BIOL 2406 Fall 2011 Topographic Maps

Transcription

1 BIOL 2406 Fall 2011 Topographic Maps A topographic map is a two dimensional representation of a three dimensional land surface. Elevation or relief is shown through the use of contour lines that are continuous points of the same elevation or vertical distance above a reference point such as sea level. There are also colors and symbols that represent other features on the land, such as water, vegetation, roads, boundaries, urban areas and structures. The US Geological Survey produces a series of topographic maps that depict sections of the Earth's surface. These maps systematically divide the United States into precise rectangles based on a latitude/longitude grid system (Figure 1). These maps are commonly referred to as quadrangles or quads. Figure 1. Latitude/longitude grid system showing quadrangle. quadrangle Latitude and Longitude The most common way to locate points on the surface of the Earth is by standard, geographic coordinates called latitude and longitude (Figure 2). These coordinate values are measured in degrees, and represent angular distances calculated from the center of the Earth. Latitude values indicate the angular distance between the Equator and points north or south of it on the surface of the Earth. A line connecting all the points with the same latitude value is called a line of latitude. All lines of latitude are parallel to the Equator, and are referred to as parallels. Parallels are equally spaced. The Equator is given the value of 0 degrees Figure 2. Geographic coordinate system. latitude. There are 90 degrees of latitude going north from the Equator and 90 degrees to the south of the Equator. The North Pole is at 90 degrees N and the South Pole is at 90 degrees S. When the directional designators are omitted, northern latitudes are given positive values and southern latitudes are given negative values. Lines of longitude, called meridians, run perpendicular to lines of latitude, and all meet at the poles. Lines of longitude are not parallel; the closer they are to the poles, the shorter the distance between 1

2 them. There is no obvious 0-degree point for longitude, as there is for latitude. By international agreement, the meridian line through Greenwich, England, is currently given the value of 0 degrees of longitude; this meridian is referred to as the Prime Meridian. Longitude values indicate the angular distance between the Prime Meridian and points east or west of it on the surface of the Earth. There are 180 degrees of longitude to the east of the Prime Meridian; when the directional designator is omitted these longitudes are given as positive values. There are also 180 degrees of longitude to the west of the Prime Meridian; when the directional designator is omitted these longitudes are given as negative values. The 180-degree longitude line is opposite the Prime Meridian on the globe, and is the same going either east or west. The 180 degree meridian is also known as the International Date Line. Counting and Converting Minutes and Seconds Degrees alone are not accurate enough to find a precise location. At best, one degree of latitude and longitude would define an area of 70 square miles. To overcome this problem, degrees are divided into minutes and minutes are divided into seconds: 1 degree (1 ) = 60 minutes (60 ) 1 minute (1 ) = 60 seconds (60 ) When counting seconds, if the count reaches 60, add 1 to the minutes count and start counting seconds again from 0. When counting minutes, if the count reaches 60, add 1 to the degrees count and start counting minutes again from 0. Latitude and longitude can be expressed as degrees minutes seconds (ddd o mm ss.s ), degrees decimal minutes (ddd o mm.mmm ), or decimal degrees (ddd.dddd o ). It is essential to know how to make conversions between these. To convert from degrees minutes seconds to decimal degrees first divide the seconds by 60 to get the degrees decimal minutes. Next divide the decimal minutes by 60 to get the decimal degrees. For example, to convert 48 20' 30" into decimal degrees: 30" 60 = 0.5' yielding ' = yielding To convert from decimal degrees to degrees minutes seconds multiply the decimal degrees by 60 to get degrees decimal minutes then multiply the decimal minutes by 60 to get degrees minutes seconds. For example, to convert into degrees minutes seconds: x 60 = x 60 = 48 20' 30" Contour Lines Contour lines connect points on the map that have the same elevation above sea level. As a result of this every contour line must eventually close on itself to form an irregular circle. Contour lines do not overlap or cross one other, or else you have two elevations in the same exact place. However there are exceptions to this rule. If there is a vertical cliff contour lines can merge to form a single line. On an overhanging cliff or a cave the contour line representing the lower elevation is dashed. Where contour lines are spread out, you have a gentle slope. Where contour lines are close together, you have a steep 2

