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Measuring Elevation Measuring Elevation
Elevation refers to the height above or below a fixed Elevation plays a crucial part in everyday life.
geographic reference point.
In engineering, elevation is used to determine the center of
In most cases, we the Earth’s sea level as our reference gravity and the type of materials to be used.
point, but in some cases the ground level is used as the
reference point. The elevation of a particular place can also help predict and
prevent disasters such as flooding.
Measuring Elevation Measuring Elevation
Mississippi River at flood stage ~ 217 ft. above sea level Mississippi River at flood stage ~ 227 ft. above sea level
Measuring Elevation Measuring Elevation
Humans adapt to their environment, as some are adapted
to higher elevations while others are adapted to low
elevations.
A majority of the world's population lives in coastal regions
with elevations of 500 feet or less.
The people of Tibet in Central Asia are adapted to living at
17,500 ft. where crops cannot grow and the oxygen is
extremely thin.
Mississippi River at flood stage ~ 237 ft. above sea level
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Measuring Elevation Differential Leveling
Importance of Vertical Positions
Elevation refers to the height above or below a fixed
geographic reference point. A vertical position is the height of a point relative to some
reference surface, such as mean sea level, a geoid, or an
In most cases, we the Earth’s sea level as our reference ellipsoid.
point, but in some cases the ground level is used as the
reference point. The roughly 600,000 vertical control points in the U.S.
National Spatial Reference System (NSRS) are referenced
One method for measuring elevation is to use differential to the North American Vertical Datum of 1988 (NAVD 88).
leveling.
Differential Leveling Differential Leveling
Importance of Vertical Positions Importance of Leveling
Surveyors created the National Geodetic Vertical Datum of
1929 (NGVD 29, the predecessor to NAVD 88), by The determination of elevations is called leveling
calculating the average height of the sea at all stages of the
tide at 26 tidal stations over 19 years Measuring relative elevations changes is a comparatively
simply process
Surveyors extended the control
network inland using a surveying Precise and accurate control of
technique called leveling. relative elevations are critical to
most construction projects
Differential Leveling Differential Leveling
The Level
Level Instrument
A level consist of a high-powered telescope
The level is attached to a spirit or bubble level that keeps
the line of sight of the telescope horizontal
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Differential Leveling Differential Leveling
Level Instrument Level Instrument
4.70
4.60 4.59
4.60 4.58
4.57
4.56
4.55 4.54
4.50 4.53
4.52
4.51
Red numbers indicate 4.50
foot intervals
Differential Leveling Differential Leveling
Definitions Definitions
BS = 6.32 ft HI = 106.32 ft FS = 3.10 ft BS = 6.32 ft HI = 106.32 ft FS = 3.10 ft
Point B Point B
E = BS - FS
Point A Point A
Elevation = 103.22 ft Elevation = 103.22 ft
Starting point Starting point
(elevation 100.00 ft) (elevation 100.00 ft)
Differential Leveling Differential Leveling
Definitions Definitions
Bench mark (BM) - relatively permanent point of known Backsight (BS) - a sight taken to the level rod held at a
elevation point of known elevation (either a BM or TP)
BS = 6.32 ft HI = 106.32 ft FS = 3.10 ft BS = 6.32 ft HI = 106.32 ft FS = 3.10 ft
Point B Point B
Point A Point A
Elevation = 103.22 ft Elevation = 103.22 ft
Starting point Starting point
(elevation 100.00 ft) (elevation 100.00 ft)
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Differential Leveling Differential Leveling
Definitions Definitions
Height of instrument (HI) - the elevation of the line of Foresight (FS) - a sight taken on any point to determine
sight of the telescope its elevation
BS = 6.32 ft HI = 106.32 ft FS = 3.10 ft BS = 6.32 ft HI = 106.32 ft FS = 3.10 ft
Point B Point B
Point A Point A
Elevation = 103.22 ft Elevation = 103.22 ft
Starting point Starting point
