Kayarchy - the sea kayaker's online handbook and reference

 

Sea kayak navigation (2)

Chartwork

Lighthouse on Mull

 

Maritime charts

All over the world, nautical charts use the same conventions specified by the International Hydrographic Organisation. The major symbols and abbreviations are identical. Some colours and some written abbreviations vary with the country and language.

For British waters most seafarers use official Admiralty charts prepared by the UK Hydrographers' Office. For a list of UK charts and many worldwide charts go to www.ukho.gov.uk and download the digital catalogue. For popular parts of the British coast go to the Admiralty Leisure website. A thick A4 booklet entitled "Symbols & Abbreviations Used On Admiralty Charts" is sold separately as Chart 5011. A yacht chandler will stock the charts for the local area and will be able to order charts for other regions. You can obtain others by mail order. See Kayarchy Shopping.

If you are in an area which gets a lot of leisure traffic you could use a similar yachting or kayaking chart from an independent publisher. These usually have more information about the layout of small creeks and harbours, whether the foreshore is sand or mud, and the location of waterside shops, and they often include a simple tidal atlas of the area.

Maps & charts compared

Ideally a sea kayaker will have both an Ordnance Survey map and a maritime chart covering any unfamiliar area (s)he intends to explore. A chart shows the depth of the sea, gives some information about currents, and shows navigation marks and maritime hazards such as shipping channels and military exclusion zones that will not appear on a map. This imaginary chart extract shows an area with unusually strong tidal streams, as you can tell from the tidal stream arrow and the symbols marking powerful eddies and overfalls.

Map and chart comparison

On a map the sea is shown in plain blue with no indication of depth, and only a hint about the places where sand or mud will be exposed at low tide. However a map is often more use than a chart. It shows the shape of the land very clearly with contour lines which join points of equal height. Where the contour lines are close together, the slope is steep. The shape of the land is also shown on this map by darker colouring for higher land, spot heights and conventional "shadows", and more information is given about land features. This makes it easy to identify a landmark such as a village, bridge, island, wood or mountain. If you can identify a landmark, you can easily find your own position at sea.

Kayakers and walkers seldom go long distances so they prefer their charts and maps on quite a large scale. 1:50,000 is usually fine for both. A 1:25,000 map will show more detail including rock outcrops, vegetation, paths, fence lines and campsites. A 1:25,000 chart has more space to show features and hazards that may affect small craft.

  Metric Non-metric
Hiking, normal sea kayaking 1:50,000 (2 cm = 1 km) 1:63,360 (1 inch = 1 statute mile)
Mountaineering, kayaking in area with fast currents or complex topography 1:25,000 (4 cm = 1 km) 1:24,000 (1 inch = 2,000 feet / 0.379 land miles)
Detailed harbour plans 1:10,000 or larger 1:10,000 or larger

A smaller scale of 1:75,000 scale (1.5 cm = 1 km) is less useful and your entire day trip may fit into a five-centimetre square. For many sea kayaking areas, charts are available only a 1:75,000 scale. See also Distances On A Chart.

Below you will see an extract from an Admiralty Leisure chart, reproduced by kind permission of the UK Hydrographic Office. It shows the tip of Pembrokeshire in Wales. This is an area with excellent scenery and wildlife, strong tidal currents which create challenging sea conditions, surf beaches, and plenty of nice pubs and walks if it's too windy for kayaking. It also has some truly exceptional hazards for the sea kayaker.

The scale of the chart is 1:75,000, and the extract is about one-thirtieth of it. On paper this extract is 16 centimetres wide. The area covered is a little over 6 nautical miles (11 kilometres) wide.

For a higher-resolution image, click here.

Chart of Ramsey - low resolution

To the west you will see a dozen islands called the Bishops & Clerks. The South Bishop is one of the largest, and has just enough room for a lighthouse.

