Topographical Maps and Map Reading — Page 223, Lesson 203

I want to walk you through a critical part of map reading for professional flying — the difference between Maximum Elevation Figures and Minimum Safe Altitudes, and how they are calculated. This is genuinely a life-and-death distinction, so let's get it right from the start. First, let's define the two terms. MEF stands for Maximum Elevation Feature. MSA stands for Minimum Safe Altitude, which is also called Safety Altitude. The chart you are using will show one or the other, and you must check which one is displayed because the difference is huge. The basic calculation of the dominant obstacle is the same for both. The chart is divided into rectangles of latitude and longitude, sized conveniently for the scale. On CAA and TPC 1:500,000 charts, the rectangles are half a degree of latitude and longitude. On the ONC — that's the Operational Navigation Chart — they are every whole degree. On some of the Jeppesen small-scale charts, they are every 5 degrees, and so on. Inside each rectangle, we find the highest natural feature — that could be a mountain, hill, or ridge. Then we find the highest man-made obstruction — things like a TV transmitter mast, a crane, or a tall building. Now here is the key rule: every structure with a height AGL — that means Above Ground Level — of 300 feet in the UK and US, or 100 metres (which is 328 feet) in continental Europe, is required by law to be notified to the national aviation authority, such as the CAA in the UK or the FAA in the US. So structures less than 300 feet or 100 metres may exist without being recorded. To account for these possible unknown obstructions, we apply two rules. First, for a man-made obstruction, the elevation is accurately known, so no further allowance is necessary. Second, for a natural feature, someone may have built an obstruction of up to 300 feet — or 100 metres — on top of a hill quite legally without telling the CAA. Therefore, a safety factor of 300 feet — or 100 metres — is added to the elevation of natural features. Then we take the higher of the two values — the man-made obstruction's known elevation, or the natural feature's elevation plus the 300-foot safety factor — and we round it up to the next whole hundred feet. For example, 968 feet becomes 1000 feet. Let me walk you through a concrete example. Within a particular rectangle, there is a TV mast with an elevation of 1432 feet, and a hill with an elevation of 1268 feet. We add 300 feet to the hill, so it becomes 1568 feet. We round that up to 1600 feet. The man-made mast is 1432 feet, which is lower. So the dominant figure is 1600 feet. If the chart displays Maximum Elevation Features, or MEF, the figures 16 would appear in that rectangle — meaning 1600 feet. Now, if the chart shows Minimum Safe Altitude, or MSA — also called Safety Altitude — a further safety factor is added. If the rounded-up figure is 5000 feet or more, we add 2000 feet. If it is less than 5000 feet, we add 1000 feet. In our example, the rounded-up figure was 1600 feet, which is less than 5000, so we add 1000 feet. The MSA would become 2600 feet, shown as 26 in the rectangle. Here is the critical warning: CAA charts, TPCs, and ONCs all show MEFs, not Safety Altitudes. The book puts it in capital letters: THE MEF IS NOT A SAFETY ALTITUDE. That is a hard rule you must never forget. Let's move on to other chart features. Water — whether sea, lake, or river — is shown in blue. Woods are shown in green. Roads are shown as red lines. Railways are shown as black lines. Finally, a note on measuring distances. In many cases, using an appropriate navigation ruler will suffice, but you must be sure the chart you are using is constant scale. In any event, you should know the various ways that chart scale is depicted.