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Fields
A field is a region with a measurable
quantity at every location.
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The picture to the right is an example of a field map.
In this example, the numbers on the map represent the
concentration of gasoline found at that location. So the map
simply shows how much of something (in this case gasoline) is
found at many locations. |
Isolines
Isolines are lines that are drawn on a field
map to connect all of the points on that map that have
the same value.
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The image on the left is an example of a
gasoline field map with the 40 isoline drawn in.
Notice that the line connects points that all have
the same value. In this example, the circle connects all
of the points with a value of 40. That means that
every point on that line has a value of
exactly 40. |
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This map has all of the isolines drawn in, at
an interval of 10 (each line is 10 units apart) is shown
to the right: |
Topographic
Maps
Contour lines are isolines that are drawn on
an elevation map. These lines connect all points with
the same elevation, and give a great picture of the lay
of the land. The resulting map is called a
Topographic Map.
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Above is an example of a Topographic Map.
Notice that isolines are drawn every 20 meters to
connect points with the same elevation. |
There are several important rules to remember
when reading a topographic map:
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Close line =Steep Slope
The
steepest slopes on the map can be seen by looking for
lines that are very close together. Since each line
represents a change in elevation of a set amount (20
meters in this example), lines that are close together
indicate steep gradients. The area circled in red is
very steep, because it has many lines crowded close
together. |
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Contour lines bend upstream
when crossing a river
Since contour lines must remain at the
same elevation, they must bend when they cross
riverbeds. A riverbed drops downward, so the contour
line must bend uphill to stay at the same elevation. The
highlighted lines show that the direction of the Mill
River must be northeast. |
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Highest
possible elevation of a hill
The
highest elevation of a hill can be calculated by finding
the last (highest) contour line on that hill, and then
figuring out the next line that would be drawn. The
highest possible elevation of the hill is just below
the value of that next line. The highest possible
elevation of the hill indicated by the arrow is 239
meters.
The last line indicated on the map is 220 meters. The
next line would be 240 meters (remember that the contour
interval is 20 meters). Since there is no 240
meter line, the hill cannot be higher than
239 meters!
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Depressions are shown by
small marks
pointing inward off the contour line
The arrow
is pointing to a depression, or hole. The
contour line with the marks, or hatchers, has the
same elevation as the line before it. In this case the
hatchered line has a value of 140 meters, and the
depression must be less than 140 meters. |
Profiles
A profile is a picture of what the landscape
would look like from the side. It is a cross section of
the landscape. You should be able to identify a correct
profile, as well as draw one.
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Identify a
Profile
Given the topographic map above,
which of the four choices below accurately depicts
the correct profile between points X and Y?
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| The best way to approach this is by the process of
elimination. First of all, point X appears to be
approximately 290 meters in elevation. Let's eliminate
all the choices that don't begin at 290
meters:
That eliminates choices 1 & 3 right off the bat!
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| Next, we look at point Y, which appears to be
approximately 130 meters.
That eliminates choice 4. That means choice 2 must
be correct!
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Draw
a Profile
Given the topographic map above,
lets draw a profile map from point P to point Q.
First, we need a horizontal grid that represents the
distance from P-Q
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Next, transfer the points from the contour map
to the grid.
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Then connect the points with a smooth line to
draw the profile. |
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And there! We have a cross section of the landscape.
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