How to Calculate Courses and Distances at Sea

You will learn about how to calculate courses and distances by plane sailings, parallel sailings, Mercator sailings, great circle sailings and composite great circle sailing.

Each calculation features a “how to” guide and a worked example video for each question type.

You will also learn about the magnetic and gyro compass.

This online training for Deck Cadets is aligned with the Standards of Training, Certification and Watchkeeping (STCW) and the UK MCA / Scottish Qualifications Authority (SQA) Level 7 Navigational Maths and Science unit.

This learning can be used to study and prepare for:

Navigational Maths and Science assessment
SQA Navigation assessment
UK MCA Orals assessment.

You can see the contents of what is contained in this unit in the contents list below, and subscribe here.

Calculating Courses and Distances

Difference of Latitude (D’Lat)

As you move from one latitude to another, you will change latitudes. This is known as the difference in Latitude, or D’Lat.

D’Lat can be ‘N’ if you are moving in a Northerly direction but does not necessarily mean you are in the Northern Hemisphere. You name D’Lat in the direction you are moving.

How to calculate the direction of D’Lat using the lines of latitude

Step-by-step instructions for calculating D’Lat

Mean Latitude

Finding the mean latitude is not quite as simple as adding both latitudes and dividing by two, it depends on which hemisphere you are in.

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Calculating Courses and Distances

Difference of Latitude (D’Lat)

Mean Latitude

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