The technique that is used worldwide is called **The Intercept Method.** It is also sometimes called the Altitude-Intercept Method or Marcq Saint Hillaire Method. It is a graphical technique; that is, it utilizes a chart—often paper—to show lines of position and a **fix**.

Its use is so extensive largely because the steps involved are straightforward, and a navigator does not need to perform complicated math. It can only be used if we have some idea about where we are, to within a hundred miles or so. This is almost never a problem because we usually have some idea about our location.

The process of taking a sextant measurement, called a **sight**, and making it into a line of position is called a **sight reduction**.

Before going into detail of the sight reduction steps used in the Intercept Method, let's look at how the method works in a broad sense. For this process, we’ll assume that we have access to a celestial navigation calculator, which will provide some of the output values needed for mapping a line of position.

The main steps of the Intercept Method are:

1. Determine the "**Observed Altitude**", also called "**Height observed**" or **Ho**, of the star. This is our sextant measurement plus some corrections.

2. Select a position close to where we think we are. This is called our ``**Assumed Position”**, or **AP**.

3. Get the star's **"Computed Altitude"**, also called **"Height computed"** or **Hc,** and the star's **azimuth**, **Zn**. These are the altitude of the star and direction to its geographic position if we were exactly at the Assumed Position.

4. On a Universal Plotting Sheet, plot our AP and draw a line through it in the direction of the azimuth, Zn. This **azimuth line** is the direction towards the star's geographic position. Our LOP will be perpendicular to the azimuth line.

5.Determine where our LOP **intercepts **the azimuth line. This uses the Observed Altitude, Ho and the Computed Altitude, Hc. Mark this intercept position on the azimuth line.

6. Draw our **line of position**. It is simply a line perpendicular to the azimuth line at the intercept point.

7. Observe more than one star to get crossing LOPs; our** fix **is at the intersection.

Broadly speaking, these steps form the basis of the Intercept Method. In the next sections, let's look at each step in a little more detail, demonstrating the Intercept Method using a sextant observation of the star Capella as an example.