Getting your bearings while navigating is essential. But it’s even more important knowing exactly where you are.
Good news: it is enough to know how to correctly consult the terrestrial coordinate grid, made by meridians (the vertical lines) and parallels (horizontal lines).
Meridians
Meridians are semicircles that join the earth’s poles.
The main one is the Greenwich meridian or Meridian Zero, at the opposite end of which is the Greenwich antimeridian or Meridian 180, or “date change line”, which passes near the Fiji Islands.
Conventionally, the meridians that we consider are 360 and are counted from Greenwich 0 to the 180th Greenwich Meridian.
In nautical charts, meridians are represented by vertical lines.
Parallels
Parallels are circles parallel to the equator.
The equator divides the Earth in northern hemisphere and southern hemisphere.
So, while the equator is the 0 parallel, the poles are the 90th. The total number of parallels is 180, 90 of which are north of the equator and 90 south of the equator.
On nautical charts, the parallels are the horizontal lines.
Geographic coordinate system
Geographic coordinate system defines the distance of a point from the equator and the Greenwich meridian.
Latitude
Latitude is expressed by the Greek letter φ (fi) and is read vertically on the scales that are on either side of the nautical chart.
It has a value from 0 to 90 North to or 0 to 90 South.
The correct definition of latitude is “angular distance” measured on the arc of the meridian between the equator and the parallel passing through a point on the surface of the Earth.
Longitude
Longitude is measured in longitude scales can have a value from 0 to 180 East or 0 to 180 West.
It is expressed by the Greek letter λ (lambda) and is read on the nautical chart through the graduated meridian scales on the chart.
Longitude is the angular distance measured on the arc of the equator between the Greenwich meridian and the meridian passing through the given point.
How to read geographic coordinates
As per general convention, we will express latitude by the letter φ, then longitude by λ.
To check the latitude of a given point, you should just scroll to the side of the nautical chart where the values are represented.
Longitude, on the other hand, is expressed in the scales at the top and bottom of the nautical chart, as mentioned in the paragraph above.
To figure out whether north or south latitude or to figure out whether east or west longitude, check where the zero point is (equator or Greenwich meridian) and adjust accordingly.
How to read scales
To represent any map in more detail, scale representations are used. This is also true for nautical charts, so you should know how to read scale map representations, first.
In a scale representation, the meridian and parallel within which the cartographic representation lies are indicated at the sides of the representation and at the top and bottom.
You will frequently come across the 5 IIM map (an acronym for Hydrographic Institute of the Navy). The area that is represented by the 5 IIM chart is between the 42nd and 43rd parallels north and between the ninth and 11th meridians east.
Another important map is the 42 IIM, which lies between the 40th and 41st parallels north and between the eighth and ninth meridians east.
Sexagesimal calculation
Remember: for all calculations involved in the measurement of nautical distances you must use a sexagesimal base.
We therefore speak of nautical mile referring to the length of the arc of the maximum circle (circle=each meridian and its own antimeridian at the height of the Equator) that subtends an angle to the center of the earth equal to one prime. So: one prime, one mile, one knot= 1852 m.
By convention, the circumference of the maximum circles is considered to be equal to 40 thousand kilometers.
Consequently, on the basis of division into 360 parts we obtain the measurement value of one degree, which is 1852 m.
By convention, the circumference of the maximum circles is considered to be 40 thousand km, and consequently on the basis of division into 360 parts, the measurement value of one degree is obtained, which is 1852 m.
Prime (symbol: ′)
In the latitude scale, the prime is the unit of measure designating the amplitude, that is, the mile measure of the node.
A prime of longitude corresponds to a mile thus to a node only on the equator, while 1° of the meridian and 1° of the equator correspond to 60′.
Mile
The mile is the unit of distance measurement and corresponds to the magnitude of a prime.
Node
The node is the unit of measurement of speed. To sail at a node of speed is to travel one mile in one hour.
Where are the primes marked on a nautical chart?
In the nautical chart, simply consult the scales at the side of the chart representation, where you will see the prime symbol (′).
From there you can follow the little squares to find the magnitude of the prime based on the notches you have counted.
The scales on either side of the map representation do not show the subdivision into seconds, so it should be considered that the prime is divided into ten small notches.
Each of these notches represents a tenth of a prime, or six seconds.
This sounds like an unnecessary complication, but it’s not!
Consider that it is physically impossible to divide the prime into 60 parts; it would require a powerful magnifying glass and would unnecessarily crowd the nautical chart.
In any case, for the sake of simplicity it is convenient to give importance along the latitude and longitude scales only to the numerical values bearing the former’s symbol (′).
We will discuss in another article which scales of nautical charts you should use – depending on the type of trip you want to make.
How to read and report the coordinates of a point on a nautical chart
It is essential to know not only how to read geographic coordinates but also how to report them on a chart.
Latitude is measured on the sides of the chart, so you can place the compass with one tip on the place whose coordinates you want to know, while the other tip can stand on a parallel that you use as a reference line. By sliding the compass with this opening in a line going to the right or left, you reach the scale at the sides of the map.
From here, you can mark a dash and read the latitude value. Now, use the same process for longitude, with the upper and lower scales.
The best way to learn how to do this simple task is to practice.
How to find the coordinates of a point on the nautical chart
In the latter case, first the coordinate values are marked on the relevant scales with a pencil, and only then you position the tip of the compass, open it, and slide it to the desired point on the nautical chart.
Use of a ruler at the first measurement (e.g., of latitude) by placing it in line with the value you have found.
It will be easier then to search for longitude and stop at the same time as the line indicated by the ruler. The intersection of the two lines is the point you were looking for!