Plotting Positions & Courses

 

Coastal navigation (piloting) is the ability to find your vessel's position and lay out safe courses, within sight of land, to get to any destination desired. For this, we use everything we've talked about so far; buoys, lights, landmarks, the compass, and the chart. We'll be going through these important tasks one by one; the DR, finding your position, and plotting safe, navigable courses to be used in steering the boat.

The tools of a modern navigator include:

Charts along with updates from the Local Notice to Mariners. You'll want to have harbor charts onboard to cover any harbor you might want to enter. For convenience there are chart kits sold that cover large regions with several pages including various scales of charts. You can also get by with a small scale chart (large area) and a good cruising guide. The cruising guide should include harbor chartlets for most of the harbors you might enter. The cruising guide will also include much more helpful information such as anchorages, mooring areas, and marinas to name a few.

Tide and Current Tables will be very helpful in planning your trip. There are areas you will want to navigate only with a falling tide or a rising tide.

Coast Pilot and Light List are great references if you have any questions about how to negotiate a harbor entrance or the particular phase characteristics of any lighted aid to navigation.

Pencils, sharpeners, and erasers along with a notebook. Many times you will want to write down bearings, headings, and other notes.

Binoculars are extremely useful for identifying buoys and landmarks. If you have the kind with an internal compass it can be used to take bearings on objects and is especially useful to see if that ship up ahead will cross in front of you or not. If the bearing keeps changing over a period of time you should be ok. Remember though, it's always best to take early and obvious action to avoid any danger.

Wristwatch or other time keeping device will prove invaluable onboard. For timing the light phase characteristics of buoys and lighthouses at night it is a good idea to practice timing seconds by saying one thousand one, one thousand two, and so on.

Hand-held calculator. This will come in very handy for all those speed, time, and distance calculations. For info on how to solve these see: Speed, Time & Distance.

GPS with up to date nautical charts is not essential but quite a help. Do not rely solely on this as your only source of information. While the GPS system is extremely accurate and almost never goes down the same cannot be said for the GPS receiver. Not only do they require batteries but in the harsh marine environment they are quite prone to breaking down.

Hand-bearing compass of some sort for taking accurate bearings. This could be a dedicated hand-bearing compass or binoculars with a built-in compass. This device can also be used to check the ship's compass for any deviation. For a greater understanding refer to True & Magnetic.

Depth sounder in good working condition will provide accurate readings of the water depth. We will use this in more ways than one for our navigation work.

Parallel ruler and dividers are shown below. We'll talk more about just how these devices are used. You may order both of these from our Ship's Store.

Chart with chart tools.

 

As we discussed in True & Magnetic almost all mariners use magnetic headings and bearings when within sight of land. After all, this is what the ship's compass reads directly. If we use true degrees then we must convert to magnetic at some point, the less confusion we introduce the less chance for mistakes. To keep things simple we'll be using magnetic bearings and headings throughout coastal navigation.

The ship's log is a log book where numerous items may be recorded; it's like a diary for the boat. Time of departure and arrival, crew and guests onboard, weather, engine hours (invaluable to determine fuel consumption), course changes and headings, notes, etc. It's good to get in the habit of jotting these down in an organized manner. Several log books are sold in marine stores or you can download our version free from the Downloads page.

Bearings and headings are readings from the compass or the chart. The heading of the boat is simply what the ship's compass reads at any given time, it is the direction in which the boat is headed. Bearings are compass readings to objects outside the boat as in; That freighter is bearing 035° or, that lighthouse is bearing 245°.

Heading

Boat with compass.

The image at left shows our boat and a magnified ship's compass. According to the compass we are on a heading of 60°. Magnetic north is straight up. If we turn the boat the compass doesn't turn, it will always points to magnetic north. So, in effect, we turn the boat around the compass. If you wanted to steer a course of 30° you would turn the boat to port (left) towards 30°. Or, if you wanted to steer a course of due east you would turn to starboard (right) towards E (due east or 90°).

Now you know how to steer the boat on a compass course! This is a major accomplishment and the basis of navigation. With just a few more elements you'll know how to get yourself around the bay or across the Caribbean. If you need a refresher on how to read buoys or many other items on a nautical chart please refer to Unlocking The Chart.

