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Geology Lab: Topographic Maps and Profiles
Terms in this set (28)
Aerial photographs, satellite images, and topographic maps are important research tools that provide insight into the various processes that shape the surface of the Earth. At one time, these materials were more difficult to obtain and interpret, but with the advent of the internet, Google Earth, and GIS applications, these materials are accessible by a wide variety of users. Today more than ever, these images and maps contain vast amounts of data reduced to a scale that can be easily managed and digested by scientists, researchers, and students. The ability to "read" the features and understand the geologic phenomena that helped sculpt the landscape is imperative for any person with interest in understanding Earth processes.
Topography means the "shape of the land," and refers to the surface configuration of features on the Earth such as hills, valleys, lakes, slopes, streams, etc. Every topographic map shows, to scale, the width, length, and variable height of the land above a datum or reference plane. These maps, referred to as quadrangles, are two-dimensional representations of the three-dimensional surface of the Earth. Their primary value to the Earth scientist is for determining locations, landform types, elevations, and other physical data.
United States Geological Survey (USGS)
Topographic maps have been produced by the United States Geological Survey (USGS) since the late 1800s and most areas of the United States have been accurately portrayed on these commercially available maps. In order to facilitate their use, all USGS topographic maps follow a similar format. Each map uses standard colors and symbols, and contains information about where the area mapped is located, the date when the mapping was done or revised, scales, north arrow, and the names of adjoining quadrangle maps.
A scale model is a miniature representation of an actual object, and topographic maps are essentially scale modes of the Earth's surface. Every map will have a map scale that expresses the relation between measurements on the map to the true distance on the Earth's surface and different map scales depict an area on Earth with more or less detail. On a topographic map, scale is usually indicated in the lower margin and is expressed in three ways:
Fractional scales, Graphic scales, or Verbal scales.
Fractional scales, sometimes called ratio scales (1:24,000 or 1:62,500) expresses a distance of one unit on the map represents some distance of the same unit on the surface of the Earth. For example, using the 1:24,000 scale, one inch of the map would equal 24,000 inches on the Earth's surface. Maps with small fractional scales such as 1:250,000 cover large areas; those with large fractional scales such as 1:1,000 cover small areas. The USGS publishes maps at various scales to meet both the need for broad coverage and detail.
Graphic scales, often referred to as bar scales, are a series of bars divided into segments that show the relation between measurements on the map to actual distance on Earth. Scales showing miles, feet, and kilometers are usually included. Graphic scales are useful for measuring the distance between points, and can be used to make a "map ruler" for measuring distances on the map using a piece of paper or string.
Scales can also be expressed as verbal scales such as "one inch on the map equals one mile" or "one centimeter on the map equals one kilometer."
One of the most useful functions of a topographic map is determining the precise location of a feature on the Earth's surface. Three frequently used methods for designating location are:
Latitude and longitude to determine the location of a point
Public Land Survey system to define an area
In reference to prominent features
These methods of location can be used to provide information helpful to industry professionals such as engineers, surveyors, and realtors; as well as recreational users for such activities as biking, hiking, boating, geocaching, and many others. The advent of personal GPS systems and advanced mapping techniques have put this technology in the palm of your hand using small, hand-held GPS units and/or your telephone.
Latitude and Longitude
This means of location uses an imaginary grid system which covers the surface of the Earth. Both latitude and longitude have reference (zero) lines: the equator is 0 degrees latitude, and latitude is referenced as north or south of the equator; 0 degrees longitude is the prime meridian, which runs through Greenwich, England. Longitude is referenced east or west of the prime meridian. Latitudinal lines are also referred to as parallels, and run parallel to the equator. Longitudinal lines are sometimes referred to as meridians and converge towards the poles. This imaginary grid is projected onto a sphere (the Earth) and locations are reported in degrees, minutes ('), and seconds (") in reference to the equator and prime meridian.
Topographic maps are bounded by parallels of latitude on the north and south, and meridians of longitude on the east and west. The latitude and longitude represented on a topographic map are printed at the four corners of the map and are indicated at intervals along the margins. Maps that cover 15 minutes of latitude and 15 minutes of longitude are called 15 minute series topographic maps. A 7 1/2 minute series topographic map covers 7 1/2 minutes of latitude and 7 1/2 minutes of longitude. There are 60 minutes of arc in one degree, and 60 seconds of arc in one minute of arc. Therefore, 1/2 minute is equal to 30 seconds.
