Learn Marine Navigation – Why?

Learn Marine Navigation – Why?

I asked Noel why he did all his Maritime Courses, his response was…  “I wouldn’t go to sea without knowing everything!”

Of course, anything to do with boating is a continual learning exercise, so here’s some basic info on why learning how to Navigate properly is important and why we did all the courses we could, including proper chart work!

Learn Marine Navigation – Why?

As a commercial skipper who’s worked internationally, a recreational sailor (circumnavigating the planet one-and-a-half times, plus many inland waterways), and maritime teacher, I’d like to explain why you shouldn’t just rely on electronic equipment AND why it is prudent to have several options.
Errors in GPS
GPSs do contain. You should regularly check your position via other methods, e.g. 3-bearing fix or bearing and range.

Types of errors:

  • Systematic: Some errors are caused by the changing satellite geometry. This is called Dilution of Precision (DOP). Four satellites are need for a three-dimensional fix, one of the satellites should be directly overhead, with the other three separated by 120 degrees of azimuth. This is obviously an ideal situation and any other arrangement than this ideal situation will cause DOP inaccuracy. A good value of HDOP (Horizontal DOP) is between two and four, values in excess of this size are of poor accuracy. Check the values on your GPS.
  • Moisture and salt in the air, poor weather conditions or lightning can cause environmental errors.
  • Multipath error: A satellite signal may be reflected off the water, or off metal objects on board your vessel. This may cause the GPS receiver antenna to receive the signal by two or more paths.
  • Receiver errors: Mismatch of satellite signals.
  • Selective Availability: SA is the military’s ability to scramble the GPS readings. At the time of writing, the USA is not scrambling the signal. DGPS (Differential GPS) was developed to reduce SA.
  • Ionospheric delay: Signals from satellites bend on entering the ionosphere (this is known as refraction) and their speed varies. (The ionosphere is the outer region of the Earth’s atmosphere)
    Learn Marine Navigation - Why?

Click here for Common Errors Using the GPS

Electronic charts: are useful but they contain errors. They are classed as an “aid to navigation”. Good mariners never rely on just one piece of equipment.  You can use electronic charts in conjunction with paper charts. Of course, we can lose our power with a flat battery and lightning strike, whereas paper charts are reliable in accuracy and design.

Zooming in may reveal all reefs and obstructions. BUT not zooming in enough may hide them. Also, zooming in too far distorts distances!

Chart Errors - training software
Sample of the charts we go through (minimised)
Paper Chart Errors
Many cruisers purchase secondhand charts. We did the same. But you must check that they are still in production and update them via Notice to Mariners. On much older charts, you may find that the GPS co-ordinates put you on an island. This is where the GPS is more accurate than the chart. All positions will need to be offset by the amount given in the title of the chart. Every modern chart will have a note stating whether the GPS co-ordinates can be plotted directly or an offset is to be followed.

Ensure you study your chart prior to casting off. That means all the title information, legends, and notes.

Here’s an example of off-set charts

(1): Chart AUS 252 Whitsunday Group. Under the chart title it states:
SATELLITE DERIVED POSITIONS
Positions obtained from the Global Positioning System (GPS) in the WGS 1984 Datum can be plotted directly onto this chart.Example 2: Chart AUS 802 Cape Liptrap to Cliffy Island, states:

Positions are related to the Australian Geodetic Datum (1966)
(see SATELLITE DERIVED POSITIONS Note).

Next to title it states: SATELLITE DERIVED POSITIONS
Positions obtained from the Global Positioning System (GPS) in the WGS 1984 Datum must be moved 0.09 minutes SOUTHWARD and 0.08 minutes WESTWARD to agree with this chart.

Making the corrections: In the case of chart AUS 802 the correction would be as follows:

GPS (WGS 1984) position
39° 00.00’ S
146° 15.00’ E

+ 0.09’ S
—————-
+ 0.08’ W
—————–
39° 00.09’ S
146° 14.92’ E

Corrections vary: When we traversed the Pacific Ocean we found that some island charts noted corrections that, if left un changed, would have put the island out by a nautical mile!

You’re the Skipper You’re Responsible: Learn Marine Navigation – Why?
You must understand that all equipment has limitations. You don’t have to purchase paper charts in every scale. A small scale chart (covering a large area) will show ALL reefs and hazards. And a few extra charts will seem very cheap if you start finding water around your ankles in the boat!
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2 thoughts on “Learn Marine Navigation – Why?”

  1. Pingback: Common Errors when using GPS - SisterShip Training

  2. Pingback: Common Errors when using GPS - and how to fix them

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