Distances at sea are measured in nautical miles which are different to land miles.
To measure distance on nautical charts, you will need a nautical ruler or dividers, which are specifically designed for use with charts. Calculating nautical miles on a chart involves measuring distances between given points using a tool called a navigational ruler or dividers.
Before we get started it’s handy to remember the following conversions:
- 1’ (minute) of latitude = 1 nautical mile
- 1° degree of Latitude = 60’ (minute) or 60 nautical miles
- Each line of longitude or latitude is called a degree
- Each degree is broken into 60 minutes
- Each minute is broken into 60 seconds
Distances are always measured from the Latitude scale and not from Longitude. This is because distances between latitude lines are equal whereas longitude lines get smaller the closer you get to the earth's poles.
On digital devices such as savvy navvy you can tap the screen of your digital chart to drop a pin at your start location, you can then tap to drop another pin at your end destination (and any waypoints in between) and the app will give you an automatic distance calculation as well as an option to plot an automatic route. It is however always recommended to use traditional and modern navigation in conjunction with each other. The app also provides a handy ruler tool for quick distance measurements.
Calculating nautical miles on a chart - key steps
In this instance let's discuss calculating nautical miles on a paper chart. Here are some key steps. 👇
Determine the scale of the nautical chart
What is a scale on a chart? To put it simply chart scales make information easy to read by shrinking the perspective scale. This is because it would be very difficult to use real-world measurements (due to large sizes) which is why charts scale that data down into more manageable data.
For example, a scale that is 1:10,000 means that anyone one unit of measure on the chart is 10,000 times larger in real life.
- One mile = 0.86 nautical miles
- One nautical mile = about 1.15 miles
On your chart look for the scale bar which indicates the distance on the chart corresponding to a given distance on the ground. For example, if the scale is 1:50,000, then one inch on the chart represents 50,000 inches on the ground.
Measure the distance on the chart
Using your dividers, measure the distance between two points on the chart. Be sure to measure along the intended route, taking into account any bends or turns in the course. Make a note of the distance. Then take this distance and transfer it to the latitude scale on the side of your chart. This will tell you the distance in degrees and minutes of latitude.
Once you have your distance in degrees and minutes you can convert it into nautical miles by using the conversion:
1 minute of latitude = 1 nautical mile
It is important to note that this method assumes that the chart is accurately scaled and that the navigational ruler or dividers are properly calibrated. Additionally, the method does not take into account factors such as tidal currents, wind, and other environmental conditions that can affect the actual distance travelled.
Once you have your distance you can start to work out your passage plan and estimate your time of arrival. For more information on this visit our other useful articles:
- How to create a passage plan using a nautical chart
- The ultimate guide to nautical charts symbols
- How to read a nautical chart depth