3 slope. Individual contour lines are usually brown in color. Heavy brown lines are referred to as index contours because they often have elevations printed on them. The distance (or elevation) between contours is referred to as the contour interval and is often specified on most topographic maps in feet or meters. Moving from one contour line to another always indicates a change in elevation. To determine if it is a positive (uphill) or negative (downhill) change you must look at the index contours on either side. On a hill with a consistent slope, there are always four intermediate contours for every index contour. If there are more than four intermediate contours it means that there has been a change of slope and one or more contour line has been duplicated. This is most common when going over the top of a hill or across a valley. At certain points on a topographic map, individual elevation points apart from the contour lines are measured. These more precise measures are called bench marks. On the ground these points are marked by a brass plaque. On the map they are marked by an X with the elevation labeled beside it. Sometimes the bench mark has the initials BM in front of the measurement. Rules of Contour Lines Contours do not cross or intersect each other, except in the rare case of an overhanging cliff. All contours eventually close, either on a map or beyond its margins. Contours are widely spaced on gentle slopes. Contours are closely spaced on steep slopes. Evenly spaced contours indicate a uniform slope. The inside of a closed contour line is the high side. A series of closed contours represent a hill. The top of a hill is higher than the highest closed contour. Hollows (depressions) without outlets are shown by closed, hatched contours. Hatched contours are contours with short lines on the inside pointing downslope. The bottom of the hollow is lower than the lowest closed contour. Where a contour line crosses a stream or valley, the contour bends to form a "V" that points upstream. In the upstream direction the successive contours represent higher elevations. Elevation increases as you move away from a stream. In crossing a valley, the last contour met before reaching the stream is the first one encountered on the other side of the stream. A single higher elevation contour never occurs between two lower ones, and vice versa. A change in slope direction is always determined by the repetition of the same elevation either as two different contours of the same value or as the same contour crossed twice. Contour lines never run into a body of water. Reading the Margins Information about the map is found in the margins (Figure 3). For example, the names of the eight adjoining quadrangles are in the corners and sides of the map. 3

4 Agency or Author Who Created Map In the upper left corner is the agency or author who created the map. This same information can also be found in the bottom left corner. Map Title The title of the map is in the right upper corner. This corner section provides the name of quadrangle, state (and sometimes the county) where the quadrangle is located, and map series. Quadrangles are often named after a prominent city, town, or natural feature that is within the quadrangle. adjoining quadrangle map author longitude tic mark adjoining quadrangle longitude tic mark map title adjoining quadrangle longitude & latitude reference coordinates latitude tic marks date created & projection used map scale contour interval quadrangle location legend revision date Figure 3. Plano quadrangle showing margin information. Latitude and Longitude Latitude and longitude lines are indicated with fine black tick marks. These graticules are intersections of latitude and longitude lines and are marked on the map edge, and appear as black cross-marks (+) at 4

5 four points in the interior of the map. Topographic maps do not show the latitude/longitude lines just the tick marks. The numbers next to the tick marks indicate degrees ( ), minutes (') and seconds ("). On 1:24,000 scale maps, latitude and longitude tick marks are indicated every 2.5 minutes. Longitude tick marks are on the top and bottom edges of the map and latitude tick marks are on the right and left edges. Note that the degrees may be left off (as an abbreviation) and you may only see the minute and/or second designations. Reference coordinates for latitude and longitude (degrees, minutes, and seconds) are black and located on the four corners of the map. Legend In the right lower corner there is a legend that contains symbols for the roads. Slightly to the left of this area there is the quadrangle location. The location is pinpointed on a map of the state. Also in this corner is the revision date, which is when the map was last updated. If the map is old, it may not be accurate. Refer to the map production information in the bottom left corner for additional information on map dates. Contour Interval Contour lines can be drawn for any elevation, but to simplify things only lines for certain elevations are drawn on a topographic map. These elevations are chosen to be evenly spaced vertically. The contour interval is the difference in elevation between two adjacent contour lines. A typical interval is 10 feet. This means that every time you go from one contour line to another the elevation would change 10 feet. Index lines would therefore occur every 50 feet. Figure 4. Typical 7.5 minute topographic map scale. Map Scale The scale of the map is found at the bottom center. Map scale represents the relationship between distance on the map and the corresponding horizontal distance on the ground. Scale is represented in two different ways on a topographical map (Figure 4). The first is a fractional or ratio scale. A typical map scale is 1:24,000. This means is that one inch on the map represents 24,000 inches on the ground. These maps are commonly known as 7.5-minute quadrangle maps; each map covers 7.5 minutes of latitude and 7.5 minutes of longitude, which is approximately 8 miles (north/south) and 6 miles (east/west).the USGS produces maps using the 1:24,000 scale, but also produces maps using 1:62,500, 1:100,000, and 1:250,000 scale. The 1:24,000 scale provides larger and clearer details than the 1:250,000, but it does not cover as large an area (Figure 5). Below the ratio scale is a bar or graphic 5

