Topographical Maps and Map Reading — Page 223, Lesson 201

I want to walk you through the start of Chapter 13, which is all about topographical maps and map reading. This is a foundational skill for any pilot, because even though modern aircraft navigation systems are very reliable and accurate, they do occasionally fail or suffer degraded accuracy. Also, many light aircraft simply aren't equipped with sophisticated navigation systems at all. So there is a real need for you, as a pilot, to develop and maintain visual navigation skills using the basic aids: map, compass, stopwatch, eye, and brain. It's essential that you can interpret the conventional symbols — what we call the legend — of all the aeronautical charts you use. A set of legend decodes is included at the end of this chapter for reference. Now, aeronautical charts show three basic characteristics: relief, which is the terrain; other features; and scale. Let's start with relief, or terrain. If you're going to navigate an aircraft safely in poor weather, you must take great care to avoid high ground. You do that either by flying around it or over it at a safe altitude. However, terrain features can also make excellent visual references in good weather. So you need to understand how these features are depicted on a chart. Information about the location and height of high ground is given on maps in several ways. The first is contours. Contours are lines drawn joining all places that are at the same elevation above mean sea level. The intervals at which they are drawn can be read from the chart. You must always check whether the contours are depicted in feet or metres. Contours that are close together indicate steeply sloping ground, whereas widely spaced contours indicate gently sloping ground. The second method is layer tinting, also called colour gradient. A different colour is used for ground in different elevation bands. For example, land between mean sea level and 250 feet may be coloured white, between 250 feet and 500 feet light yellow, and so on. A key to the colour code is given in the margin of the map. It's important to realize that both contours and layer tinting — which is effectively contours with the spaces between them coloured in — are based on elevation, meaning the vertical distance above mean sea level. The third method is spot elevations. The elevations of prominent peaks are shown in figures. For easy recognition, the highest spot height is printed on a white rectangle that has a black perimeter, and its position is given in the margin. Other spot elevations are simply a dot for the exact position, with the elevation printed alongside. Finally, there's the Maximum Elevation Figure, or MEF, which is also known as 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 Minimum Safe Altitude, which may also be known as Grid Minimum Off-Route Altitude, or Grid MORA. So to summarise: you have contours, layer tinting, spot elevations, and the Maximum Elevation Figure or Minimum Safe Altitude — these are the tools the chart gives you to understand the terrain and keep your aircraft safe.