Most charts have a few tidal diamonds which are symbols printed in magenta that refer to a table in the margin. The table states the speed and direction of the tide at that location at various times. This chart has only two, neither of which appear on this chart extract. Both are near the mouth of Milford Haven for the benefit of oil tankers going to the oil refinery. Exceptionally, because there are very fast currents in the area covered by the chart extract, there is a tidal stream arrow. See the arrow halfway up the left side of the extract. The arrow has feathers, showing that it indicates the direction of the incoming tide at that location. It states that the flood tide flows at 2 knots during neap tides and 5 knots during spring tides.

The sea around the Bishops & Clerks and in the channel between them and Ramsey Island is littered with the twin wiggly lines that mean overfalls and tide rips. That's useful information for a kayaker, enabling you to avoid them either by a sprint across, by going at neap tides, or by crossing the channel at slack tide. In general, see Tides.

The most striking landmark is the St Davids lifeboat station at St Justinian, located just south of Point St John. It's a white building with a red roof which stands on high concrete legs and has a long, steep ramp for launching the lifeboat. The only sign of it on the chart is a small black symbol consisting of two cones stuck together end to end. (OK, it does look just like the Dagger Kayaks logo!)

The magenta box on the chart and the number 1482 indicates that UKHO also publishes a 1:25,000 chart covering only that area. This would be very useful for a kayaker making a first visit to this challenging region.

For more about what charts do show, see Features On A Chart.

What a chart does not show

Charts are intended mainly for use by ships and yachts so they have very little information about roads, car parks and paths, and they leave out many things which a kayaker would regard as a maritime hazard.

For example if a headland thrusts out a long way into the sea, it can create a small tide race or turbulent eddyline which is nothing to a ship but troubling to a novice or intermediate kayaker. Likewise, an underwater reef running out five metres from the tip of a headland won't be on the chart, but in a big swell it may unexpectedly create a breaking wave large enough to capsize or loop a kayaker. Another entertaining hazard you may encounter is the blowhole, in the form of a long thin horizontal tunnel into the base of a cliff. When the swell runs into one and fills it up, it may eject a horizontal jet of mixed air and water with the strength of a fire hose. Not on the chart.

An experienced sea kayaker who knows the speed of tidal currents in the area can look at a map or chart and accurately fill in a lot of information by guesswork. See the Reef Point illustrations at Kayaking On Fast-Moving Water.

The 1:75,000 Pembrokeshire chart extract we have used as our example does not show:

The B-tches, which are either a serious hazard or a white water playspot depending on the state of the tide and your point of view. A line of little islands with shallow water between them, they are not named on the chart but appear as a green finger sticking out of the east coast of Ramsey Island. The green colour indicates that at very low tides you can walk dry-shod along the line of islands.
• The Horse Rock whirlpool, which is that very, very rare thing, a whirlpool that can take down a sea kayak. Horse Rock is not named on this 1:75,000 chart but it is shown as a tiny round dot in the middle of Ramsey Sound, just east of the deep water channel. The chart indicates that it is exposed at very low tides. Its drying height is marked as 0.9 metres above the level of the lowest tides. Note the ship wreck marked immediately alongside it.
•. Information about the shape and height of the land. It is often possible to identify an island or section of coast by its shape. On a map, relief is shown by contour lines, by different colours used for land of different heights, by symbols for cliffs, etc, and sometimes by overprinting shadows to give a three-dimensional effect. On a chart, the land is usually shown in one colour, and contour lines are either omitted or given only at intervals which are too large to be much use.
• The nature of the shoreline. You can't see them on the 1:75,000 chart, unless you use a magnifying glass and some imagination, but there are low to medium cliffs right round the coast except at Whitesands Bay and Porth Clais, with the occasional low-tide-only beach.
• Paths you could use to get up from the water in emergency.
• Most of the farms, camp sites and homes that might be useful in an emergency.
• The St Justinian car park and steps down to the beach.
• The Whitesands beach car park and café.
• The small town of St Davids, which surrounds the cathedral marked halfway up the right side of the extract.

