Navigating safely : Understanding your chart plotter

Feb 19, 2020

Navigating Safely: Understanding your chart plotter


Chart plotters, mobile phone apps, and open source software programmes are becoming increasingly popular as the primary form of position fixing on many recreational vessels. It’s easy to see why; on many small boats there is little room to do chart work and the motion at speed is not conducive to paper & pencil navigation. Strictly speaking these devices should be seen as aids to navigation – it’s still important to maintain situational awareness and not to become over reliant on ‘the screen’.

It’s important to remember that electronic equipment can fail and over confidence, or lack of competence, on the part of the user can lead to errors with serious consequences. The well-publicised grounding of Team Vestas Wind in the Volvo Ocean Race, due to “deficient use of electronic charts” should serve as a warning - if the pros can make this sort of error then the recreational boatie should be extra vigilant. 

Make sure you know the limitations of the type of chart your plotter uses. For example have you, or another user, removed crucial ‘layers’ of information to avoid clutter? Don’t confuse the ability to ‘zoom in’ with greater accuracy - if the original chart lacks details ‘zooming in’ won’t help. Also don’t make the mistake of losing your night vision by not being able to adjust the brightness of screen – toggling between day and night settings should be automatic.

Some plotters are almost ‘plug and play’ while others will require some technical nous to set up. For example you’ll need to set the units and language.  If HDOP and WGS84 sound like features on your TV then you might find our GPS course useful. For more details go to

Plotters combine the position information provided by the GPS receiver with the vessel’s position superimposed on an electronic representation of a chart. It’s important for users to understand that there are two fundamentally different types of charts used on plotters: ‘Raster’ and ‘Vector’.


Raster charts

Raster charts are a ‘scanned’ copy of a paper chart. Raster charts look the same as paper charts and contain the same information. While you can ‘zoom in’ and increase the size of the chart image, you are still working with the same image (resolution and scale) of the original chart. Zooming in is essentially like looking at a paper chart through a magnifying glass.

Raster charts are ‘quality assured’; they are copies of existing charts and are produced by the relevant national hydrographic office. In NZ you can source raster charts free of charge via the LINZ website – coupled with open source software you can turn your laptop into a plotter 


Instead of being a scanned copy of a paper chart, vector charts can be visualised as layers of digital information stored in a database, displayed on a screen providing a representation of a chart. The fundamental difference is that vector charts can be ‘zoomed’ in or out to a far greater extent. 

The display can also be customised, with layers of information added or taken away. For example, you may wish to remove depth sounding figures above a certain value (e.g. not show depths greater than 10m), or remove all place names to reduce clutter. Be cautious that you haven’t de-selected vital information!

On many plotters features can be ‘interrogated’ with the cursor.  For example, placing the cursor over a lighthouse will display an information box containing information regarding the light and the structure.

Many Vector charts are also produced in conjunction with hydrographic departments and so come with a ‘quality assurance’ – but be aware, not all come with this guarantee of accuracy. 

Electronic Chart Corrections

To be reliable, electronic charts, just like the paper versions, must be kept up to date. How up to date are your electronic charts? Some suppliers will give you free updates while others may have a fee. Charts for mobile devices may be updated automatically and some fixed units provide a blue tooth or Wi-Fi system for updates.

And finally  - don’t forget that good old paper charts are still produced and are an inexpensive ‘idiot proof’ way to help you navigate safely. 

For more on the basics of navigation checkout our website at  or call 09 361 4700 for more information.


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