united states geological survey topographical maps |
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The intricate details on a topographic map can be befuddling, even frustrating, if you are learning while out in the backcountry. But once you get familiar with the markings and symbols, reading maps will become a friendly aid, rather than a dreaded assignment. Topographic Maps Grid System Lines of latitude run parallel to the equator (east and west), and lines of longitude run perpendicular to the equator (north and south). These lines form a pattern of squares across the surface of the earth like a checker board. One square, formed by lines of longitude and latitude, measures 90 degrees x 90 degrees. Degrees are further subdivided into minutes and seconds. • 1 degree = 60 minutes • 1 minute = 60 seconds The important thing to understand, when we look for a topographic map for our hiking adventures, is that it will be located within this larger context of degrees, minutes, and seconds. Fortunately, it will also, usually, be identified with a place name. An alternative method of notation within the Geographic Coordinate System is the decimal degree system. In the decimal degree system, minutes and seconds are replaced by a percentage of a degree, expressed as a decimal up to four places long. It is often used for Geographic Information Systems (GIS) applications, Topographic Maps Scale Explained A topographic map lists its scale in the margin or legend. The scale is the relationship between distances on a map and the corresponding distance on the ground. For example, a map with a scale of 1:50,000 means that 1 unit of measurement on the map equals 50,000 equivalent units on the ground. Units can be in inches, feet, centimeters, or meters, depending on what scale map you are looking at and what measuring system the country uses. Most USGS maps are 1:24,000, also known as 7 ˝ minute maps. On this map, 1 inch = 24,000 inches on the ground. In Canada, most hiking maps are 1:50,000 scale, where 1 cm = 50,000 cm on the ground. A small scale map (ex. 1:100,000) covers a large area; it shows less detail with fewer features. A large scale map (ex. 1:10,000) covers a small area; it shows great detail and small features. A 1:2,500, large scale map would show individual houses and street lights. Topographic Maps Symbols and Colors The symbols and colors used on Topographic maps are fairly straight forward: • Black - man-made, “cultural,” features such as roads, buildings, etc. It is also used to show geographical names (toponymy), certain symbols, geographic coordinates, precise elevations, border information and surround information. • Blue –water, lakes, rivers, streams, etc. The names of bodies of water and water courses are also shown in blue, as are magnetic declination and UTM (Universal Transverse Mercator) grid information. • Brown - contour lines, contour elevations, spot elevations, sand. • Green - areas with substantial vegetation like a forest, orchard, or vineyard • Grey - used on the back of the map where the different symbols and a glossary of terms and abbreviations can be found. • Orange - unpaved roads and unclassified roads and streets. • Purple - features added to the map since the original survey. These features are based on aerial photographs but have not been checked on land. • Red - major highways; boundaries of public land areas. A red tint is used to show an urban development. • White - areas with little or no vegetation; also used to depict permanent snowfields and glaciers Topographic Maps Glossary • Bearing - Angular direction expressed in degrees. • Contour Lines – A line on a map joining points of equal elevation. These lines are used to illustrate relief on a map. • Distortion - The difference in shape between the actual curved surface of the earth and the flat representation of a topographic map. • Grid - A system of horizontal and vertical lines that provide coordinates for locating points on an image. • Magnetic Declination - The angle between magnetic north and true north, expressed in degrees and minutes, east or west from true north. • Magnetic North - The direction your compass needle points. • Projection - A method by which the curved shape of the earth is represented on a flat surface, overcoming distortion. • Relief – The variations in elevation of the earth’s surface. • Scale – The relationship between distance on a map and the corresponding distance on the ground. For example, a map with a scale of 1:50,000 means that 1 unit of measurement on the map equals 50,000 equivalent units on the ground. • Topographic Map – An illustration of the earth’s different elevation levels by means of contour lines. They present both natural and manmade features. • True North - Direction of the northern rotational axis of the earth – the No ... Author: Will Robertson |
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