TO MAP READING
by Bill Tong
Map reading is an extremely
useful skill for just about everybody, whether you are a person who likes
hiking outdoors, or just a driver who needs to occasionally look up a road
map. Because geology is a field based science, geologists not only must
know how to read specialized topographic and geologic maps, they must often
construct and draw their own maps based upon their own field observations.
The lab unit on topographic map interpretation will teach you how to read
and interpret these maps to find land elevations, interpret topographic
contour lines, measure distances, read longitude and latitude, and locate
objects and landmarks using the same coordinate system used by land surveyors
and geologists. Before you can understand how to read a topographic map,
you will be introduced to maps in general, including simple hand drawn
maps, road maps, globes, physical relief maps, etc., with emphasis on what
these maps have in common, as well as their differences and limitations.
A map is a drawing or graphical representation of the land, as viewed from the air from a great height. This perspective is called the "aerial view" or "map view." As land bound creatures who usually walk across the land rather than fly above it, the map view is not the natural perspective for humans; rather, we have more of a "side view" of the land surface.
A. "Treasure Map": The simplest and most basic type of map may have just two points or landmarks - let's call them "Point A" and "Point B." This simple map shows the distance and path relationship between "A" and "B." - in other words, it shows you how to get from Point A to Point B (or vice versa) and how far it is to travel this path.
B. Street Map: The most commonly used map is probably the street map, which is a local map showing boundary lines, streets and roads, landmarks, and usually includes a graphical scale that can be used to calculate map distances between points "as the crow flies."
C. Globe: A globe is the name of a spherical map depicting the entire earth's surface. Note that while an enormous area of land of represented by this type of map, fine details are nearly impossible to read.
D. Physical relief map: Some maps deliberately show raised bumps to indicate mountainous or hilly areas. These maps typically show very large areas, including entire states or countries on a single map. Note that this type of map exaggerates the vertical dimension (elevation) and is not drawn to the same scale as the horizontal (if this were not true, then Mt. Rainier in Washington State would be 200 miles high instead of about 4 miles high!). Relief maps cannot show much precision, especially in the mountainous areas. They are also expensive to produce and inconvenient to carry around, so they are usually displayed on walls.
E. Topographic map: This type of map combines features found in road maps, combined with lines and symbols that give us detailed information regarding the topography (changes in elevation of the land surface). These special lines, called topographic contours, are drawn based upon surveying information, and can give a very accurate and detailed view of both the vertical (elevational) and horizontal (map) dimensions, all in a convenient, flat, 2-dimensional sheet of paper.
F. Geological map: This type of map is a topographic map which also shows the underlying geological structures, rock layers, or rock formations. Geological maps are used to interpret often complex geological histories and sequences of events that took place to produce the current landscape. These maps are used by geologists to locate oil & gas, minerals, groundwater, dinosaur skeletons, fossils, etc.
1. A reference point/reference direction
2. A consistent map scale
3. Boundary lines
1. Reference Point & Direction: To be useful, a map should have a reference point - better yet, it should also have a reference direction. Without a reference point, you will have difficulty locating your present position on the map. Without a reference direction, you will not know what path to take to get to your destination. The reference direction is usually shown as an arrow which indicates which way is North; however, even when not explicitly shown, the standard convention for maps is that "the top of the map sheet points North."
2. Map Scale: A good map should have a consistent scale, which is a mathematical relationship between the map and the land that it represents; in other words, distances should be shown in their correct proportions. Let's say that on a given map, the distance between Point A and Point B is 1,000 feet. If we draw a Point C on the map, and the distance from Point A to Point C is 2,000 feet, this line should be twice as long as the line between Point A and Point B.
Map scales can be expressed in a number of ways. These include the verbal scale (such as "1 inch = 1,000 feet"), the representative fraction or R.F. (such as "1:12,000"), and the graphic scale, which looks like a measuring stick drawn at the bottom of the map. It is important for you to know how to interpret and convert between these different scales, and know what they are used for. For a detailed explanation of map scales, see the handout "Map Scales & Units."
