Unit 14

1. Read and translate the following text

Treatment of horizontal datums by electronic charting systems (ecs) / ecdis

There are many different makes and types of ECS/ECDIS equipment and thus the following information is intended as general guidance only. Users need to consult their system manuals in conjunction with these notes.

Most electronic charting systems process and display positions using WGS 84 Datum for the horizontal reference, largely because they have been designed to work with GPS receivers as the primary positioning source. In systems that allow other settings, WGS 84 Datum is normally the default. As stated earlier, a significant proportion of paper charts and thus their digital equivalents are not yet accurately related to WGS 84 Datum. This obliges the ECS/ECDIS to be able to identify when position source and chart are working on different datums and take action either to make appropriate adjustments or provide a warning to the user.

Admiralty Raster Charts (ARCS)

ARCS charts (facsimile copies of the Admiralty paper chart) contain the same position shift information as that shown on the paper chart. Where a shift to WGS 84 Datum is available, the ARCS chart carries this information. This enables the ECS/ECDIS to automatically make the appropriate adjustment to allow the ship's position from GPS to be correctly displayed on the ARCS chart. Where the shift to WGS 84 Datum is unknown, this is explicitly recorded in the ARCS chart data and the ECS/ECDIS should display a warning that the plotted position is liable to be inaccurate.

Other Electronic Chart Data

Electronic chart data is available from a wide variety of sources. Users are advised to check the user documentation with regard to the horizontal datum used by these products. ECS/ECDIS systems designed to use WGS 84 Datum will normally expect position inputs to be related to that datum. These inputs could be vessel position data direct from a navigation aid such as GPS or user entered data such as route/waypoint information from keyboard or floppy disc. If data referred to other datums is used, it is likely to result in the vessel position or any overlay information being offset relative to the displayed chart data.

Many GPS receivers have the capability to provide positions referred to a number of user selected datums. It is advisable to only use WGS 84 Datum settings when the GPS is interfaced to ECS/ECDIS. This is because other settings often use average shifts and more importantly, in most cases, the GPS does not tell the ECS/ECDIS what datum is being used. The use of settings other than WGS 84 Datum will likely result in ownship position being incorrectly displayed on the electronic chart.

Recommendations to the mariner

It is unwise to:

Assume that the vessel position shown on an electronic chart is always correct. Ignore datum shifts when navigating close to the coastline or other dangers. Make assumptions when the datum of the chart is not stated.

Rely on precise GPS positions when a chart is old or carries a "cannot be determined" note; traditional position fixing methods relative to the coastline are much safer.

It is wise to:

Confirm periodically that the vessel position (as derived from GPS etc) is correctly plotted on the electronic chart by use of relative navigation techniques such as visual bearings, radar range and bearings or transferred ARPA target.

Keep the GPS receiver set to WGS 84 datum and use the note quoted on a specific chart.

Be aware of the age and accuracy of the data used in the construction of the charts.

2. Answer the following questions:

1. How do most electronic charting systems process and display positions?

2. What do ARCS charts contain?

3. What inputs can be for ECS/ECDIS systems designed to use WGS 84 Datum?

3. Summarize all the recommendations given above to the mariners while using electronic charts from the viewpoint of datums.