Bearing

Boat with compass and buoy

Here, we have turned our boat to port to come to a heading of 30°. Notice how the magnetic compass still points straight up to the top of the page. We have turned the boat while the compass remains stationary. We are sailing in the vicinity of a mid-channel buoy marking the entrance to a channel and wish to take a bearing to the buoy. In this case we are sailing into Buzzard's Bay off southern New England and looking for the entrance to New Bedford harbor. We see through our binoculars it is labeled with "BB". To take the bearing we look over the compass toward the buoy and try to visually line up the center of the compass with it and note what we see on the compass. Here, the buoy has a bearing of 330°. Because we can find the buoy on a chart we can now locate our boat on the chart. We have one bearing which will give us one line of position (LOP). We call it a line of position because we know, with some certainty, we are located somewhere on that line. If we were on a different line there would be a different bearing to the buoy.

Let's see how to transfer this reading of 330° to the chart. We will first need to find our buoy on the chart. We look for a red and white vertically striped buoy labeled "BB" on the chart and find it center left below. Next we take our parallel ruler and place it on the line of 330° magnetic (inner circle) on the compass rose, then carefully 'walk' it over to the buoy and draw the line shown.

Chart showing one LOP

 

The Fix
We know we are somewhere along the lower blue line. How can we determine our position with more accuracy? There are a number of different ways. One way is to look at our depth sounder and see what it's reporting the depth to be. We see that along the LOP the depth varies from 55' to about 40' very close to shore. Another, more accurate way, is to plot another LOP and where the two cross is where we must be. Let's look around and see what we can find that appears both on the chart and somewhere on the horizon.

There are a number of other buoys we might be able to use. However, the one just beyond our "BB" buoy, R "2", would not be a good choice. It lies to close in line with "BB" to be of any value to us. Even a very small error in our work would lead to a large error on the chart. Better to minimize the error and look for another buoy about 60° to 120° away from our 330° bearing to "BB". We see an excellent candidate at the top center of the chart labeled R "10" Fl R 4s GONG. So, we know it is a red lighted buoy flashing red every 4 seconds and it makes a gong sound. Rather than sighting over the ship's compass to find its bearing we use our hand-bearing compass which is made specifically for this task. We hold it up and pointing it at R "10" we get a reading of 45°. We follow the same procedure as before by first finding 45° magnetic on the compass rose and then drawing a line parallel to that and starting at the buoy until it crosses our first line.

Chart showing two LOPs.

 

We have now found our position on the chart. For even more accuracy we could draw a third LOP. This would be a check of our work so far, it should come very close to the current intersection. If it doesn't then we need to go over our work carefully.

Let's properly label our work so far. We have two LOPs that could be labeled or simply left alone because it's obvious where they were drawn from. We should have our notes of the headings in a supplemental log book or note book we keep for these occasions. But, we do need to label the intersection as a fix, so draw a circle around the point which shows we have fixed our position. Also, it would be very good to put the time of the fix on the chart alongside. This will be a good record of our progress into Buzzard's Bay and would function as a reference in case we need it for any reason later in our trip.

Chart showing a fix

 

Notice we use 24 hour time on the chart which will avoid the confusion of am and pm. Now that we have fixed our position we can plot a course for our destination. Unfortunately, the chart above doesn't show enough area to know where to go so we'll have to zoom out, pan up and to the left to see the bigger picture.

Chart showing entrance to New Bedford Channel

 

We have our fix at the bottom of the chart and can see where to go to enter the New Bedford Channel. This channel starts with the green and red buoys labeled "1" and "2" just above (north of) the "BB" buoy. If we can see these buoys from the cockpit then we can simply aim the boat in that direction. But, this is New England and fog has just rolled in obscuring our view. Let's plot a course to the first buoys of the channel. We start by simply drawing a straight line that runs from our fix to the buoys then, we calculate the course heading by placing an edge of the parallel rule on the course line, walk it over to the center of the compass rose, and read the magnetic heading.

Chart showing course to New Bedford Channel

 

We then label our course line with its heading and place a capitol 'M' after it to signify it is degrees magnetic. The course, and nothing but the course, goes on top of the line and parallel with it. Everything else relating to the course goes below the line and parallel with it. One more important piece to the puzzle is the distance to the channel entrance from our fix. If we know the distance we will then be able to calculate our ETA. Remember, on a mercator projection chart such as this one, each minute of latitude is equal to one nautical mile. For a further explanation please see Latitude & Longitude. If you are observant you may have noticed that our course takes us directly over an obstruction on the chart. However, if we look closely we see there is 40 feet of water over the obstruction so no need for concern.

We measure the distance or length of the course line by using the dividers. We spread the tips of the dividers to the same length as the course line then bring them to the side of the chart to find the length in nautical miles.