3. Boundary Lines: Without boundary lines, it would be difficult to navigate and locate points on a map, or measure distances easily. Besides marking the perimeter boundaries at the edges of a map, boundary lines also subdivide the land that is represented on a given map. Different types of maps often use different types of boundary lines, which often depend upon the scale of the map and/or the amount of land represented by the map. Boundary lines on most maps use geographical directions and angular measurements, because the earth is a roughly spherical planet.
a. Longitude Lines: These are boundary lines that are drawn to north to south, but divide the earth's land surface from east to west. On a spherical map of the entire earth, called a globe, you can see that longitude lines converge (come together) at the north and south poles. This means that the distance of land between longitude lines is greatest at the earth's equator and is smallest at the poles. Longitude lines are generally drawn for every 15o of angular measure. A complete circle around the earth is 360o, so there are 24 major longitude lines. There is a line of origin, or "zero line" where the angular measurement is 0o longitude - this is called the Prime Meridian, and is located near Greenwich, England. If you start at the Prime Meridian and travel westwards half-way around the world (up to the 180o longitude line, also called the International Date Line), you have defined the extent of the "western hemisphere" of the earth. The other half of the earth, measured eastwards from the Prime Meridian, is then the "eastern hemisphere." So, the Prime Meridian and the International Date Line are really connected together as part of the same circle on the globe. Longitude measurements are expressed as a number of degrees up to 180o and the direction is given as "W" if located in the western hemisphere, or "E" if located in the eastern hemisphere.
b. Latitude Lines: These boundary lines are drawn from east to west, but divide the earth's land surface from north to south. Unlike longitude lines, latitude lines never converge - they remain the same exact distance apart from each other around the entire globe; in other words, latitude lines are parallel to each other. The line of origin or zero latitude is called the Equator, which is located 90o from the poles of the earth. All land located north of the Equator defines the "northern hemisphere," while all land south of the Equator is the "southern hemisphere." Latitude numbers are expressed as a number of degrees up to 90o, with a direction of "N" if in the northern hemisphere, and "S" if in the southern hemisphere. It should be apparent that you cannot go farther north than 90o N (the North pole), because if you walk past the North pole, you are going south again, and the latitude numbers begin to decrease below 90o.
c. Intersections of Longitude and Latitude lines: Note that the intersections of these boundary lines differ depending upon latitude. Looking at a globe, it is apparent that near the Equator, the parcel of land defined by the intersection is shaped like a rectangle; however, the parcels of land near the polar regions look more triangular, like pie wedges. This means that maps of land near the polar regions will show more frequent changes in longitude than on equatorial maps, because the longitude lines are closer together in the former than in the latter. However, you should note that most maps typically cover only a tiny fraction of the earth's surface, so that the curvature of the land surface as well as the convergence of longitude lines is ignored, such that longitude boundaries appear parallel, and the earth appears to be flat.
d. Township and Range Lines: A land surveying system established long ago by the U.S. Government and still in use today is called the Township and Range System. Township lines are equivalent to latitude, while range lines are equivalent to longitude. Township and range is a subdivision of longitude and latitude and any point on earth may be described by a set of location coordinates under this system. The standard parcel of land, called a township, is 6 miles north-to-south by 6 miles east-to-west, measuring 36 square miles. For surveying purposes, a township is subdivided into 36 equal sub-parcels called sections each measuring 1 mile X 1 mile (1 square mile). Sections may be subdivided into four sub-parts, or "quarter sections" - a NW 1/4, NE 1/4, SE 1/4 and a SW 1/4. An object or point may be located within a particular quarter section by subdividing once more into four subparts. For details on how to read and interpret township and range lines, see your other lab handouts, such as "Maps Scales & Units," and "Using Township & Range Coordinates."
e. Topographic Quandrangle Maps: Most topographic maps cover a standard area defined not by surface area, but by the angular measure of the earth's curvature. If you take a very small slice of the earth, say 1o out of the complete circle (360o), and then subdivide this 1o slice into 60 equal sub-slices, one of these sub-slices (1/60 o) is called a minute. If you took a minute, and subdivided it into 60 sub-slices, 1/60 minute is called a second. It should be apparent to you that if 60 minutes = 1o , then 30 minutes = ½o , 15 minutes = 1/4 o, 7.5 minutes = 1/8 o; and 30 seconds is ½ minute, 15 seconds = 1/4 minute, etc. The two most common types of topographic quadrangle maps are the 7.5 minute series, which covers 1/8o north-to-south and east-to-west on the map; and the 15 minute series, which covers 1/4o north-to-south and east-to-west. You can confirm this by comparing the longitude and latitude coordinates of each of the four corners of the map. The name "quadrangle" refers to the four corners of this rectangular shaped map. Note that because latitude lines maintain the same distance apart from each other, the north-to-south distance covered by these maps is always consistent. However, because longitude lines converge towards the polar regions, topographic quadrangles of land closer to the polar regions will appear narrower from east to west than do maps from the equatorial regions.