The first is decimal minutes, written like 5150.2N — that's 51 degrees, 50.2… — Page 20, Lesson 27

I want to walk you through how we quote positions with greater accuracy than just whole minutes of latitude and longitude. We've already seen that a whole minute of latitude is about one nautical mile, but except at the Equator, a minute of longitude is smaller, so we're quoting to about the nearest 0.6 of a nautical mile. To keep things simple at this stage, we'll focus just on latitude when we talk about resolution accuracy. There are two ways to quote a position more accurately than the nearest whole minute. The first is decimal minutes, written like 5150.2N — that's 51 degrees, 50.2 minutes North. The second is degrees, minutes, seconds, or DMS, written like 51°50'12"N. In decimal minutes, we only use the first decimal place. If we need greater accuracy than that, we switch to DMS. Now, in modern automatic navigation systems where the pilot enters positions, decimal minutes are the standard. If you enter a position into an Inertial Navigation System, an Inertial Reference System, a Flight Management Computer, or a Global Positioning System, it will be to the nearest decimal minute — for example, 5150.2N. What that means is that the position is nearer to 5150.2 than it is to 5150.3 or 5150.1. By quoting to decimal minutes, you are declaring an accuracy of 0.1 of a nautical mile. That's 608 feet, or 185 metres. But where greater accuracy is required — on large-scale maps and charts — positions will be given in DMS. For example, 515013N means 51 degrees, 50 minutes, and 13 seconds North. In this case, we are declaring an accuracy to the nearest second. That means the position is nearer to 515013 than to 515014 or 515012. One second is 1/60th of a minute, which gives an accuracy of about 101 feet, or 30 metres. To give you a real-world example, the Aerodrome Reference Point (ARP) for Oxford Airport — that's the midpoint of Runway 01/19 — is quoted in the official CAA publication, the UK Aeronautical Information Package (AIP), as 515013N 0011912W. We can achieve even greater accuracy by using decimals of a second. The first decimal place of a second implies an accuracy of 1/10th of a second, which is 10 feet, or 3 metres. Using two decimal places of a second implies an accuracy of 1/100th of a second — that's 1 foot, or 30 centimetres. For instance, the UK AIP quotes the position of the OX locator beacon — that's the low-powered homing beacon at Oxford — as 515000.28N 001924.45W. Taking into account both latitude and longitude, that gives an overall accuracy of within 50 centimetres. That might be a bit more accurate than we really need for a homing aid, but it's an appropriate level of accuracy for a landing aid.