Shart showing latitude scale with dividers

 

Above, we can see the course is 1.9 nautical miles long. We add this to the course line in the completed chart below.

Chart showing course line with heading and distance

 

Calculating The ETA
Please see Speed, Time, & Distance for a full explantion. If our speed is 5 knots and we have 1.9 nautical miles to go this will take us: 1.9 nautical miles/5 knots = .38 hours, which in minutes is .38 x 60 = 23 minutes, for an arrival time at 1433. Knowing the distance is 1.9 nautical miles we could also reset the trip meter on the ship's log to zero and watch it slowly advance as we travel to our destination. The trip meter on the boat's log acts just like the one on your car's odometer.

We have seen the position fix is where two or more bearings intersect. We can also fix our position by being very close to a buoy that is on the chart. If we were just a couple of boat lengths from "BB" buoy in the chart we could have called that our fix. We can also use our GPS to create a fix.

The DR
What if we are in open water and can't find any buoys or other landmarks from which to get bearings? We us dead reckoning (the DR) to locate, as best we can, our position on the chart. The good thing about the DR is that we need no outside references to locate ourselves. Of course, this won't be as good as a real fix but it still serves us very well. We can get along on just the DR until we can obtain a fix.

Onboard our boat we have all the essentials required; a log which shows distance run, our compass which shows the heading we are sailing on, and a wrist watch to know how much time has gone by. The DR is simply a combination of these items. Let's take a look at how it works.

Chart showing a DR plot.

 

At 1210 we knew our position to be the lighted buoy at 'Brenton Reef'. We change course to 120°, after one hour on this course we note the log trip meter reads 6.3 so we plot our 1310 DR using these numbers. To differentiate the DR position from a fix we use a semi-circle rather than a full circle. We are probably somewhere near the 1310 DR but there are many things that affect our position; current is a large factor. Leeway may also be affecting us if we are sailing close hauled (beating) we won't be going directly where the boat and compass are pointing. To some extent we will be blown sideways by the wind. Keels are designed to counteract this effect but there will still be some amount of this sideways motion present. Leeway may be anywhere from non-existent to as much as 15 to 20 degrees off course. The DR is the best we can do until we can get a fix or a better estimate of our position.

The EP
We have seen that a fix consists of two or more bearings taken simultaneously and that the DR does not need any bearings or outside information. If we add one piece of information, from outside the boat to the DR, we get an EP or estimated position. This is better than the DR alone but not as good as the fix. This additional information is usually a single bearing we are able to get but it might also be a known current or a depth sounding.

If the DR and EP correspond, at the time they are taken, then the EP is placed along the DR. We use a square to denote the EP. If the DR and the EP do not correspond then we place the EP so it is at a right angle to the DR position. By doing it this way we should be more correct because we are placing the EP closer, if even a little, to the DR.

Chart showing an EP plot.

 

The Running Fix
Let's say we are going by a lighthouse that we can see for a period of time but we have no other buoy or landmark to give us a fix. We can two sighting of the lighthouse at two different times to give us a running fix. Still not as good as a regular fix but substantially better than just a DR and probably better than an EP.

We will take an initial bearing to the lighthouse and put it on the chart in the normal fashion. Then, as we progress along our course, we take another bearing on the lighthouse at a later time. The trick here is that we simply advance the first bearing along our course to catch up to the second bearing.

We are sailing south on a compass heading of 190°. At 0935 we take bearing 'A' on Cape Florida lighthouse and reset our trip meter to zero, see chart on left. At 1000 we are nearly abeam of the lighthouse and take another bearing 'B'. We note the trip meter on the ship's log says we have traveled a distance of 3.2 nautical miles between the two bearings.

Chart showing running fix

Chart showing running fix

 

 

 

 

 

 

 

 

 

 

 

 

 

We draw bearing 'B' the usual way so that it starts at the lighthouse and intersects our course line. Then we advance bearing 'A' 3.2 nautical miles along our course line and are careful to keep it parallel to the original. The intersection of the two bearings is our running fix. This is probably very close to where we actually are. Of course, being this close to land we would be watching our depth sounder closely.

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If you are ready to find out for yourself what it's like to sail a 35-50' boat, receive meaningful sailing lessons, and get a taste of the sea then check out an excellent resource: Blue Water Sailing School. All sailing lessons lead to ASA certification and are taught by experienced instructors who are licensed by the U.S. Coast Guard (Captain's license).