Topographic Maps. What is the elevation of points A & B?

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Loren Joseph
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1 Topographic Maps Topographic Maps show the topography, or the surface features of the earth. -- Show natural features * ex.: rivers, hills, valleys, etc. -- Show constructed (man-made) features as well * ex.: buildings, roads, major gas or power lines, etc. *These maps will also give you information on the height (elevation) of the land, the steepness of the slopes, and the shape of the land ==================================================================================== Elevation the height of the land above sea level -- it is measured from a point that is known as mean sea level Mean Sea Level the point midway between the highest & lowest tide levels of the ocean -- the elevation at this point is 0 (zero) -- it serves as the reference point for elev. [similar to the equator (lat.) or prime meridian (long.)] Contour Lines lines on a topographic map that connect points of equal elevation -- when going from one contour line to the next the elevation changes (it either goes up or down) -- the shape of the contour lines will reflect the shape of the land Contour Interval The difference in elevation between one contour line and the next -- contour intervals may differ from one map to another ( 5 ft, 10 ft, 20 ft, 50 ft, 100ft ) -- the contour interval that is used is dependent upon the relief of the land Relief The difference in elevation between the highest and the lowest points of the area being mapped High Relief : very mountainous, great changes in elevation, large contour interval (ex. 50 ft) Low Relief : fairly flat, little change in elevation, small contour interval (ex. 5 ft) =================================================================================== What does the topography of the right side of the island look like? 0.B A What does the topography of the left side of the island look like? What is the contour interval of this map? What is the highest possible elevation found on the map? What is the elevation of points A & B? Index Contours occur at every 5 th contour line -- Darker than the other contour lines (they stand out more and are easier to locate) -- They are labeled with an elevation (they are used to determine the elevation of the other lines) 50

2 Quadrangles the name given to topographic map sheets. There are 2 main types of Quadrangles series * This map shows 15 of latitude and 15 of longitude * The information is reduced a great amount and as a result this type of map is less detailed series * This map shows 7.5 of latitude and 7.5 of longitude * The information is not reduced very much and as a result this type of map has greater detail Symbols used on Topographic Maps -- Buildings that are occupied -- Buildings that are not occupied -- School -- Church x BM 1321 BENCHMARK -- a reference point on the map used for determining elevations and establishing exact locations for surveying purposes. It is a brass marker that is embedded in a slab on concrete with information stamped into it. Determining Landforms using Topographic Maps * Closely spaced contour lines rapid change in elevation (steep slope) * Widely spaced contour lines gradual change in elevation (gradual slope) * Evenly spaced contour lines The slope increases at the same angle over a great distance (the same slope) Uniform, steep slope Contour lines evenly spaced and close together indicate a uniform, steep slope. The closer the contour lines are to each other, the steeper the slope Uniform, gentle slope Contour lines evenly spaced and wide apart indicate a uniform, gentle slope

3 Anytime the contour lines form into a V shape this indicates a valley. (a) Valley; (b) Draw A valley is a course of a stream that has at least a limited extent of reasonably level ground bordered on the sides by higher ground. The valley generally has maneuvering room within its confines. Contours indicating a valley are V-shaped and tend to parallel a major stream before crossing it. The more gradual the fall of a stream, the farther each contour parallels it. The V-point of the contour crossing always points upstream. A draw (sometimes called a gulley) is a less developed course of a stream in which there is essentially no level ground and, therefore, little or no maneuvering room within its confines. The ground slopes upward on each side and toward the head of the draw. Draws occur frequently along the sides of ridges at right angles to the valleys between them. Contours indicating a draw are V-shaped, with the point of the V toward the head (or top) of the draw. Often times (although not all the time) you will find a stream or a river in the bottom of the valley. Two ways to determine the direction of the river flow: -- Rivers will always flow from a higher elevation to a lower elevation (they will flow downhill). -- The V shape of the contour lines will be pointing upstream. So the river will flow the opposite way that the point of the V is pointing. GRADIENT The amount that the elevation drops between 2 points that is averaged out into feet per mile For example: if the elevation drops 80 feet over 4 miles, the gradient would be 20 ft / mile The gradient tells you the slope of the land between two points. It is calculated by dividing the relief by the path distance. Path distance is the distance measured along the path travelled (NOT necessarily the straight line distance!). For example, if you were planning a canoe trip, you may want to know the gradient between your starting point upstream and your stopping point downstream. If your starting elevation was 50 feet above sea level, and your ending elevation was 20 feet above sea level, the relief would be (50-20) or 30 feet. Although the straight line distance between your starting and ending points may only be 2 miles, with the curvature of the river the actual path distance is greater--let's say 3 miles. The gradient therefore = 30 feet / 3 miles = 10 feet per mile!

4 Hilltops will be represented with the contour lines forming into smaller and smaller closed loops. Contour Interval The Darker lines that are labeled with an elevation The max. elev. that could occur on this map would be Hill This is a point or small area of high ground. When you are located on a hilltop, the ground slopes down in all directions. Depressions (holes in the ground) will be shown using hachure lines The Rule for numbering hachure lines: When you go UP in elevation and go from a regular contour line to a hachure line, the elevation will STAY THE SAME

5 ** Extra, more advanced topographic mapping information. (You do not need to study this info. for your test) ** Concave slope (you do not need to know this term) Contour lines closely spaced at the top and widely spaced at the bottom indicate a concave slope. Considering relief only, an observer at the top of a concave slope can observe the entire slope and the terrain at the bottom or concealment from observers or weapons at or near the top; also, farther up the slope, the climb would be more difficult. However, a unit attacking up a concave slope would have no cover Convex slope (you do not need to know this term) Contour lines widely spaced at the top and closely spaced at the bottom indicate a convex slope. An observer at the top of a convex slope would have no observation of most of the slope or of the terrain at the bottom. But a unit attacking up a concave slope would have a much greater degree of cover and concealment than on a concave slope; also, the climb farther up the slope would be easier. Cliff A cliff is a vertical or near vertical slope. When a slope is so steep that it cannot be shown at the contour interval without the contours coming together, it is shown by a ticked "carrying" contour or contours. The ticks always point toward lower ground. (a) Ridge; (b) Spur RIDGE -- Normally, a ridge is a line of high ground with minor variations along its crest. The ridge is not simply a line of hills; all points of the ridge crest are appreciably higher than the ground on both sides of the ridge. SPUR -- A spur is a short, continuously sloping line of higher ground normally jutting out from the side of a ridge. A spur is often formed by two roughly parallel streams cutting draws down the side of a ridge. Saddle SADDLE -- A saddle is a dip or low point along the crest of a ridge. A saddle is not necessarily the lower ground between two hilltops; it maybe simply a dip or break along an otherwise level ridge crest.

Map Patterns and Finding the Strike and Dip from a Mapped Outcrop of a Planar Surface

Map Patterns and Finding the Strike and Dip from a Mapped Outcrop of a Planar Surface Topographic maps represent the complex curves of earth s surface with contour lines that represent the intersection