ECDIS using different chart datum

 

 

Chart makers construct their charts by using a grid of co-ordinates. This grid is called a datum. Chart makers in different parts of the world have sometimes used different datums, or data, to draw up their charts. However, over the last 10 years there has been an attempt to get all chart makers to use the same datum, the World Geodetic System 1984 (WGS 84). All ENC charts and all GPS conform to WGS 84.

 

 

In some parts of the world the official paper charts used to produce electronic charts were based on datums which do not conform to WGS 84. So if you are using a GPS, which conforms to WGS 84, in conjunction with a raster or paper chart which has been produced from non-WGS 84 datum, you may get an incorrect position for your vessel. Some GPS receivers can allow the user to change datum to match that of the chart being used, but this must be done with great care to ensure the GPS and chart datums do match precisely.

 

 

Some RNCs may not conform to WGS 84 and horizontal datums and chart projections may be different between RNCs. Mariners should understand how the chart horizontal datum relates to the

datum of the vessel's GPS. In some cases this may appear as a shift in position. This difference may be most noticeable at grid intersections and during route monitoring.

 

 

An ECDIS in RCDS mode should have the software to be able to shift, or convert, a non-WGS 84 RNC into a datum compatible with the GPS. If the ECDIS does not have such software then it should be able to display the message 'Shift not known', or a similar indication that it is unable to match the WGS

84 datum of the GPS to the RNC. In such a case, to avoid the dangers of a false position being shown on the chart or of a miscalculation of distance being made, the operator should instead fall back on conventional navigational techniques and use paper charts instead. The lesson to be kept in mind is to check which datum your chart conforms to. You should be able to interrogate the ECDIS to reveal this information. If the chart does not conform to WGS 84 and the ECDIS cannot shift it to match that datum then ECDIS display will need re-calibration. You must consult office prior doing anything to re- calibrate the ECDIS.

 

 

Scale

 

 

Make sure that information from radar or other external sensors being displayed on the ECDIS is on the same scale as the electronic chart being displayed. Most ECDIS systems can adjust the scale of an additional layer of information which has been input in a different scale so as to match that of its ENC; if not, it should be able to alert the watchkeeper so that he can make the adjustment manually. This procedure is known as 'moving up'. Always make sure you know which scale your ECDIS is operating on, and be aware of additional input perhaps being in a different scale, such as 12 mile range, 24 mile range, 1:50,000, or coastal instead of ocean.

 

 

Accuracy

 

 

In confined waters the accuracy of chart data - whether paper charts, ENC or RNC data - may be less than that of the position fixing system in use. This may especially be the case when using differential GNSS.

 

 

Clutter and not understanding what the ECDIS is showing

 

 

The ECDIS is always monitoring the information in the database and will usually display all data on the largest scale chart available. Try not to add too much information on the screen at the same time: more information does not always mean better information. Displaying all the ENC information available on the ECDIS screen at the same time can cause information overload and confuse the operator. Having too much information on-screen can sometimes make an important feature less visible. You should select only the information layers you need, and make sure you understand all the symbols on view, such as navigational aids and contours.

 

Updates

 

 

Make sure you are using the latest ENC or RNC update. You should be able to interrogate the ECDIS to check when the electronic chart you are using was last updated and what information was added. You should also check as to whether there is a more recent update available.

 

 

Only CD’s from authorized suppliers must be used to safeguard against corruption of ECDIS due to virus or other data files.

 

 

Manufacturers suggest deleting all charts from the ECDIS before the base-DVD charts are copied onto the ECDIS to avoid data failure. The copying of the Base-DVD to the ECDIS can only be done one unit at a time to ensure that at least one ECDIS is in operational mode.

The IHO ENC / ECDIS performance test is to be carried out whenever the base-DVD charts were copied to the ECDIS units.

 

 

NOTE: If the Navtex and T&P notices are not automatically transferred to the ECDIS then the Officer on Watch need to enter the data manually into the ECDIS. All cancelled messages need to be manually removed by the responsible officer. Masters are to include this in their bridge self audits.

 

 

PASSAGE PLANNING

 

 

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