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DGPS

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Differential Global Positioning System (DGPS)

GPS / DGPS Made Easy

Nautical Charts

Note: The following section describes how a mariner can make use of GPS/DGPS in conjunction with nautical charts. The same principles apply to land-based GPS/DGPS, except that land maps are used instead of charts.

If charts or maps are not available, you may relate your present position to objects around you and use this correlation to return to backtrack to previous positions. (See practical application section.)

A nautical chart, a pair of dividers or compass (divider with pencil) and a straight edge (ruler) are essential tools of safe navigation.

A nautical chart is a map of water areas and adjoining land which is intended primarily for marine navigation. It provides the mariner with a vast amount of information such as depth of water, elevation of land, Horizontal Datum, nature of the bottom and the positions of aids and dangers to navigation.

There are several scale levels of charts: a large-scale chart covers a small geographical area while a small-scale chart will cover a larger area. Larger scale charts are recommended for their greater accuracy.

These charts are produced by the Canadian Hydrographic Service (CHS). When you first look at a nautical chart, you will notice that it uses numerous symbols and abbreviations to convey important information to the user. To help you decipher these symbols and abbreviations, CHS publishes a manual called Chart 1. Information about CHS and where to obtain charts is available on the Internet at www.chs-shc.gc.ca/

Another very important publication which you will require is the Notices to Mariners which advises about any changes that may have occurred with respect to navigation, and explains how to update your charts accordingly. This publication is also available on the Internet at www.notmar.com

When using GPS or DGPS you must be able to relate the position that it provides to the world around you, otherwise the information given by your receiver will have little meaning. To confirm your position, the latitude and longitude readings from the GPS or DGPS should be plotted on a chart. Conversely, the chart can provide latitude and longitude information which you can enter into a GPS or DGPS unit for trip planning.

Once you have your chart, the first step is to make sure that you have selected the right chart datum on your GPS/DGPS receiver. This information is shown in the Horizontal Datum note near the bottom of the chart's title note and looks like this: NAD 27 & NAD 83. Some receivers may show WGS 84 instead of NAD 83.If the Horizontal Datum Note says that the chart is on North American Datum 1983 (NAD 83), then select NAD 83 or WGS 84 on the GPS/DGPS receiver. Then you may plot positions directly on the chart. Alternatively, you could input positions scaled off the chart, inputting these to the receiver as "waypoints".

If it says the chart is on North American Datum 1927 (NAD 27), you have two choices:

You can select NAD 27 on the GPS/DGPS receiver and then either plot the positions on the chart or input the positions to the receiver. With this procedure, however, there is the possibility that, in some parts of Canada, you may end up introducing an additional positioning error of up to about 50 metres.

You can select NAD 83 or WGS 84 on the receiver and manually follow the instructions of adding or subtracting published constants to both the latitude and longitude before plotting on the chart. To input coordinates scaled off the chart, one needs to reverse the mathematical operation before inputting the value to the GPS/DGPS receiver.

All Canadian charts are being converted to NAD 83 as new charts or new editions are produced.

Example of information found on chart:

Horizontal datum : North American Datum 1983 (NAD 83). Positions on NAD 27 must be moved 0.30 seconds northward and 3.59 seconds eastward to agree with this chart.

Let's start...

Scales for measuring longitude are from east to west, while the scales for latitude are from north to south.

Latitude and Longitude are defined in terms of degrees, minutes and seconds

(i.e. ° = degrees, ’ = minutes and ” = seconds)

Note:

1 degree contains 60 minutes;

1 minute contains 60 seconds; and

1 minute of latitude = 1 nautical mile (1852 metres or approximately 6076 feet).

(Please note that the Canadian Coast Guard recommends that mariners not use aids to navigation as waypoints).

Let's take a position from a chart (i.e. Cape St. Francis).

To find the latitude of Cape St. Francis, you need to place your dividers on the nearest latitude line.

Still keeping one point of the dividers (or compass) on the latitude line, extend the other point to the position of Cape St. Francis.

Once you've done this, take your dividers and place one point on the latitude scale (on the side) and read the latitude off the scale. Please be as precise as possible because errors will come back to haunt you later.

The latitude for Cape St. Francis should read 47° 48 minutes 32 seconds North

Repeat the same process for longitude by using the longitude line and longitude scale (at the top or bottom).

The longitude for Cape St. Francis should read 52° 47 minutes 09 seconds West.

The complete coordinates for Cape St. Francis are 47° 48 minutes 32 seconds North / 52° 47 minutes 09 seconds West.

Remember...

When taking a position, please be as precise as possible.

Mariners are cautioned not to use aids to navigation as waypoints - due to the risk of colliding with the aid or grounding on the danger the aid is marking.