10a: Latitude and Longitude
The imaginary lines running horizontally around the Earth that measure distance north or south of the Equator.
Lines of latitude
This north is based on the Earth’s axis and never changes.
True north
GPS stands for this.
Global Positioning System
1700 hours is what time in 12-hour format?
5:00 p.m.
The method of estimating your position based on course, speed, and time.
Dead Reckoning
The imaginary lines running vertically that measure distance east or west of the Prime Meridian.
Lines of longitude
The difference/angle between true and magnetic north
Variation
This satellite-based system provides your location using latitude and longitude.
GPS
This time system is used by navigators worldwide and based on Greenwich, England.
Universal Coordinated Time (UTC) or Zulu Time
The formula used to calculate distance traveled.
Distance = Speed × Time
1 degree of latitude equals this many nautical miles.
60 nautical miles
The error caused by magnetic influences on the vessel itself.
Deviation
Water passing hull
The difference of hours between UTC and PDT as of October 2025
7 hours behind (UTC – 7)
The type of position found by updating your last known fix using heading and speed.
Dead Reckoning position
This coordinate pair gives your position on Earth.
Latitude and Longitude
Formula to get Compass Heading
Magnetic Heading ± Deviation
The waypoint feature on a GPS does this.
Marks and saves a specific position
Why do navigators use 24-hour time?
Avoid confusion between a.m. and p.m.
If you travel at 6 knots for 45 minutes, how far have you gone?
4.5 nautical miles
(6 × 0.75 hr = 4.5 nm)
Explain step-by-step how to plot a point using latitude and longitude.
Find latitude on the chart’s sides, longitude on the top/bottom, and mark where they cross?
When the compass reads 090°, the deviation shows +2°, and the variation is 15° East, what is your true heading?
073° True?” (090 – 2 – 15 = 073)
Two advantages of GPS over traditional navigation.
Accuracy and real-time tracking
If it’s 1500 UTC, what time is it in California? Answer in 24-hour time.
0800 PDT
Name all the columns that belong in a Dead Reckoning table.
Leg, Course, Speed, Time, Elapsed Time, Distance, and Remarks
If you start at 47° 36.0’ N and sail 3 nautical miles due north, what is your new latitude?
47° 39.0’ N
Your true course is 085°.
Your local variation is 14° East.
Your deviation on this heading is 3° West.
What is your compass course?
074° Compass
(085°T – 14° (E variation) + 3° (W deviation) = 074° Compass)
Why should you know how to dead reckon even with GPS?
GPS can fail or lose signal
Your log reads departure at 1530 UTC, arrival at 2330 PDT, and you traveled 48 nautical miles.
If your zone time is UTC–7, what was your average speed in knots?
3.2 knots
1530 UTC → 0830 local. 0830 → 2330 = 15 hours. 48 ÷ 15 = 3.2 knots.
You depart your fix at 0900, steering 090°T at 6 knots. You travel for 30 minutes.
How far have you traveled, and in what direction?
3 nautical miles east