6 scale representing distance in miles, feet and meters. The bar is divided into segments, each of which represents an equal distance on the map. The left side of the bar is usually divided into smaller units of equal size to make it easier to measure shorter distances. The bar scale is useful when a user does not have a ruler or measuring scale; the distance between two points can be transferred to a piece of paper and then compared to the bar scale. 1:250,000 1:100,000 1:24,000 Figure 5. The effect of map scale Estimating Slope Slope is the degree of inclination or steepness and it is usually expressed in percent. A one percent slope indicates a rise or drop of one unit over a distance of 100 horizontal units. For example, a one percent slope rise would indicate a one foot rise over a 100 foot horizontal distance. Slope along with soil texture (sand, loam, clay) and ground cover determines how fast water will drain from an area. Water drains quickly from steep slopes; however erosion may be a problem. Flat surfaces may result in saturated soils. To calculate slope using a topographic map, you will need to determine the following: Vertical Distance (also referred to as Rise) This is the difference in elevation between two points; it is calculated by subtracting the elevation of one point from the elevation of the other point. Horizontal Distance (also referred to as Run) This is the distance from one point to the other and is calculated by measuring distance with a ruler and applying the map scale. For example, if the map scale is 1:24,000 and the distance between the two points when measured with a ruler is ½ inch, the horizontal distance would be 12,000 inches or 1,000 feet. Slope can then be calculated using the slope formula: Vertical Distance Horizontal Distance X 100 = %Slope 6

7 Estimating Distances First, refer to the scale at the bottom of the map. On a 7.5 minute topographic quad 1 inch equals 2,000 feet. Straight line distances can easily be measured using a standard ruler or engineer s 20 scale ruler that has 20 graduations per inch. The side of the ruler used to measure latitude previously is an example of such a scale. Measure the distance in decimal inches and then multiply by 2000 to convert it to feet. Use the conversion factors in the adjacent table to change units. Since most roads and trails are not straight, measuring instruments are helpful. A map wheel (Figure 6) is such a device. It has a counter which measures distance as the wheel moves along the route. Some measuring devices are electronic and must be set to the map s scale before they are used. Figure 6. Mechanical measuring wheel. Steps for using a mechanical map wheel: Set the counter to zero by adjusting the wheel. Notice that moving the wheel causes the arms to rotate within the display. Place the edge of the wheel at one end of the distance to be measured. Orient the map wheel so that moving it causes the arms to rotate clockwise. Slowly move the device along the route. Stay as close to the path as possible. Zigzagging adds distance. Repeat the process 3 times and average the values. Convert results into desired units. 7