For those things, you need to look at a 1:50,000 or maybe 1:25,000 chart alongside a yachtsman's or kayaker's pilot guide and the 1:50,000 Ordnance Survey map.

Chartwork ashore

A yachtsman goes to sea with a chart table and a rack of tools,. A kayaker has to do the chartwork before (s)he goes afloat. On the water (s)he will have time only to check to see whether (s)he is still on course and on time. See Navigation Checks While Afloat.

You can use your chart at home to work out times, distances, bearings and escape routes for your trip. To take with you in the kayak you can make a waterproof extract of the chart and a note of the times of high tide and slack water.

We will look at the following topics:

• "North", headings, bearings & reciprocals
• Magnetic variation
• Latitude & longitude
• Distances on a chart

• Features on a chart

• Co-ordinates on a chart

• Navigation tools ashore

• Navigation tools afloat

• First, take your chart...

Adjusting your plot for currents
• Using tidal streams
• Adjusting your plot for wind
• Aiming off

"North", headings, bearings & reciprocals

North. Maps and charts are laid out with true north at the top. True north is the direction to the North Pole, which is one end of the axis of planetary rotation. A compass points to the Earth's magnetic north pole which is not a fixed point. See Magnetic Variation.

Heading. This is the direction in which your kayak is pointing, which a navigator would describe in terms of its angle from north. Your kayak may be heading 90 degrees, but tidal currents and wind meand that this is not necessarily the direction in which you are going.

Course ("intended path"). Your course could be defined is the direction you think you will have to paddle to reach your destination, after allowing for wind and tide, and the time for which you think you will have to keep going.

Track ("course over the ground"). Your track is where your kayak actually goes in relation to the sea bed. If you're in a race it may be a straight line, if you're not it probably has a few angles where you stopped to take a photo or eat a sandwich. You'll never know, unless you like using GPS.

Bearing. A bearing is the angle from north of a line joining two points. Conventionally, a circle is divided into 360 degrees, so the bearing of something precisely north of you is zero degrees, east of you is 90 degrees, something directly to your south is at 180, and to your west is 270.

To help users calculate a bearing with only a small orienteering compass, a terrestrial map has a grid of north-south and east-west lines printed on it. On the usual 1:50,000 walker's map the grid lines are at intervals of 1 km (2 cm on the map).

A chart also has a grid but the lines are much further apart, usually at intervals of 4 nautical miles, so a chart has a compass rose to help you calculate bearings.

A compass rose is a disc about 5 cm in diameter, printed on the chart. It has an outer ring divided into 360 sections, where zero / 360 is true north, and either an arrow showing magnetic north or an entire inner ring where zero / 360 degrees is magnetic north.

Bearing from northThe compass rose in the image is from a recent chart of south-west Wales. The writing along the magnetic north line says "5 degrees 10 minutes W 2001 (9 minutes E)". The figure in brackets is the annual variation. To find the magnetic variation for 2010, deduct 9 minutes x 9 years, which is 1 degree 30 minutes, so magnetic variation for this chart in 2010 is 3 degrees 40 minutes.

Let's say you are planning a trip from Mythical Harbour (on the left of the image) to Seal Island. The island lies roughly due east, so plainly the bearing will be roughly 90 degrees. To find the precise bearing, draw a line from the harbour to the island. Transfer that line to the compass rose to see the angle it makes with magnetic north. The easiest way of doing this is to use one of the navigation tools listed here. You find that the angle is 80 degrees magnetic, so that is your heading for a trip to Seal Island - if there is no significant tide or wind.

Reciprocal. For the return journey, your heading is the reciprocal bearing. In other words, add 180 degrees. So your heading on the return journey will be 260 degrees magnetic.

Magnetic variation

Your compass points to the Earth's magnetic north pole, not the true North Pole which is the northern end of the Earth's axis of rotation. The magnetic north pole is presently wandering around in northern Canada. The Earth's magnetic field also has flaws and swirls which move around from year to year. The only simple way to establish the direction of magnetic north in a particular location is to use a recent chart or map.

