Laboratory #5: Topographic Maps

Materials Needed:

1.Pencils

2.Metric/Standard Ruler

3.Calculator

4.Triangles or right edges

Map Contours and Scale

Topographic maps indicate the three dimensional shape of the Earth’s surface on a flat piece of paper. The most precise method of indicating the shape of the Earth’s surface along with the elevation of different areas above sea level is by means of contour lines. Contour lines are lines that connect points of equal elevation; similar to bathtub rings that form when a bathtub is emptied.

The vertical distance between contour lines is a fixed number chosen by the people that make the map. This distance is known as the contour interval and is an even number such as 10, 50, or 100 feet (or meters).

The scale of a map indicates the relationship between a linear distance on the map and the corresponding linear distance on the Earth’s surface. For example, a scale shown on a map as 1:62,500 indicates that one unit length on the map represents 62,500 of those same units on the Earth’s surface. The ratio is called the representative fraction or R. F. Shown below in Figure 1 are two maps drawn at different R.F.s.


Topographic Profile

A topographic profile shows the intersection of the land surface with a vertical plane. Such views of the land surface can be seen in road-cuts, quarries, and canyon walls. A profile can be constructed from a topographic map along any straight line by the method shown in the figure to the right.

Contour Lines

Contour lines are generally drawn as black or brown lines on a map. Usually, every 5th contour is printed with heavier print than the others and is labeled with the elevation of the contour above sea level. In addition to contour lines, elevations of single points that have been accurately surveyed to the nearest foot are also shown. These points, called a bench mark or B.M., have been surveyed by a survey crew and are usually set on the Earth’s surface in the field with an approximately 3 inch diameter brass marker set in concrete. Bench marks are stamped with the date of construction and survey, as well as the elevation of a mark on the brass plate.

The following are useful general statements regarding contour lines:

  • When contour lines cross streams, they bend upsection. The line forms a “V” with the apex pointing upstream.
  • Contours do not cross.
  • Closed contours appearing on a map as circles or ellipses represent hills.
  • Closed contours with “hachures” (short lines points down slope) represent closed depressions or basins.
  • Closely spaced contours represent steep slopes.
  • Widely spaced contours represent gentle slopes.

  • The difference in elevation between the lowest and highest point of a given area is the maximum relief of that area.

Laboratory #5: Topographic MapsName:

Part I: Scale Conversion

Convert the following representative fractions to ground units:

1:24,0001 inch = 2000 feet

1:31,0001 inch = 2583.33 feet

1:48,0001 inch = 0.76 miles

1:62,5001 inch = 0.99 or ~1 miles

1:250,0001 inch = 3.95 or ~4 miles

1:1,000,0001 inch = 15.78 miles

A map of unknown scale shows two radio transmission towers. On the map, the towers are 3.6 inches apart and the actual ground distance between the two towers is 1,000 feet. What is the R. F. of the map?

3.6 inches: 1000 feet → 1000 feet x (12 inches/1 foot) = 12,000 inches

3.6 inches: 12,000 inches

1: 3,333

A map of Mexico has a 1:500,000 Representative Fraction. How many kilometers on the ground are represented by 10 centimeters on the map? (100 cm = 1 meter, 1000 meters = 1 kilometer)

1:500,000→ 500,000 cm x (1 m/100 cm) x (1 km/1000 m) = 5 km

1 cm = 5 km

10 cm = 50 km

A map of the CSULA campus shows two water towers exactly 4 inches apart. The ground distance between the two towers is exactly 1 mile (5,280 feet). What is the R. F. of the map?

4 inches: 5,280 feet → 5,280 feet x (12 inches/1 foot) = 63,360 inches

4:63,360

1:15,840

A map of the local area has an R. F. of 1:24,000. Determine the area in square feet of a rectangular piece of property that is 2 x 3 inches on the map.

1 inch = 24,000 inches → 24,000 inches x (1 foot/12 inches) = 2000 feet

A = L x W → (2 x 2000) x (3 x 2000) → 4000 x 6000 = 24,000,000 ft2
Laboratory #5: Topographic MapsName:

Part II: Topographic Map and Profile

  1. Study the map below and note the elevation of the bench mark (BM 982). This mark (x) is 982 feet above sea level. Label each contour line with the proper elevation.
  1. Determine the approximate elevations of points a, b, c, and d.

a = ~1010 ft.b. = 970 – 975 ft.c. = ~1030 ft.d. = ~1110 ft.

  1. Complete the topographic profile across the line A – B. Be sure to put the vertical scale on both margins of the profile.

Laboratory #5: Topographic Maps Name:

Part III: Topographic Map Making

The map shown below has survey spot elevations and drainage lines. Using a contour interval of 5 feet, construct a reasonable topographic map of this area. Start with the 605 contour line and proceed to the 640 line.