8 Common Topographic Map Features A hill is an area of high ground. From a hilltop, the ground slopes down in all directions. A hill is shown on a map by contour lines forming concentric circles. The inside of the smallest closed circle is the hilltop. Hill A saddle is a dip or low point between two areas of higher ground. A saddle is not necessarily the lower ground between two hilltops; it may simply be a dip or break along a level ridge crest. If you are in a saddle, there is high ground in two opposite directions and lower ground in the other two directions. A saddle is normally represented as an hourglass. Saddle Valley A valley is a stretched-out groove in the land, usually formed by streams or rivers. A valley begins with high ground on three sides, and usually has a course of running water through it. If standing in a valley, the three directions offer high ground, while the fourth direction offers low ground. Depending on its size and where a person is standing, it may not be obvious that there is high ground in the third direction, but water flows from higher to lower ground. Contour lines forming a valley are either U- shaped or V-shaped. To determine the direction the water flows, look at the contour lines. The closed end of the contour line (U or V) always points upstream or toward high ground. Ridge A ridge is a sloping line of high ground. If you are standing on the centerline of a ridge, you will normally have low ground in three directions and high ground in one direction with varying degrees of slope. If you cross a ridge at right angles, you will climb steeply to the crest and then descend steeply to the base. When you move along the path of the ridge, depending on the geographic location, there may be either an almost unnoticeable slope or a very obvious incline. Contour lines forming a ridge tend to be U- shaped or V-shaped. The closed end of the contour line points away from high ground. 8

9 A depression is a low point in the ground or a a sinkhole. It could be described as an area of low ground surrounded by higher ground in all directions, or simply a hole in the ground. Usually only depression that are equal to or greater than the contour interval will be shown. On maps, depressions are represented by closed contour lines that have tick marks pointing toward low ground. Depression A draw is a less developed stream course than a valley. In a draw, there is essentially no level ground and, therefore, little or no maneuver room within its confines. If you are standing in a draw, the ground slopes upward in three directions and downward in the other direction. A draw could be considered as the initial formation of a valley. The contour lines depicting a draw are U-shaped or V- shaped, pointing toward high ground. Draw A spur is a short, continuous sloping line of higher ground, normally jutting out from the side of a ridge. A spur is often formed by two roughly parallel streams, which cut draws down the side of a ridge. The ground is sloped down in three directions and up in one direction. Contour lines on a map depict a spur with the U or V pointing away from high ground. Spur A cliff is a vertical or near vertical feature: it is an abrupt change of the land. When a slope is so steep that the contour lines converge into one carrying contour of contours, this last contour line has tick marks pointing toward low ground. (8) Cliffs are also shown by contour lines very close together and, in some instances, touching each other. Cliff 9

10 A ridgeline is a line of high ground, usually with changes in elevation along its top and low ground on all sides. Along the ridgeline there are hilltops with saddles between them. The term ridgeline is not interchangeable with the term ridge. A ridge is on either end of the ridgeline. Ridgeline Appendix Distance Conversions 1 foot m 1 mile 5,280 ft 1 mile km 1 kilometer 3, ft 1 kilometer mile Area Conversions 1 acre 43,560 ft 2 1 acre 4, m 2 1 hectare 107, ft 2 1 hectare 10,000 m 2 10

11 Check Your Understanding Questions 1-12 refer to the Plano Quadrangle. 1. When was the map produced? When were the revisions? How are revisions indicated on the map? 2. What lines (ddd o mm ss ) of latitude and longitude form the borders? N S E W 3. What is the highest and lowest elevation? highest lowest The total relief would therefore be ft. 4. Due to the aspect of this relief, streams on the map are flowing in what direction? 5. What cities are shown on the map? 6. What major creek flows through the quadrangle? 7. What heavy-duty roads are present? 11

12 8. What medium-duty roads are present? 9. What railroads are present on the Plano Quadrangle? 10. What is approximate elevation of the Spring Creek Campus in Plano? feet meters 11. Find a depression on the Plano Quadrangle. What is associated with the depression? 12. What is the straight line distance between the P in Plano and the A in Allen? miles kilometers 13. What does it indicate when contour lines are close together? 14. What does it indicate when contour lines are far apart? 12

13 15. What is the slope between points A. and B.? (1 inch = 1000 ft.) A. B. For questions refer to the map below. B. A. F. C. E. D. 13

14 16. What is the contour interval of the above map? 17. What contour line has the highest elevation? 18. What contour line has the lowest elevation? 19. The total relief of the map is ft. 20. Identify the topographic features. A. B. C. D. E. F. 14