Topographical Maps and Map Reading — Page 223, Lesson 203

Let’s pick this up right where the chart’s safety figures come from. We’re looking at how a chart tells you the highest thing in a block of airspace, and the critical difference between two figures that look similar but mean very different things: MEF and MSA. First, the acronyms. MEF stands for Maximum Elevation Features. MSA stands for Minimum Safe Altitude, and it’s also called Safety Altitude. The book makes a blunt point: check which one is shown on the chart you’re using, because the difference can quite literally be a matter of life and death. Now, the calculation behind both is the same at the start. The chart is divided into rectangles of latitude and longitude, sized to suit the chart’s scale. On the CAA and TPC 1:500,000 charts, those rectangles are half a degree of latitude and longitude. On the ONC, they’re every whole degree. On some Jeppesen small-scale charts, every 5 degrees. So the rectangle size depends on the chart. Inside each rectangle, you find the highest natural feature — a mountain, hill, or ridge — and you also find the highest man-made obstruction, like a TV transmitter mast, a crane, or a tall building. Now here’s the legal bit: any structure with a height AGL — that’s Above Ground Level — of 300 feet in the UK and US, or 100 metres, which is 328 feet, in continental Europe, must by law be notified to the national aviation authority, the CAA or FAA and so on. So anything below that threshold might exist without being recorded. That’s the uncertainty we have to guard against. So the rules to handle that uncertainty are these. For a man-made obstruction, the elevation is accurately known, so no further allowance is needed. But for a natural feature, someone could have legally built an obstruction up to 300 feet, or 100 metres, on top of that hill without telling the CAA. So you add a safety factor of 300 feet, or 100 metres, to the elevation of natural features. Then you take the higher of the two values — the man-made elevation, or the natural elevation plus the safety factor — and you round it up to the next whole hundred feet. So 968 feet becomes 1000 feet. Let’s run the example. In a rectangle, there’s a TV mast at elevation 1432 feet, and a hill at 1268 feet. Add 300 feet to the hill: it becomes 1568 feet. Round that up to 1600 feet. That 1600 is the figure that goes in the rectangle. Now, if the chart displays MEF, Maximum Elevation Features, the figures 16 would appear in the rectangle — meaning 1600 feet. But if the chart shows MSA, Minimum Safe Altitude, a further safety factor is added. If the rounded-up figure is 5000 feet or more, you add 2000 feet. If it’s less than 5000 feet, you add 1000 feet. In our example, 1600 is less than 5000, so we add 1000, giving 2600 — and the MSA would become 26. Here’s the crucial warning: CAA charts, TPCs, and ONCs all show MEFs, not Safety Altitudes. And the book states it in capitals: THE MEF IS NOT A SAFETY ALTITUDE. So don’t treat the MEF number as a safe altitude to fly at — it’s just the highest feature plus the natural-feature allowance, without the extra MSA margin. Then we move to the other chart symbols. Water — sea, lake, or river — is shown in blue. Woods are shown in green. Roads are red lines. Railways are black lines. There’s an exercise at the end of the chapter to consolidate these symbols. Finally, distance measurement. Beyond symbols, the pilot needs to measure distances on the chart. Often a navigation ruler will suffice, but you must be sure the chart is constant scale. And in any event, you should know the various ways chart scale is depicted. That’s the next piece we’ll look at.