How to read nautical charts like a pro

But there’s not much point in fitting a sophisticated, powerful chartplotter if you don’t know how to read it. That said if the boat’s electronics ever fail and you’re forced to rely on paper charts to plot a course home, you need to know what you’re looking at.

So what are the most common features on a nautical chart, and how do you read them?

Water depths are defined by numbers, called soundings, on a chart. However, unlike on land, where profile rings show height above sea level, soundings show something quite different.

Rather than sea level, which is calculated by averaging the height of the water at low and high tide over a 19-year period, soundings on a chartplotter use something called “mean lower low tide”.

This is the average depth, in metres, of the two lowest tides. What this sounding does is give skippers an idea of the minimum amount of depth they will have under their boat on any given day. This means that at some point during a tidal cycle, the seabed may be slightly shallower than what is shown on the chart – something handy to keep in mind.

On charts, shallow underwater dangers are usually shown as crosses with a ring of dots outlining them, while on chartplotters the same thing is usually defined by a bespoke icon, usually something looking like a black dot fringed in red. These often don’t show the depth of the obstruction, so best steer clear of them.

Another big difference between electronic and paper charts is how the soundings are displayed. Soundings are based on regular surveys of the sea floor, with some surveys more up-to-date than others that may be decades, or even centuries, old. Paper chart may also give hints at whether the sea floor is sand or rock, important when choosing which anchor to drop.

You can easily tell if a paper chart is using soundings from a recent survey. Newer data is italicised, so if the numbers lean over, it’s more reliable than the more upright numbers.

A chartplotter doesn’t show any difference between what is old data and what is new. However, more modern chartplotters can remap the seabed in real time and overlay the information over the historical chart, meaning your boat has the most current underwater map possible.

Both paper charts and chartplotters will show aids to navigation. These are the systems of buoys and markers that before the age of electronics helped you safely navigate.

The difference is that while paper charts rely heavily on difficult-to-recognise icons, chartplotters can display the same information in colourful, easily identified ways.

Paper charts can show information about the direction of speed and current, but chartplotters trump this because they can show the tide height and direction of flow in real time.

You’re still going to need to know the difference between starboard (green) and port (red) navigation marks, though, as many channels may only define one edge. Passing to the correct side of the mark – defined by if you’re heading into port, or out – may be the difference between running aground and getting home.

Paper charts are fixed scale. A chart used for navigating a large body of water may use a scale as small as 1:100,000 (1.0cm on the chart will equal 1.0km in the real world) while when you’re close to shore, the chart may be scaled as low as 1:20,000 (1.0cm on the map equals 2.0km in the real world).

In contrast, a chartplotter can use any scale you want. Some will zoom in and out automatically if you’re speeding up or slowing down, giving you a good look ahead while up on the plane but showing more detail around you while picking your way through the water.

Paper charts will show distances in nautical miles, but a chartplotter can often be set up to show distances in any unit you like, as long as it’s metric, imperial or nautical. Pick your poison.

Paper charts will use a key somewhere on the map that will explain what each of the symbols used on the chart means.

In contrast, chartplotters will rely on the user moving a cursor on the screen to hover above the symbol and either automatically suggest what you’re looking at, or wait until the user clicks on the symbol to reveal what it is.

The more modern chartplotters using cameras can use augmented reality to identify and show the skipper what the symbols mean using a real-world overlay on the screen.

Electronic chartplotters are not a substitute for paper charts. Batteries can fail, and electronics can short out and die, but, generally speaking, paper charts will outlive anything that needs a battery to power it.

You can buy paper charts from a number of online chandleries.

The more techno-savvy skipper can buy a chartplotter and then buy an annual subscription to any one of a number of electronic charts packages available on the market.

Each subscription offers different levels of map detail and features, so what you spend will dictate how much data your chartplotter will show. That means you can display everything from basic charts to data-rich displays showing satellite image overlays and community edits.

Some services also offer web-based versions of their mapping systems that allow you to plot a route on a desktop, tablet or smartphone and then upload it to your boat.

However, nothing beats a switched-on brain and a sharp set of eyes at the helm when it comes to safe navigation. As in most things in life, being in the moment is always the best course to run.