Chart

If you are navigating with a chart, printed on it in various places will be a compass rose like this one, showing both magnetic and true north. When you have drawn a line on your chart to show your intended route, you can easily find its magnetic bearing by using a straight-edge to transfer it to the nearest compass rose printed on your chart. If the nearest compass rose is printed some distance away you can prepare your chart extract in advance by drawing a grid of north-south lines parallel to the magnetic north line on the nearest rose. When you set off in your kayak, simply use your compass to follow that bearing.

So there's no real reason to use true north at all. If you do everything in magnetic you won't have to do mental arithmetic to convert your bearings between true and magnetic. Easy. And if you do your kayaking somewhere with a large magnetic variation such as Maine, there's no risk you'll convert the wrong way and end up paddling 40 degrees off your intended course.

Map

Terrestrial maps don't generally have a graphic indication of magnetic north. Instead they have a grid of squares printed over the entire map, pointing more or less true north. (In fact they point to "grid north" which is a cartographer's convention and may be a degree or so off true north. See www.ordnancesurvey.co.uk/oswebsite/aboutus/reports/misc/north.html)

Two centuries ago, magnetic variation in Britain was about 30 degrees west. It is now so small in much of Britain that hikers and kayakers don't bother to adjust for it. For 2010 in south-west Wales, click here.

However in the US state of Maine, magnetic variation is approximately 20 degrees west. This is often written as -20 degrees. Up in Newfoundland it is -20 at St Johns and apparently -40 at Battle Harbour. To go from A to B in Maine, if the chart indicates a course of 90 degrees true you must adjust for variation or end up far to the north of B. Since the variation is west, to convert from true to magnetic you must add it. To get from A to B you will steer 110 degrees magnetic.

To convert a bearing from magnetic to true, you subtract a "west" variation. Say you are on the water in Maine and you can see a conspicuous lighthouse. You want to know where you are in relation to the lighthouse, so you use your compass to measure its bearing as 270 degrees from you. Its true bearing on a map would be 250 degrees.

On the water, it isn't easy to remember whether to add or subtract so people tend to use a mnemonic. Where there is a significant westerly variation, hill walkers traditionally say "grid to mag, add; mag to grid, get rid" and sailors remember GUMA-MUGS for "grid unto magnetic add; magnetic unto grid subtract". Any of these will work fine for west variation. You'd need a different mnemonic for parts of the world where magnetic variation is east. For example Texas where it is about 10 degrees east (+10 degrees) and Puget Sound where it is 18 degrees east.

Latitude & longitude

Charts are based on a grid of lines superimposed on the globe.

Lines of latitude are like parallel slices through an orange. They are all parallel to the equator and show how far north or south you are. They are longest at the equator and get shorter towards the poles.

Lines of longitude are like segments of an orange. They all run from North Pole to South Pole, and are all the same length. They show how far east or west you are of the International Prime Meridian, the line of zero longitude. For historical reasons this runs through Greenwich Royal Observatory in London.

Distances on a chart

We have said that a nautical mile is the length of one minute of latitude. There are 360 degrees in a circle and 60 minutes in a degree. Charts have a horizontal scale of degrees and minutes along the top and bottom, and similar vertical scales at right and left edges. To measure distance, use ONLY the vertical scales.

This is important. Lines of latitude are parallel but lines of longitude converge to a point at the poles. On a chart of English waters, this means that the length of a minute on the horizontal scale has shrunk to only 60% that of a minute on the vertical scale. The horizontal scale is there so seafarers can calculate co-ordinates.

 

 

 

 

 

 

    Go to next page for:
Features on a chart
Co-ordinates on a chart

Navigation tools ashore

Navigation tools afloat

First, take your chart...

Adjusting your plot for currents

Using tidal streams

Adjusting your plot for wind

Aiming off
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