Transcript No Slide Title

Elevation:
Definitions:
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
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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 DESCRIPTIONS
AND ELEVATIONS
BROWN 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 Profiles:
show 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 1:24,000, the profile
horizontal scale is 1:24,000).
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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 1:2,400.
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’ =
1:1,200; 1” = 50’ = 1:600 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
460 m
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B
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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
= 0.326
which, 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. Harry Williams, Cartography
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