In aviation, the standard units for **distance and speed** across the Earth are **nautical miles** and **knots**:

A knot is one nautical mile per hour (1kt = 1nm/hr).

However, the lines of longitude are further apart at the equator and closer together as you move towards the poles. This means the distance between each degree and minute of longitude changes depending on your latitude.

Therefore, one minute of longitude is ** not **equal to one nautical mile.

A nautical mile is slightly more than a statue mile:

The standard unit used for vertical distance and **altitude** is **feet**.

We can use a navigational computer (such as the CRP-1) to convert between nautical miles, statute miles, kilometres and feet. The navigational computer is talked about in more detail in later lessons, but we will look at how to convert between these units now.

Let’s look at an example of how to convert between units using the CRP-1:

**What is 3000 feet in metres and in nautical miles?**

**1 Align 30 on the inner scale with the FEET marker along the outside**

**2 Without rotating the scale, read off the metres from under the METRES marker along the outside**

Under the metres marker is 9.2

We need to know the approximate value to know where to put the decimal point.

In other words: is the answer 9.2m, 92m, or 920m?

There is roughly 3.3 feet in a metre (or 3300ft in a km) so the answer will be roughly 1000m. Therefore, we know the answer must be that **3000ft = 920m**

**3 Again without rotating the scale, read off the nautical miles from under the NAUT.M marker along the outside**

Under the nautical miles marker is 5.

Again, we need to know the approximate value to know where to put the decimal point.

There is roughly 6000 feet in a nautical mile (or 1.8km in a nm) so the answer will be roughly half a nautical mile. Therefore, we know the answer must be that **3000ft = 0.5nm**

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