Aerodromes - Physical Characteristics — Page 377, Lesson 510

Let’s start with the Aerodrome Reference Point, or ARP. This is the designated geographical location of the aerodrome, given in latitude and longitude, and it must be reported to the aeronautical information services authority in degrees, minutes, and seconds. So the ARP is the official "address" of the aerodrome in coordinates. It should be located near the initial or planned geometric centre of the aerodrome, and it will normally remain where it was first established. In practice, it is usually the centre of the longest runway. So when you see an aerodrome's coordinates published, that's the ARP. Next, the pre-flight altimeter check location. An aerodrome is required to have one or more of these. They should be located on an apron, so that you can perform an altimeter check before you get taxi clearance, which eliminates the need to stop for that purpose after leaving the apron. Normally, an entire apron can serve as a satisfactory altimeter check location. The elevation of this location is given as the average elevation, rounded to the nearest metre or foot, of the area in which it is located. And here's the tolerance: the elevation of any portion of a pre-flight altimeter check location must be within 3 metres, or 10 feet, of that average elevation. So if you're standing anywhere on that apron, your altimeter setting should be within that tolerance of the published average. Now, aerodrome and runway elevations. The aerodrome elevation and the geoid undulation at the aerodrome elevation position—which is the highest point of the landing area—are measured to the nearest half metre or foot. But for precision runways, they are measured to the nearest 0.25 metre or foot. So the precision runway gets a finer measurement. And the note here is important: the geoid is the gravitational level of mean sea level extending continuously through the continents. It is irregular due to local gravitational disturbances, hence the term "geoid undulations." So the geoid is not a perfect sphere; it's the actual level of mean sea level, and it varies because of gravity. That's why we talk about undulations. Let me make sure I've covered everything. The ARP is the designated geographical location, reported in degrees, minutes, and seconds, near the geometric centre, usually the centre of the longest runway. The pre-flight altimeter check location is on the apron, average elevation rounded to nearest metre or foot, with a 3-metre or 10-foot tolerance. And the aerodrome elevation and geoid undulation at the highest point of the landing area, measured to the nearest half metre or foot, but 0.25 metre or foot for precision runways. The geoid is the gravitational level of mean sea level, irregular due to local gravitational disturbances. That's the core of this section. Let's move on to the next part when you're ready.