Elevation: Definitions: Elevation - height above some arbitrary datum; on USGS maps this datum is mean sea level (

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ElevationDefinitions Elevation - height above
some arbitrary datum on USGS maps this datum is
mean sea level (0 feet). This datum was
carefully defined as the National Geodetic
Vertical Datum 1929 (NGVD29), based on averaging
sea level over a period of many years at 26 tide
stations along the coasts of the US and Canada.
More recently, this datum has been updated to the
more accurate North American Vertical Datum of
1988 (NAVD88), although this datum is not yet in
wide usage.Relief difference between maximum
and minimum elevation in a given region (e.g. a
map area).
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Past attempts to Portray Elevation on Maps
Pictorial representation
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Hachures
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Layer Shading
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Hill Shading
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Bench marks, spot heights and contours Bench
Marks
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Bench marks are bronze disks, usually set in
concrete. They are accurately surveyed geodetic
control points. Traditionally, geodetic control
is categorized as primary, secondary, or
supplemental. Primary (First Order) control is
used to establish geodetic points and to
determine the size and shape of the earth.
Secondary (Second Order) Class I control is used
for network densification in urban areas and for
precise engineering projects. Supplemental
(Second Order, Class II and Third Order) control
is used for network densification in non-urban
areas and for surveying and mapping projects.
Accuracy classifications such as these provide a
common means to judge the quality of a point and
its appropriateness for use in other work.
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Bench marks are shown on USGS maps by BM x 567
Example of Bench Mark Record BENCH MARK
DESCRIPTIONSAND ELEVATIONSBROWN COUNTY,
INDIANA USGS BM TT 26 SC 1942 In Brown County,
Elkinsville Quad, in the NW ¼ of Section 1, T. 7
N., R. 1 E., 2 nd P.M. about 5.5 miles west of
Elkinsville at the Chambers Bridge over Middle
Fork Salt Creek, at the T road intersection of
Paynetown Road and Knightsridge Road set in the
top of a 8-inch by 8-inch concrete post, 230 feet
north and 50 feet east of the east end of the
bridge, 120 feet south and 70 feet west of the
center of the intersection, 30 feet west of the
centerline of Paynetown Road, along an east-west
fence line, 0.3 foot above the ground a U.S.
Geological Survey bronze tablet, stamped TT 26
SC 1942 532. 531.956 feet N.G.V.D. 1929 3 rd
ORDER.
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Spot heights are accurately surveyed points shown
on maps, but do not have physical markers on the
ground. Intended to help map reader usually
found on hill tops, road intersections shown by
x 555 (x not always present).
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Contours imaginary lines of constant elevation.
Every 5th contour is bold to facilitate tracing
index contours.
Index contours are numbered at a break in the
line, with the number upright if possible.
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The difference in elevation between adjacent
contours is the Contour Interval.
The contour interval varies depending on the
relief of the map it is usually a multiple of
10 feet.
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Contour spacing indicates slope e.g. steeper
slope, gentler slope
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Topographic Profilesshow the shape of the
surface between two points. Contour elevations
are transferred from the map (usually using a
piece of paper as shown) to graph paper. The
elevations are plotted on the graph paper with
reference to a Y-axis showing elevation. Because
distances on the map are transferred directly to
the graph paper, the horizontal scale of the
profile is the same as the map (i.e. if the map
is 124,000, the profile horizontal scale is
124,000).
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However, unlike the map, the profile also has a
vertical scale determined by the Y-axis. For
example if the Y-axis is 1 inch 200 feet, this
is a vertical scale of 12,400. Because of this,
profiles usually have vertical exaggeration
(vertical scale is larger than horizontal scale).
In the example above, the vertical exaggeration
is 24,000/2,400 10x. How is the vertical scale
chosen? It is arbitrary i.e. 1 to 100
11,200 1 50 1600 and so on. Topographic
profiles must have a title, the horizontal and
vertical scales, the vertical exaggeration, the
UTM coordinates of the end points and labeled
axes.
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Slopes or gradients slope expresses the
relationship between the change in height of the
surface ('rise') with respect to a horizontal
shift in position ('run'). There are a variety of
ways in which slope can be expressed. For
example, the slope AB, rises 150 meters over a
distance of 460 meters.
A
150 m
B
460 m
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A statement this is simply a statement of the
vertical change in height and the corresponding
horizontal distance, usually expressed in terms
of feet per mile or meters per kilometer. In our
example, the slope would 150 m per 0.46 km, which
gives 150/0.46 m per 0.46/0.46 km or 326 m per
km. A ratio this is the ratio of the rise to the
run, which must be in the same units, with the
left-hand side of the ratio reduced to 1 i.e.
150 460 150/150 460/150
1 3.07 this may also be expressed as
1 in 3.07 A fraction similar to the ratio, but
with the rise divided by the run (in the same
units) to provide a fraction i.e.
150/460 0.326
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A percentage as with all percentages, the
fraction is simply multiplied by 100 to give a
percentage i.e. (150/460) x 100
0.326 x 100
32.6 An angle one of the most familiar yet most
difficult ways of expressing slope by
trigonometry, tangent B rise/run
150/460
0.326which,
by the tangent function on a calculator, 18o
It is also possible to directly measure the
slope on a topographic profile using a
protractor, but only if there is no vertical
exaggeration (the vertical scale is the same as
the horizontal scale). If there is vertical
exaggeration, angles are also exaggerated and
will not give the correct value.