Let's pick this up right where the precision of position quoting gets… — Page 20, Lesson 27

Let's pick this up right where the precision of position quoting gets interesting. We've been talking about quoting positions to the nearest whole minute of latitude, and I mentioned that except at the Equator, that's only accurate to about the nearest 0.6 of a nautical mile. To keep things clean at this stage, we're going to focus purely on latitude when we talk about resolution accuracy. Now, there are two ways to quote a position with greater accuracy than the nearest whole minute. The first is decimal minutes, written like 5150.2N. The second is degrees, minutes, and seconds, which we abbreviate as DMS, written like 51°50’12”N. Here's the key rule: in decimal minutes, we only use the first decimal place. If we need accuracy beyond that, we switch to DMS. In the real world of modern aviation, the automatic navigation systems where you, as the pilot, enter positions—these all use decimal minutes. So whether you're entering 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, like 5150.2 N. What that statement really 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, which is 608 feet, or 185 metres. Now, where greater accuracy is required—and this happens on large-scale maps and charts—positions are given in DMS, like 515013N. Here we're declaring an accuracy of the nearest second, meaning the position is nearer to 515013 than to 515014 or 515012. That's an accuracy of 1/60th of a minute, which works out to about 101 feet, or 30 metres. Let me give you a concrete example. The Aerodrome Reference Point for Oxford Airport—that's the ARP, and it's the midpoint of Runway 01/19—is quoted in the official CAA publication, the UK Aeronautical Information Package, or AIP, as 515013N 0011912W. But we can go even finer. We can use 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. If we go to 2 decimal places of a second, that implies an accuracy of 1/100th of a second, which is 1 foot, or 30 centimetres. And here's a great real-world example: the UK AIP quotes the position of the OX locator beacon—that's our low-powered homing beacon at Oxford—as 515000.28N 001924.45W. That gives an overall accuracy, taking into account both latitude and longitude, of within 50 centimetres. Now, that's perhaps a bit more accurate than we really need for a homing aid, but it's an appropriate level of accuracy for a landing aid. So the progression is clear: whole minutes, then decimal minutes for 0.1 nautical mile accuracy, then DMS for 1/60th of a minute, then decimals of a second down to centimetres. Each step is a declaration of how precisely you know where you are.