(elevation 100.00 ft) (elevation 100.00 ft)
Differential Leveling Differential Leveling
Computation of Elevations – Find Elevation of BM2 Computation of Elevations – Find Elevation of BM2
BS BS FS
12.64 BM2 12.64 3.11
BM1 BM1
HI = BS + Elevation TP1 HI - FS = Elevation
Elevation 100.00 Elevation 100.00
Point BS HI FS Elevation Point BS HI FS Elevation
BM1 12.64 112.64 100.00 BM1 12.64 112.64 100.00
TP1 3.11 109.53
Differential Leveling Differential Leveling
Computation of Elevations – Find Elevation of BM2 Computation of Elevations – Find Elevation of BM2
BS BS FS
BS FS 10.88 BS FS 10.88 2.56
12.64 3.11 12.64 3.11
BM1 BM1 TP2
TP1 TP1
Elevation 100.00 Elevation 100.00
Point BS HI FS Elevation Point BS HI FS Elevation
BM1 12.64 112.64 100.00 BM1 12.64 112.64 100.00
TP1 10.88 120.41 3.11 109.53 TP1 10.88 120.41 3.11 109.53
TP2 2.56 117.85
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Differential Leveling Differential Leveling
Computation of Elevations – Find Elevation of BM2 Computation of Elevations – Find Elevation of BM2
BS BS FS
9.72 9.72 3.10
BS FS BS FS
BS FS 10.88 2.56 BS FS 10.88 2.56
12.64 3.11 12.64 3.11 BM2
BM1 TP2 BM1 TP2
TP1 TP1
Elevation 100.00 Elevation 100.00
Point BS HI FS Elevation Point BS HI FS Elevation
BM1 12.64 112.64 100.00 BM1 12.64 112.64 100.00
TP1 10.88 120.41 3.11 109.53 TP1 10.88 120.41 3.11 109.53
TP2 9.72 127.57 2.56 117.85 TP2 9.72 127.57 2.56 117.85
BM2 3.10 124.47
Differential Leveling Differential Leveling
Computation of Elevations – Find Elevation of BM2 Computation of Elevations – Find Elevation of BM2
BS FS Check the summation of the backsight and the foresight
9.72 3.10
BS
10.88
FS
2.56
with the change in elevation
BS FS
12.64 3.11 BM2
BM1 TP2 Point BS HI FS Elevation
TP1 BM1 12.64 112.64 100.00
Elevation 100.00
TP1 10.88 120.41 3.11 109.53
Point BS HI FS Elevation TP2 9.72 127.57 2.56 117.85
BM1 12.64 112.64 100.00 BM2 3.10 124.47
TP1 10.88 120.41 3.11 109.53 +33.24 -8.77
TP2 9.72 127.57 2.56 117.85
BM2 3.10 124.47 Change in elevation = 33.24 - 8.77 =24.47
Differential Leveling Differential Leveling
The initial backsight (BS) is taken to a point of known
elevation
The backsight reading is added to the elevation of the
TopHat Problems
known point to compute the height of the instrument
(HI)
The level may be moved to a temporary point called a
turning point (TP)
The elevation of a point is the height of the instrument
(HI) minus the foresight (FS)
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Differential Leveling Differential Leveling
Computation of Elevations - Group Problem 1 Computation of Elevations - Group Problem 1
Prepare a set of level notes for the survey illustrated BS FS
1.27 4.91
below. What are the elevations of points TP1 and TP2? BS FS
2.33 6.17
BM
BS Elevation 356.68
FS
1.27 4.91 TP1
BS FS TP2
2.33 6.17
BM
Point BS HI FS Elevation
Elevation 356.68
TP1
TP2
Differential Leveling Differential Leveling
Computation of Elevations - Group Problem 2 Computation of Elevations - Group Problem 2
BS FS
Prepare a set of level notes for the survey illustrated BS FS 8.78 3.06 BS FS
1.02 5.67
below. What are the elevations of points TP1 and TP2? 6.46 3.11
BM1 TP2
BS FS
8.78 3.06 BS FS TP1
BS FS
6.46 3.11 1.02 5.67 Elevation 110.42 BM2
BM1 TP2 Point BS HI FS Elevation
TP1
Elevation 110.42 BM2
Differential Leveling Differential Leveling
Common Mistakes Common Mistakes
Misreading the rod - reading 3.54 instead
of 3.45 Level rod not vertical
Moving the turning point - use a well- Settling of leveling rod
defined TP Leveling rod not fully extended or
incorrect length
Field note mistakes - work within your
group to check you records Level instrument not level
Instrument out of adjustment
Mistakes with extended rod - make sure Environment - wind and heat
the leveling rod is fully extended
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Differential Leveling Differential Leveling
Suggestions for Good Leveling
Anchor tripod legs firmly
Check the bubble level before and after each reading
Any Questions?
Take as little time as possible between BS and FS
Try to keep the distance to the BS and the FS equal
Provide the rodperson with a level for the rod
