Finding the coordinates of a position
- Standard is first the width and then the length
- Take the dividers and measure the distance from the object to a parallel, in the example below, the parallel of 51°20,0'N
- Read the latitude at the vertical side of the nautical chart.
- Next, measure the distance from the object to a meridian and read the longitude at the horizontal side of the nautical chart
Measure long distances
- Draw a line from the starting point to the end point
- Grab a some miles with the dividers at the vertical side
- Walk with the dividers along the line from the starting point to the end point and count the steps
- Measure what remains with the dividers
- Measure how many miles were remaining and add that to the distance of all steps you measured by walking over the line with the dividers.
Measure a course
- First, make an estimation of what you think the course will be using the variation rose. This is to prevent you use the course plotter in a wrong way resulting in a 180 degree error. This mistake happens very often in the beginning.
- Lay the side of the plotter from the starting point to the end point. Make it a habit to use the side as sometimes we need to draw a line.
- Turn the round disk to the north, ie the top of the card.
- Make sure that the north on the round disc is pointing to the north, with the lines on the round disc exactly parallel to the meridians and parallels on the nautical chart.
- Read the course and check if it matches your estimation, that you made in the first step.
Below is an example of measuring the course on the Zeebrugge leading lights, that is 136 degrees. Note that the little boat that is drawn on the Portland plotter points in the direction of the course or bearing. The lines on the round disc are parallel to the meridians and parallels.
The rest of this lesson is for Day Skipper only. Next lesson Essential Navigation & Seamanship is lesson 7 Position Fixing.
The length of a vector (arrow) indicates the speed, power or distance. The direction of the arrow is also the direction of the force, speed or distance traveled.
Converting from compass course to ground course and back again
Converting from the compass course to the ground course means that we get rid of all the errors that are in the compass course. The course is already sailed in the past. While sailing, we read the compass and convert it to the ground course and draw that ground course in the sea chart to determine where we are.
The opposite, calculating the compass course is something we do while making a planning. We are calculating a course for the future. We first measure a ground course in the sea chart and convert it to the compass course by adding all the errors like tidal current, drift, deviation and variation.
Converting a compass course to ground course
The compass course is just what we see on the compass. We convert this to the true course with the calculation that we learned in one of the previous lessons. Remember? "True virgins make dull company" and convert it to the CTW by adding or subtracting the drift. Determine the distance traveled through the water by looking on the ships log. It can be calculated in two ways. Firstly, by keeping track of the log, this is comparable to an odometer in the car. Secondly, the average speed in knots, e.g. 5 knots, during 1 hour, means 5 nautical miles per hour. Plot the Course Through Water CTW from the departure position and measure the distance through the water, using your dividers. The position at the end of the CTW line, would have been your estimated position if there had been no tidal current. Determine direction and rate of the tidal current by looking it up in an tidal almanac or the nautical chart. Plot the tidal current vector(downstream of the position we found in the previous step) by drawing a line in the direction of the tidal current with the length that corresponds with the tidal current rate. Determine the estimated position. By drawing a line from the point of departure to the estimated position you will find the course over ground. The course can be measured with the Portland Plotter. The estimated position can be found by using the horizontal and vertical sides of the nautical chart, to be written down in the logbook.
Converting a course over ground to a compass course
Plot the COG into the nautical chart by drawing a line from your departure point to your destination. Determine the tidal current rate and direction, that you will experience during your trip by looking it up in the nautical chart or tidal almanac. Plot the Tidal current into the nautical chart from the start point by plotting a vector in the tidal current's direction and mark the rate on the vector. The end of the tidal current vector is the estimated position where you would be if you would allow the boat to float with the tidal current for an hour without making any boat speed. But we want to be somewhere on the COG, so we take the estimated speed of the boat through the water with the dividers at the vertical side of the nautical chart and circle this distance / speed through the from the end of the tidal current vector through the course over ground. Now you find the position on the ground course where you can be after one hour of sailing. The vector from the end of the tidal current vector to the intersection of the boat speed circle with the COG is the course through water CTW . Measure this CTW with the Portland plotter and then convert the CTW to the compass course with the formula.