Finding Unknown Elevations

1
Finding Unknown Elevations
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Tools Needed

• Survey Transit - A basic surveying instrument
• used to measure
• horizontal and vertical angles
• Survey Rod - A thirteen foot stick with
• measurement graduations used with a
  transit to measure height
• Survey Chain Steel measuring tape used to
  measure distance

3
Abbreviations

• B.M. Bench Mark
• H.I. Height of Instrument
• B.S. Backsight
• F.S. Foresight
• ELEV. - Elevation

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Instrument Setup

• Find a known elevation (Bench Mark)
• Place rod at this Bench Mark (B.M.1)
• Find the location of the unknownelevation
  (B.M.2)
• With the chain measure the distance between the
  two bench marks
• Determine the halfway point ofthis distance
• Place the transit at this halfwaypoint or as
  close as possible

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Instrument Placement
Transit set level for horizontal reading
B.M.2 Unknown Elevation
B.M.1 Known Elevation
Equal Distance
Equal Distance
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Take Bench Mark Reading

• 1. Place rod at known bench mark

2. Take reading though transit site this is
the backsight (B.S.) reading
Backsight Reading 6.25
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Field Notes
Place known elevation (B.M.1) in field notes
Place B.S. reading in B.M.2 data
B.M.1
-
-
-
350.00
B.M.2
6.25
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Finding Height of Instrument
Add B.S. to known elevation to find H.I.
6.25 350.00 356.25
B.S. 6.25
B.M.2 Unknown Elevation
H.I. 356.25
B.M.1 Elevation 350.00
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Update Field Notes
Place Height of Instrument (H.I.) in field notes
B.M. (known elevation) B.S. H.I.
B.M.1
-
-
-
350.00
B.M.2
6.25
356.25
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Take Unknown B.M. Reading

• 1. Place rod at unknown bench mark

2. Take reading though transit site this is
the foresight (F.S.) reading
Transit set level for horizontal reading
Foresight Reading 4.15
H.I. 356.25
B.M.2 Unknown Elevation
B.M.1 Elevation 350.00
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Update Field Notes
Place F.S. reading in field notes
B.M.1
-
-
-
350.00
B.M.2
6.25
356.25
4.15
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Finding Unknown Elevation
Subtract F.S. from H.I. To find unknown elevation
356.25 4.15 352.10
Transit set level for horizontal reading
F.S. 4.15
H.I. 356.25
B.M.2 Elevation 352.10
B.M.1 Elevation 350.00
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Update Field Notes
Place new elevation in field notes H.I. F.S.
unknown elevation
This elevation becomes the known elevation For
the next bench mark (B.M.3).
STA.
B.S. ()
H.I.
F.S. (-)
ELEV.
B.M.1
-
-
-
350.00
B.M.2
6.25
356.25
4.15
352.10
B.M.3
and the calculation process begins again
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ONCE THE ELEVATIONS ARE FOUND HOW ARE THEY
USED?..
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.To create Topographic Maps
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Known elevations are charted on a map and
connected with contour lines to create a
Topographic map
Known elevation Bench Mark
Contour Lines
Elevation Height
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TRY IT !!!
Its your turn. Try calculating the height of
the instrument and the unknown elevations in the
worksheet and return here for the answers..
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Abbreviations Answers

• B.M. Bench Mark
• H.I. Height of Instrument
• B.S. Backsight
• F.S. Foresight
• ELEV. - Elevation

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How did you do?.
415.69
408.47
411.58
405.50
410.75
407.14
412.74
411.22
417.37
410.85
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The End