Session 3

Measuring Latitude and Longitude

We are located West of the

Prime meridian and North of the

Equator.

LATITUDE

SCALE

The latitude scales are

found at the left and right margins of a

chart.

LATITUDE SCALE

One minute of latitude is equal to one nautical mile.

The latitude scales can be used to

measure distance.

Latitude is measured from zero degrees at the equator to 90

degrees at the poles.

In our area, the latitude scale, at the right or left-hand margin, increases from the bottom of our nautical charts toward the

top.

090 N

090 S

000

The Longitude Scales are

located at the top and

bottom of a nautical chart..

LONGITUDE SCALE

LONGITUDE SCALE

Longitude is measured from zero at the prime meridian in

Greenwich, UK, east and west to a point in the Pacific Ocean.

In our area, the longitude scale, at the top or bottom margin, increases from right to left

General Rules

On a chart, LAT/LON is read in: Degrees Minutes Seconds

Set your GPS to read out in this LAT/LON expression.

LAT/LON Expressions

On a Nautical Chart: 042 deg 56 min 45 sec North 072 deg 45 min 25 sec West

Typical GPS or LORAN read out: 042 56.750 N 072 45.417 W

Rules for reading LAT/LON

On a Nautical Chart:Read the Degrees from the

LAT/LON scales.Read the Minutes from the

LAT/LON scales.Measure the Seconds.

Finding Your Latitude Position

Latitu

de S

cale

Longitude Scale

W Or ‘A’Fl 4sec BELL

SMALL SCALE CHART

Measuring the Seconds of Latitude.

Step TWO - Stretch the other point of the dividers to nearest Parallel of Latitude.

Step THREE - Without disturbing the opening, move the dividers to the Latitude scale in the left or right margins.

Step FOUR - Read the seconds of latitude.

1

2

3

4

Step ONE – Place one point of the dividers on the

object or target.

11.0 min.

10.5 min

041 10.7” N

The LATITUDE SCALE on a Small Scale Chart usually depicts decimal minutes.

Degrees

Minutes

Seconds

10.0 min.

041 10’ 42” N

The LATITUDE Scale

The parallel line from the Bell.

One Minute of

Latitude

equals

One Nautical Mile

DISTANCE

Sixty Seconds of

Latitude

equals

One Nautical Mile

DISTANCE

SIXTYSECONDS

OFLATITUDE

ONENAUTICAL

MILE

6 seconds1/10 of a Mile

The Second Scale on a Small Scale Chart

Finding your Longitude Position

Latitu

de S

cale

Longitude Scale

W Or ‘A’Fl 4sec BELL

SMALL SCALE CHART

Measuring the Seconds of LongitudeStep ONE - Place one divider point on the object or

target..

Step TWO - Stretch the other point of the dividers back to the nearest meridian of Longitude.

Step THREE - Without disturbing the opening, move the dividers to the Longitude Scale at the top or bottom margin. 1

2

3

4Step FOUR - Read the seconds of Longitude.

The Longitude Scale

The vertical line from the Bell.

DegreesMinutesSeconds

071 01.4’ W or 071 01’ 24” W

Any questions about how to read and plot Latitude and Longitude?