GPS Receivers & Chart Datums

What you should know when navigating

• Why do datums affect me?
• So how does this affect me?
• So What Must I Do?
• Seafarer RNCs and Plotters
• GPS Receivers and Paper Charts
• GPS Receivers and Seafarer RNCs
• Plotting Waypoints on Seafarer RNCs
• GPS Receivers and Australian ENCs
• Plotting Waypoints on Australian ENCs
• Frequently Asked Questions (FAQs)
  

Why do datums affect me?

The latitude and longitude of places on a chart or map depend on what mathematical shape is used to represent the Earth (a roughly ellipsoidal, 3-dimension surface) on a chart or map (a flat, 2-dimensional surface). Different shapes get used for mapping different areas of the world. Each shape, known as a reference ellipsoid can be identified by its datum or origin. Australia’s charts used Australian Geodetic Datum 1966 (AGD66) datum for a long while - before it was changed to World Geodetic Spheroid (WGS) (which is the datum used for calculating Global Positioning System (GPS) positions). However, not all Australian charts are drawn on WGS yet. Gradually, all the world’s charts will be converted to WGS. When this occurs datums will no longer be a problem for the navigator. In the meantime the navigator should be vigilant when applying datum corrections.

For more detailed information on datums click here: http://www.ga.gov.au/nmd/geodesy/datums/aboutdatums.jsp

So how does this affect me?

If you have two charts of the same area and they have been drawn on different datums, then the latitude and longitude for the same place is likely to be different on each chart. In Australia the difference between an AGD66 and a WGS charted position can be up to 200 metres (0.1 miles). This could be the difference between one side of a reef and the other!

GPS receivers normally calculate positions using the WGS datum. If you plot a GPS latitude and longitude on a chart which uses a different datum (say AGD66), you must apply some corrections otherwise you will plot your position in the wrong place.

This example illustrates the problem:

Let’s say, for example, that you are using a paper chart drawn on AGD66. Your GPS says you are in position 33° 50S 151° 17E and you are heading West into harbour . . .

After you have corrected your GPS position so that it plots correctly on the chart, you will see that you are heading for the rocks (and disaster)!

So What Must I Do?

CHECK YOUR DATUMS CAREFULLY – all latitudes and longitudes which you use for navigation must relate to the SAME datum. If they don’t, then you will need to apply a correction. Corrections for all Australian charts which are not drawn on the WGS datum are printed in the relevant chart notes. For example, this is the notice for the chart above:

This correction allows you to convert all your GPS positions from WGS latitudes and longitudes to AGD. These are then compatible with the chart.

Seafarer RNC Charts and Plotters

Plotters which use Raster Nautical Charts such as Seafarer RNC or ARCS automatically work in WGS latitude and longitude. Seafarer RNC and ARCS charts have been corrected to work that way – even if the paper chart is on a different datum. This means that all the latitudes and longitudes which you want to enter into your plotter must be in WGS - especially the input from your GPS. Do NOT apply any corrections.

GPS Receivers and Paper Charts

Some GPS receivers allow you to select the datum which matches the one used on your paper charts. The receiver then makes the corrections for you. Be very careful when you use this feature - if you move to another chart which uses a different datum, you will need to change the datum setting in the GPS.

GPS Receivers and Seafarer RNCs

If you are using positions obtained from a GPS with a Seafarer RNC or ARCS compatibles ystem there are NO datum corrections to be made. You should simply set your GPS to operate in its standard mode - which should be WGS84.

Plotting Waypoints on Seafarer RNCs

If you want to plot waypoints taken from your paper charts on to your Seafarer RNCs the waypoints must be in WGS. If the waypoints are taken from a paper chart which is not WGS, then you will need to apply the appropriate corrections before you enter them into your GPS.

GPS Receivers and Australian ENCs

Prior to Australian ENCs being created from paper charts or source data, all the data is converted to WGS84.  Therefore, all Australian ENCs are on WGS84 datum.  If you are using positions obtained from a GPS with an ENC compatible system there are NO datum corrections to be made. You should simply set your GPS to operate in its standard mode - which should be WGS84.

Plotting Waypoints on Australian ENCs

If you want to transfer waypoints taken from your paper charts on to your Australian ENCs the waypoints must be in WGS. If the waypoints are taken from a paper chart which is not WGS, then you will need to apply the appropriate corrections before you enter them into your Electronic Chart Display and Information System (ECDIS) or Electronic Chart System (ECS).

Frequently Asked Questions (FAQs)

Q. The cursor position on my plotter is different from the lines of latitude and longitude shown on my Seafarer RNCs. Why is this?

A. Even though the original paper charts are not drawn on the WGS datum, Seafarer RNCs have been electronically "shifted" to WGS for use in your plotter. This means that the original latitude and longitude lines now seem to be in the wrong place, even though the plotter is working correctly.

Q. I have entered all my regular waypoints into my plotter and the positions are not the same as they are on my paper charts. What is wrong?

A. You have taken your waypoint positions from a chart or charts which are not on WGS. You must correct them to WGS datum before you use them in your plotter.

Q. My plotter shows me to be about 1 cable to the South West of my true position. What is happening?

A. You have probably set up your GPS to automatically apply a datum correction. Your Seafarer RNCs do not require corrections. Reset your GPS to use WGS84 datum with NO corrections.