Maps are a representation of a part of the earth's surface as if one were looking down from a considerable height. They are drawn to scale, meaning that any two points have the same relative position on the map as they do on the ground. There are many types of maps, but probably the most useful are topographic maps, which are invaluable, provided they have been carefully made. In the construction of highways, pipelines, railroads and canals, good topographic maps are essential for indoor planning. A geologist must have accurate maps for plotting field data, although aerial photos are now used extensively. The ability to read a topographic maps keeps a traveler on his desired route, whether it is by airplane or just a hike in the country.
The making of maps necessitates the accurate measurement of both vertical distances (elevation above sea level) and horizontal distances. The exact elevations of fixed points are found by precise surveying; each point is called a bench mark (B.M.) and is marked by a round brass plate. A triangulation station is a point where the horizontal distance and relative position from other triangulation stations is accurately established by surveying methods. When a map is made, all other points in the area are located in relation to these accurately surveyed points. A bench mark at the CPR station on Richmond Street is described as follows: "Front or north stone foundation wall, 10 feet west of doorway of general waiting room, and 2 feet 7 inches below brickwork; bolt set horizontally, No. 163-F; elevation 806.170."
The features which are shown on topographic maps
can be summarized as follows:
|1. Relief - the hills and valleys represented by contour lines||
|2. Water - all drainage, including rivers, lakes, oceans, etc.||
|3. Culture - works of man, including roads, buildings, bridges, political boundaries, etc.||
|4. Woodland - coniferous and deciduous trees||
|5. Main Roads & Highways||
In addition to the map itself, additional information can be found in the map margins, including the name of the map (usually a prominent geographic feature), the scale of the map, the contour interval, the position of the map in relation to other maps, and a reference table of symbols used in the map. This marginal detail should be consulted before any examination of the map is undertaken.
In the United States, the topographic maps are issued by the U.S. Geological Survey, and are compiled in the National Topographic Series. Topographic maps for the whole of the U.S.A. have not as yet been made, but State Geologic Map Indexes which show the areas already mapped are available from the U.S. Geological Survey or the local state geological survey. (In Illinois, the Illinois State Geological Survey is located in Champaign/Urbana.) Information on how to order topographic maps may be obtained from the U.S. Geological Survey web site at the following address:
By convention, all maps are oriented in the following way: top is North, bottom is South; right is East; and left is West. Thus, a certain locality on the map is described not as "in the top-right corner," but "in the northeast corner."
The boundaries of a map are the parallels of latitude, lines which run east-west and are measured north and south of the equator; and the meridians of longitude, lines which run north-south and are measured east and west of the Prime Meridian (which runs through Greenwich, England). The degrees and minutes of latitude and longitude are indicated at each corner of the map, with the number located opposite its respective line. The degrees and minutes of latitude and longitude are indicated at each corner, with the number located opposite its respective line. It is important to note that the east-west latitude lines are always parallel, while the north-south longitude lines converge at the north and south poles.
The scale of a map gives the relation between the actual size of the map and the size of the area represented on the map. This is usually stated in two or three different ways in the margin of the map:
Changes in the shape of the earth's surface (topography) may be represented by means of a relief map, which actually shows hills and valleys. However, such maps have the disadvantage of being large, cumbersome, and expensive. Because relief maps usually represent a very large area, it is difficult to accurately show fine details. Also, relief maps typically show a high degree of vertical exaggeration, meaning that the vertical (elevation) scale is much larger than the horizontal (distance scale), so that the mountains stand out.
Most maps are produced on flat sheets of paper, and on topographic maps, the vertical dimension (elevation change) is represented by a graphical device known as a contour, an imaginary line connecting points of equal elevation above some reference elevation (datum), or, put another way, a contour represents the intersection of an imaginary horizontal plane with the land surface. The usual reference elevation or datum is mean (average) sea level, which represents a zero contour line. If the sea were to rise 20 feet, the new shore line would be a a level marked by the 20 foot contour. This vertical distance between two adjacent contour lines is called the contour interval.
The contour interval chosen for a map depends on the size of the area that the map will cover, and the differences in elevation in the area. Large contour intervals, such as 100, 250, or 500 feet, might be used for maps of a while province or in regions which are mountainous. Small contour intervals, such as 1, 5, 10, or 25 feet, might be used for small areas such as the university campus, or the prairie regions of western Canada, where there is little relief. In order to facilitate map reading, only every 5th may be labeled or drawn in bold. The standard color for contour lines is brown.
The elevation of prominent features such as hill tops and road intersections is often indicated by a brown number on the map. The elevation of any point not located on a contour line can only be estimated (interpolated) by its relative position between adjacent contours.
In contrast to a valley, a depression is completely surrounded by ground which stands at a higher elevation than the bottom of the depression. Depression contours which are used to represent these hollows without outlets have small hachures ("teeth") on the contour lines that point toward the lower elevation.
General Rules Applied To Contours:
Complete the questions which follow:
1.(a) Express the verbal scale "1 inch = 1 mile" as an R.F.
