom Hydrographic Office (UKHO) charts are not yet referred to WGS 84 datum?
What is the horizontal datum?
What are the main differences between horizontal datums?
What regional or continental equivalents of WGS 84 Datum do you know?
How are most manufacturers of GPS receivers treating datum transformations at present?

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