Latitude and Longitude Figure 1
Left, latitudinal lines run parallel to the equator and are described based on their position in the northern or southern hemisphere. Longitudinal lines converge towards the poles and are described as being east or west of the prime meridian.
Public Land Survey
The Public Land Survey (PLS) provides a precise method for identifying the location of land in most states west of the Appalachian Mountains by establishing a grid system that systematically subdivides the land area. All lands in the continental United States have been divided and described by this grid system with the exception of the original 13 colonies and Texas. The original 13 colonies existed as separate entities and each had its own land alienation scheme. Texas was already its own republic when it entered the union, and reserved the right to alienate its own public land. All other states were divided into six square mile areas called townships, and described by a grid system with reference to established base lines and principal meridians.
More Public Land Survey
The PLS grid system begins at an initial point formed by the intersection of a major east-west line, called a base line, and a north-south line, called a principal meridian. Townships, the size square mile area, are described as being north or south of the base line. Ranges provide the eastern and western edges of the township and are described as being east or west of the principal meridian. Along the borders of topographic maps that use the Public Land Survey system, the townships and ranges covered by the map are printed in red along the margins.
Each township is divided into 36 one-mile-square parcels of land, called sections, with each section containing 640 acres. Sections are numbered beginning with number 1 in the northeast corner of the township and ending with number 36 in the southeast corner. On a topographic map, the sections are outlined and their numbers are printed in red. The unique numbering system helps the map reader determine when they have moved from one township or range to the next.
More detailed sections
If a more detailed description is required, sections may be subdivided into halves, quarters, and each quarter may be quartered again. Each of these subdivisions is identified by their compass position. When describing property or the location of a landmark such as a school or public building, the description starts with the smallest subdivision and the township is given before the range. Using T2S, R2W example from above, the black start located inside section 27 on the next page would be described by dividing section 27 into quarters, and then dividing the quarter where the star is located into quarters again. If someone wanted to locate the star, the final description for the location of the star would be: NW/4 of the SE/4 of Section 27, Township 2 South, Range 2 West.
Relation to prominent features
Often, someone will give you directions to a particular location in reference to the place you are currently located. For example, if a friend needed to know the location of Walmart in reference to the SFA campus, you would tell them that Walmart is located 2.2 miles north of the SFA campus. On a topographic map, a point on the map is located by being so many miles and in a particular compass direction in reference to another point or feature on the map.
A contour line is a line drawn on a topographic map that connects all points that have equal elevation above or below a datum or reference plane on the Earth's surface. Depicting the height or elevation of the land relative to the sea level, and thereby showing the shape of landforms, is what makes topographic maps unique. Contour lines depict the third dimension, elevation, on a two dimensional surface.
In the bottom central margin of every topographic map, the contour interval is indicated letting the reader know the amount of elevation change that occurs between each contour line. Most maps use the smallest contour interval possible to provide the greatest detail for the map area. All contour lines on a topographic map are multiples of the contour interval; if the map has a contour interval of 20 feet, each line represents a 20 foot change in elevation.
To help determine the elevation of the contour lines, on most topographic maps every fifth contour line is printed as a bold line, called an index contour. Index contours are labeled with elevation data to help interpret the map. In addition to index contours, bench marks are usually present on the map. Bench marks are points of known elevation and can be used as reference.
The spacing of contour lines on the map also provides information about the topography. Contour lines that are closely spaced together indicate a steep slope; contour lines widely spaced apart indicate a gentle or gradual slope.
Relief is defined as the difference in elevation between two points on a map. Total relief is the difference between the highest and lowest points on the map; local relief refers to the difference in elevation between two specified points.
General Rules for Contour Lines
In order to create uniformity between various map publishers, contour lines must conform to certain guidelines:
1. A contour line connects points of equal elevation.
2. Contour lines separate all points of higher elevation from points of lower elevation.
3. A contour line never branches or splits.
4. Steep slopes are shown by closely spaced contours; gentle slopes are shown by widely spaced contours.
5. Contour lines never cross, except to show an overlapping or vertical cliff.
6. Hills are represented by a concentric series of closed contour lines.
7. A concentric series of closed contours with hachure marks represents a closed depression. Hachure marks point into the depression.
8. When contour lines cross stream valleys or dry stream channels, they form a "V" that points upstream.
9. Contour lines that occur on opposite sides of a valley (or hill) always occur in pairs. Single contour lines do not lay between maximum ridge and minimum valley contour lines.
