Topographical Maps and Map Reading — Page 229, Lesson 209

I want to walk you through the concept of contrast as it applies to visual checkpoints for map reading. This is a critical idea because your eyes rely on a checkpoint standing out from its surroundings. Contrast means that a visual checkpoint should differ in colour or texture from the area around it. If everything looks the same, you cannot pick out your checkpoint. But contrast is not fixed — it changes with the seasons and with geographic location. For example, the contrast between a green field and a brown forest might be obvious in summer, but entirely different in winter when snow covers everything. Similarly, what works in a temperate region may not work in polar or desert regions. You also need to remember that you may have to map-read at night. At night, the rules of contrast are significantly different. Unless there is a good level of moonlight, your checkpoints must be lit — you need artificial or natural lights to see them against the dark background. There is also a serious flight safety risk here. In meteorological or geographic conditions of poor visual contrast, you can experience visual illusions. These illusions can disorient you and lead to dangerous situations, so you must always be aware of the contrast conditions you are flying in. Now, it is rare for any single visual checkpoint to have all the ideal attributes I just described. Your selection of checkpoints for a flight will probably be a compromise. Much will depend on three factors: the altitude you intend to fly, the seasonal and meteorological conditions, and the nature of the terrain over which you will be navigating. In the next section, the relative merits of various types of visual checkpoint are discussed, but that section cannot provide a comprehensive guide — you, the pilot, must weigh your own selections against the ideal criteria. Let's move on to some typical visual checkpoints, starting with coastlines. Coastlines generally provide excellent daytime visual checkpoints. Given reasonable moonlight, they can also be used at night. Coastlines are rarely uniform for any great distance, so you can normally achieve precise navigation by referring to specific features like estuaries, headlands, ports, or lighthouses. Next, water features — lakes, large rivers, and canals. These normally show up well, even when you account for seasonal variations such as floods and droughts. Water features tend to occur in the lowest ground in a particular area, so when you are flying, you can use them as reliable references.