Topographical Maps and Map Reading — Page 223, Lesson 201

Right, let's get into the heart of visual navigation. We're starting the chapter on Topographical Maps and Map Reading. I want to begin with the fundamental reason this entire skill exists. Modern aircraft navigation systems are incredibly reliable and accurate. But they are not infallible. They can fail, or their accuracy can degrade. And on top of that, many light aircraft simply aren't fitted with sophisticated navigation systems at all. So, as a pilot, you must develop and maintain visual navigation skills using the most basic tools you have: the map, the compass, the stopwatch, and your own eye and brain. It is absolutely essential that you can interpret the conventional symbols—what we call the legend—of every aeronautical chart you use. There's a set of legend decodes at the end of this chapter for reference. Now, every aeronautical chart shows three basic characteristics. We have Relief, which is the terrain; Other Features; and Scale. Let's focus on the first one, Relief, because it's the most safety-critical. If you're navigating in poor weather, you must take great care to avoid high ground. You either fly around it, or you fly over it at a safe altitude. But in good weather, terrain features make excellent visual references. So you need to understand exactly how these features are depicted on a map. The information about the location and height of high ground is given in several distinct ways. First, we have Contours. These are lines drawn to join all places which are at the same elevation above mean sea level. The intervals at which they are drawn can be read from the chart, and you must always check whether those contours are depicted in feet or metres. The spacing tells you about the slope. Contours that are close together indicate steeply sloping ground. Widely spaced contours indicate gently sloping ground. Next is Layer Tinting, also called Colour Gradient. Here, a different colour is used for ground in different elevation bands. For example, land between mean sea level and 250 feet might be coloured white, land between 250 feet and 500 feet light yellow, and so on. A key to this colour code is given in the margin of the map. It's crucial to realise that both contours and layer tinting are based on elevation—that is, the vertical distance above mean sea level. Layer tinting is effectively just contours with the spaces between them coloured in. Then we have Spot Elevations. The elevations of prominent peaks are shown in figures. For easy recognition, the highest spot height is printed on a white rectangle which has a black perimeter, and its position is given in the margin. Other spot elevations are simply a dot, marking the exact position, with the elevation printed alongside it. Finally, there's the Maximum Elevation Figure, or MEF, which is also called the Minimum Safe Altitude, or MSA. Most Government-produced topographical charts—for instance, the CAA charts and the US Government TPC and ONC series—show Maximum Elevation Figures. Many commercially-produced charts, such as Jeppesen's, show the Minimum Safe Altitude, which may also be known as the Grid Minimum Off-Route Altitude, or Grid MORA. So, to summarise the relief picture: contours give you shape and slope, layer tinting gives you a quick visual banding of elevation, spot elevations give you exact heights of peaks, and the MEF or MSA gives you a safe altitude figure for an area. That's the foundation of understanding terrain on a chart.