Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Perry Gresham
Today’s technology has made
finding our way through wilderness terrain fairly easy and incredibly accurate.
With just about any modern cell phone, tablet or handheld GPS receiver a person
can head out to a remote area in unfamiliar terrain with confidence that he or
she will be able to navigate back to civilization and safety. They would be able
to survey terrain ahead, find direction of travel and plot a course, freely day
or night, using powerful tools coupled to the U.S. owned global positioning
utility. Tools, such as Google Earth, TopoQuest and MapsGalaxy
just to name a few. But just how dependable is Global Positioning? How
vulnerable to failure is it? The fact is, the Global positioning system is a
prime military target for reasons such as aerial munitions targeting systems,
drone control and strategic and tactical communications. Known attack methods
include spoofing software, timing attacks(replacing incoming information with
altered or false information). The reasons for such attacks range from defense
of drone attacks on the other side of the world, to trying to prevent fines from
speeding traps here in the united States. So if you spend a lot of time
outdoors, learning how to navigate with a map and compass and without the help
of global positioning can be both rewarding possibly lifesaving.
All maps have a set of
instructions called marginal information that gives the user important
information such as; map scale. bar scales, map symbols, declination(magnetic
north), contour interval and road and trail classification. The images below are
typical of most
The contour lines of a topography map make a two dimensional representation of altitude and inclination as well as terrain features. It is admittedly a long way from good ole’ Google Earth but with practice you will be surprised at what your mind’s eye can see. In the map below there are ten terrain features. The first, a hill; is a good place to start because it will help you remember that any “U” or “V” shape that’s opening is facing you, means that your facing a down slope. The spacing of the contour lines around the hill gives you an idea of the nature of the slopes. Gentle slopes have contour lines that are wide apart and evenly spaced, while lines that are closer together and evenly spaced indicate a uniform steep slope, the closer the lines are to each other the steeper the slope.
Other types of slopes that you may see are concave and convex slopes, pictured below. Contour lines that follow a concave slope are close together at the top of the hill and become farther apart as they go down. Convex slopes are depicted with contour lines that are spaced farther apart at the top and closer together near the bottom of the hill.
A valley generally has
contours that are “U” shaped and run parallel with a stream or dry wash before
crossing it. The longer the contour runs along the side of the stream or dry
wash before crossing, the more level the stream or dry wash runs. A draw is a
minor water way that’s commonly found along sides of ridges and is positioned at
a right angle to a valley, contour lines depicting draws are sharply “V” shaped
and the open end faces a valley while the head of the draw moves towards a hill
or ridge. Depressions, sheer cliffs or manmade cut a ways are shown as contour
lines with ticks that point down. An example of the use of ticks on contour
lines is given below.
Choosing a route can sometimes be tricky, especially if you haven’t used
topography maps before. If you’re uncertain that a portion of your route is
going to be possible, a good way to get a different perspective of a chosen
route is to make a profile. All you need is a pencil, a small rule and some
lined paper (things you should keep in your map bag). The first thing that you
want to do is on your map lightly draw a straight line from point A to point B
(the part of your route that is in question). Now on the lined paper on the left
side, starting with the highest elevation on the questionable part of your route
and using the contour interval of your map, list down to the lowest elevation
noted on the questionable part of your route. Now place the top flat edge of
your lined paper on the line that you previously drew over your questionable
route. Wherever a contour line cross’ you’re paper, draw a straight line down to
its corresponding elevation as shown below.
Map Scale tells the
user what one inch represents on his or her map, in actual terrain. Most
recreational maps are scaled in 1:62,500 which means that one inch on the map,
represents almost one mile. I say almost because there is actually 63,360 inches
in one mile. likewise a map that is printed at a scale of 1:250,000 means that
one inch on the map represents about four miles.
Map Symbols and colors are used to help the user visualize the surface of the Earth that the map represents. That being said, many manmade objects are of no importance, while others would be too small to be recognized so mapmakers are forced to exaggerate some symbols, however in most cases the center of the exaggerated symbol is placed in the exact location of the actual object. A good example of both the use of an exaggerated symbol and an exception to the placement of the center of the symbol would be a rest area and vista adjacent to a major road. While the road has been exaggerated the rest area symbol has been moved away from its actual location in order to preserve its relationship to the road. Colors are used to distinguish different class’ of features, while different types of maps use different colors or colors for different designations the colors used on a standard-scale topographic map are as follows:
Blue—water features such as lakes, rivers, swamps and streams
Black--cultural or manmade features
Green—vegetation such as forests and jungles
Brown—all relief features such as contour lines
Red—main roads and built up areas
Geographic coordinates or Grids are circles
drawn around the earth both horizontally (latitude) and vertically (longitude).
Latitude lines start at exact middle and are labeled 0 degrees latitude
(equator), from there evenly spaced lines radiate to the north and south poles
so that there is 1 through 90 degrees to the north pole and 1 through 90 degrees
to the south pole, so that if you started at the equator and went in a complete
circle you would have past 360 parallel lines of latitude.
Longitude lines are the same
in number (360) but they do not run parallel to each other because each line
runs right over both poles which make the distance between them near the north
and south poles smaller than the space between them at the equator. Unlike
latitude, longitude is without a center starting point so in the late eighteen
hundreds it was established that Greenwich, England would be the starting point,
or zero degree line, better known as the prime meridian. The longitude lines
start at 1 degree east and another start at 1 degree west, going around the
Earth the lines meet on the other side of the Earth at 180 degree.
The latitude and longitude lines together form a grid that covers the Earth. Because there is a northern 180 degrees and a southern 180 degrees when giving coordinates you must specify north or south for latitude and east or west for longitude. Because one degree of latitude equals 69 miles it was broken down into 60 minutes and then minutes were further broken down into 60 seconds which works out to a second being about 2/10 of a mile. When giving grid coordinates you always give latitude first and longitude last, so as an example : 31˚ 33’ 55.35” North 110˚ 14’ 43.86” West (Professor Wellers class room).
Some interesting facts: Only the latitude effects the climate, no matter what your longitude is. The Arctic Circle 66˚ 33’ N and the Antarctic Circle is 66˚ 33’ S.
The topographic map below is of the mustang mountains, Cochise County, Arizona. The grid coordinates are: Lat. 31˚ 41’ 28.28” N. Lon. 110˚ 25’ 25.07” W. If we were fellow rock hounds, hunters or some other enthusiastic outdoors persons, you could now obtain a map of the area for yourself. That is the best use for grid lines by the private individual. Grid coordinates are more like an index of the world’s maps and a very quick and easy general purpose locater, and they work very well for that purpose. I have installed a hyperlink of the full 1:24,000 scale map for the purpose of clarity, however I have paired each feature with a corresponding image in order to give the reader a better idea of what the scale looks like in reality. According to the maps marginal information its contour intervals are 25 feet. To see the map in its entirety please CTRL+click on the link below.
United States. Department of Army. Map Reading. Washington DC: Department of the Army, 1969. Print.
"NGMDB Historic Topo Data." U.S. Geological Survey. U.S. Department of Interior, Web. 17 Apr 2014. <URL: http://geonames.usgs.gov/pls/topomaps>.