1.(b) Express the verbal scale "1 foot - 2,000 yards" as an R.F.
1.(c). Express the R.F. "1 : 250,000" as a verbal scale.
1.(d) An area has been mapped at an R.F. of 1:50,000. A new map of the same area is drawn at an R.F. of 1:25,000. What area of paper, in relation to the first map, will be needed for the new map?
||2. The diagram at the left is a contoured map.
(a) Mark the positions of North, South, East, and West with appropriate letters.
(b) Is the X boundary latitude or longitude?
(c) In which geographic direction does the stream flow? How do
(d) The gradient is defined as the vertical change in elevation (in feet) per horizontal (map) distance (in miles). Calculate the stream's gradient on this map, if the stream is 2 miles long, and the contour interval is 10 feet.
|3. (a) What is the contour interval (C.I.) of the map at the left?
(b) What is a approximate different in the elevation between the
highest and lowest points on this map?
(c) If you were standing at point "A" and wanted to walk downhill along the maximum gradient or slope (or if you spilled a bucket of water from point A), in which geographic direction is this?
(d) The graphic scale of this map (for 1 mile) is shown below the map at the left. Calculate the distance from the northeast to the southwest corner.
|4. (a) In which geographic direction does the stream flow? How
do you know this?
4. (b) If you were standing at "X," would it be easier to walk due East
or due West? Justify your answer.
4. (c) If the distance "AX" is 2 miles, and the distance "BX" is 8 miles,
which is higher above X, A or B?
In order to locate any feature on a map, some reference system is required. Certainly this can be done using coordinates of latitude and longitude, but this often becomes cumbersome. Accordingly, an alternative system, the Land Office Grid Survey, is available in many places.
The Land Office Grid Survey (also called the Township & Range System) is set up using man-made reference lines called the Principal Meridian (a "zero" line which runs north-to-south) and Principal Base (running east-to-west) Lines. Townships are 6 mile increments located in reference to the Principal Meridian, while ranges are 6 mile increments located in reference to the Principal Base line (see Figure 1)
The intersections of the township and range lines each define a square parcel of land called a township, which measures 6 miles north-to-south by 6 miles east-to-west, for a total area of 36 square miles. Each township is composed of 36 sections, each of which measure 1 square mile. The numbering system of the sections is always the same; section 1 is always located in the northeast corner of a township.
In order to locate a plat of ground within a section, it is common to subdivide the section. To precisely locate a landmark using the township and range system, the appropriate section may be subdivided into 1/4 sections, one of which is subdivided into 1/4 mile squares (about 40 acres in area). Referring to Figure 3 above, an entire township is shown in the left diagram, and an enlargement of Section 36 is shown at the right. To locate the X shown above, the correct township and range coordinates should read:
"NE 1/4 of the NE 1/4 of Section 36"
Note that Section 36 is subdivided in two steps: first into 1/4 sections (NE 1/4), then that 1/4 section is subdivided once more (NE 1/4).
On the periphery of your topographic map, look for the township and range designations. Commonly, these red boundary lines will indicate one township/range coordinate on one side of the line, and another coordinate on the other side. You can also detect a township/range boundary line by looking for an abrupt change in the section numbering system. For example, if you spot a "Section 1" on your map, and the section immediately due north of it is labeled "Section 36," you should recognize that there is a township boundary between those 2 sections.
The Land Office Grid System is just one of several coordinate systems which are common included on maps. The military services have their own system. Search the topographic map with which you are working for any evidence of another coordinate system which may be included.
For more details and practice exercises on using the Township & Range coordinate system, see the handout, "Township & Range."
Constructing A Topographic Profile
Topographic maps represent a view of the landscape as seen direction from above and give an excellent perspective for regional relations. This view, however, is unnatural, for we are accustomed to seeing hills and valleys from the side. In detailed studies of landforms, it may be desirable to construct a profile or cross section through certain critical areas, so that various features may be analyzed from a more natural view. A profile may be constructed quickly and accurately across any straight line on a map, according to the procedure outlines below.
Gradients: One can easily determine the gradient of a stream by measuring a representative section of a stream and dividing this distance (in miles) into the vertical difference (in feet) between the starting point and end point. The result is an expression of a change in elevation in feet per mile (ft./mi.)
For an exercise on constructing a topographic profile from a map, see the handout "Topographic Profile Exercise."
For teaching purposes, topographic contours are best
visualized in a series of steps, from a simplified, idealized 3 dimensional
land form. The first diagram (Figure 6) shows a computer generated
surface depicting a hilly landform surface as a "fishnet." Figure
7 shows the same landform with contour lines. Figure 8 shows the
landform with the relief removed (flat surface), but with the contours
intact. Figure 9 shows the map view of the same surface.
|Figure 6: Water takes the steepest path down the side of the hill into the valley||Figure 7: Note how the water's downhill path falls into the contours which bend upstream|
|Figure 8. Flattened surface with contours intact||Figure 9. Topographic Map (aerial view) of same surface.|