10. To report a reversal of slope, you must repeat elevations. When crossing a hill, the last contour line crossed before reaching the top is the first one crossed when descending from the other side. The same is true when crossing a valley. In the example below, a person walking upslope would encounter the 340' contour line before the summit. Once they start their descent, the first contour line they encounter would be the 340' contour line. They were going upslope and now they are going downslope - a reversal of slope - so they will be repeating their elevation.
11. Topographic maps published by the USGS are contoured in feet or meters, usually referenced to sea level.
Constructing a Contour Map
Originally, contour maps were constructed by first surveying an area and establishing the elevations of several points in the field. The surveyor then sketched contour lines on the map by estimating their location between the points of known elevations. Today, surveyors, geologists, geographers, and industry professionals have access to computer software and mapping programs to help determine elevations and contours, and to help produce professional maps for research and industry.
Topographic maps, like most other maps, depict the Earth's surface viewed from above. In some cases, a topographic profile - a side view - will provide a more useful representation of the elevations and slopes of an area. A topographic profile is a cross section along a given line which represents an outline of the physical features at the Earth's surface.
How to construct a topographic profile
1. Choose a line along which the profile will be constructed. In this lab, the profile line will be provided, and is usually labeled with letter designations such as A - A' - B - B'.
2. Lay a piece of paper along the profile line. Mark the beginning and end of the profile line on your paper.
3. Next, make a small tick mark every place a contour line intersects the paper and note the elevation of the contour line by each mark.
4. Make note of the horizontal scale of the map and the contour interval.
5. Make note of the highest and lowest elevations you recorded on your paper. This will give you an idea of how to construct your vertical scale.
6. Record your elevations on the left hand side of the one inch grid provided to create your vertical scale. Each line should represent a change in elevation that corresponds to the contour interval of the map. If the contour interval is 20 feet, each line on the grid will represent a 20 foot change in elevation.
7. Place your paper with the tick marks along the bottom of the grid and place a small dot on the grid line that corresponds with the elevations you recorded.
8. Draw a line connecting these dots to create a topographic profile.
After you have constructed a topographic profile, you must calculate the vertical exaggeration. Your topographic profile does not have the same vertical scale as the horizontal scale of your map; therefore you have "exaggerated" the vertical component of your profile. Vertical exaggeration (VE) is calculated using the following formula:
Vertical Exaggeration = Horizontal Scale/Vertical Scale
For example, if the horizontal scale is one inch on the map it is equal to 2000 feet, and the vertical scale has recorded a 100 foot change in elevation:
Vertical Exaggeration = 2000 ft/100 ft
VE = 20X or 20:1
The U.S. Geological Survey uses standard colors and symbols on topographic maps. Over the course of the next few weeks, we will be looking at USGS topographic maps and using the standard colors and symbols to interpret features. A copy of the USGS Topographic Map Symbols handout can be found on the d2l page associated with this class. This handout can also be accessed at the following website.
U.S. Geological Survey Standard Colors
Red: major roads and highways
Brown: contour lines
Green: vegetation, woods, orchards, vineyards
Blue: water features such as lakes, swamps, rivers, streams; intermittent water features are depicted with a dashed line.
U.S. Geological Survey Standard Map Symbols
Boundaries: National (large with 2 dots), State (small with 2 dots), and County (small with 1 dot)
Buildings and Related Features: Building (rectangle), School (flag) House of worship (small cross), Cemetery (Cem or Cross)
Contours: Index (number at top), Intermediate (thin), Supplementary (dashed), Depression (hutures)
Bathymetric: Index (thick), Intermediate (thin)
Land Surveys: Range or Township Line (thick), Section Line (thin)
Railroads: Standard Gauge Single Track (hash marks)
Roads and Related Features: Primary Highway (red with black), Unimproved Road (2 dashed), Trail (1 dashed)
Mines and Caves: Quarry (axes), Mine Tunnel/Cave Entrance (Y sideways), Mine Shaft (half filled box), Prospect (X)
Rivers, Lakes, and Canals: Perennial stream (Thick), Intermittent Stream (dashed), Spring or Seep (circle, circle with ponytail), Well (filled in circle)
Surface Features: Levee (3 lines), Sand (grains)
Vegetation: Woodland (green box), Orchard (Tree top dots)
Submerged Areas: Marsh or Swamp (sticks on beds)
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