Posted by Admin on

Three methods are used on aeronautical charts to display relief: contour lines, shaded relief, and color tints. Contour lines, as the name implies, connect points of equal elevation above mean sea level (MSL) on the Earth’s surface. Contours graphically depict terrain and are the principal means used to show the shape and elevation of the surface. Contours are depicted by continuous lines—except where elevations are approximate, then with broken lines—labeled in feet MSL. On sectional charts, basic contours are spaced at 500-foot intervals, although intermediate contours might be shown at 250-foot intervals in moderately level or gently rolling areas. Occasionally, auxiliary contours portray smaller relief features at 50-, 100-, 125-, or 150-foot intervals.

Figure shows how contours, shaded relief, and color tints depict terrain. Contours show the direction of the slope, gradient, and elevation. For example, in figure, valley floors have little or no gradient, while the mountains have steep gradients. The contours are labeled with their elevation. Shaded relief depicts how terrain might appear from the air. The cartographer shades the areas that would appear in shadow if illuminated from the northwest. Shaded relief enhances and supplements contours by drawing attention to canyons and mountain ridges. Color tints depict bands of elevation. These colors range from light green for the lowest elevations to dark brown for higher elevations. Color tints in figure range from light green in the valley to dark brown over the mountain ranges, supplementing the contours and enhancing recognition of rapidly rising terrain.

In addition to contours, shading, and tints, significant elevations are depicted as spot elevations, critical elevations, and maximum elevation figures. Spot elevations represent a point on the chart where elevation is noted. They usually indicate the highest point on a ridge or mountain range. A solid dot depicts the exact location when known. An “x” denotes approximate elevations; where elevation is known, but location approximate, only the elevation appears, without the dot or “x” symbol. Critical elevation is the highest elevation in any group of related and more-or-less similar relief formations. Critical elevations are depicted by larger elevation numerals and dots than are used for spot elevations. Figure illustrates the difference between spot and critical elevations.

Maximum elevation figures (MEF) represent the highest elevation, including terrain and other vertical obstacles —natural and constructed— bounded by the ticked lines of the latitude/longitude grid on the chart. Depicted to the nearest 100-foot value, the last two digits of the number are omitted. The center of the grid in the upper right portion of figure shows 128. The MEF for this grid is 12,800 feet MSL. This figure is determined from the highest elevation or obstacle, corrected upward for any possible vertical error (including the addition of 200 feet for any natural or man-made obstacle not portrayed), then rounded upward to the next higher hundred-foot level; therefore, almost all MEFs will be higher than any elevation or obstacle portrayed within the grid on the chart. Pilots should note that these figures cannot take into account altimeter errors and should be considered as any other terrain elevation figure for flight-planning purposes.

Latitude and longitude are labeled in degrees. Lines of latitude and longitude are subdivided by lines representing 10 minutes, and half lines representing 1 minute of arc. Because longitude represents the same distance anywhere on the Earth—unlike latitude, which decreases toward the poles—one minute of longitude anywhere on the Earth equals one nautical mile (nm); therefore, lines of longitude can be used for quick estimates of distance.

After large earthquakes in southern California in 1971 and 1994, perhaps the Los Angeles sectional chart should have contained the comment: “CAUTION—Terrain elevations subject to change without notice.”

Other topographical relief features considered suitable for navigation are contained in figure. They include lava flows, sand and gravel areas, rock strata and quarries, mines, craters, and other relief information usable for visual checkpoints.

Lava flows, sand ridges, and sand dunes are quite pronounced when seen from the air, and they make excellent checkpoints, especially if they are isolated by other terrain features. Unfortunately, most of these features only appear in the western United States. Strip mines and large quarries also make excellent checkpoints because of their visibility. Large craters, where they appear, also make excellent checkpoints.

« Prev Post