Leveling

1
Leveling

• Chapter 6

2
Approximate Slope Formula for Baseline Work

• D difference in elevation
• L Slope distance measured
• H Horizontal distance needed
• C Correction
• C d2 / 2 L

3
Importance of Leveling

• Determination of elevations with a surveying
  instrument
• Ex terracing on a farm or the building of a
  simple wall, the construction of a drainage
  projects, or the building of bridges etc..

4
Basic Definitions

• Vertical Line a line parallel to the the
  direction of gravity
• Level Surface a surface of constant elevation,
  that is perpendicular to the plumb bob line at
  every point
• The elevation of a particular point is the
  vertical distance above or below a reference
  level surface
• A level line is a curved line in a level surface
  all points are of equal elevation
• A horizontal line is a straight line tangent to a
  level line at one point

5
Reference elevations or datum

• The surface of a body of water in the vicinity
  can be assigned a convenient elevation. Any value
  can be assigned to the datum, for example, 100 ft
  or 1000 ft, but the assigned value is usually
  sufficiently large so that no nearby points will
  have negative elevations
• The mean sea level This datum, which is mean sea
  level, is defined as the position the ocean would
  take if all tides and currents were eliminated
• In the United States the sea-level datum is the
  value of mean sea level determined by averaging
  the hourly elevations of the sea over a long
  period of time, usually 19 years

6
First, Second, and Third-Order Surveys

• First-order survey
• are made for the primary national control
  network, metropolitan area surveys, and
  scientific studies.
• they are very accurate surveys used for military
  defense, sophisticated engineering projects,
  dams, tunnels, and studies of regional earth
  crustal movements
• Second-order survey
• are less accurately than are first-order surveys.
  They arc used to densify the national network as
  well as for subsidiary metropolitan control.
• they are used for control along, tidal
  boundaries, large construction projects,
  interstate highways, and small reservoirs

7
First, Second, and Third-Order Surveys

• Third-order survey
• are done somewhat less accurately than are
  second-order ones. They are general control
  surveys referred to the national network.
• In detail they are used for local control
  surveys, small engineering projects, small-scale
  topographical maps, and boundary surveys

8
Methods of Leveling

• Trigonometric leveling
• Barometric leveling
• Spirit leveling

9
Trigonometric leveling

• is leveling in which horizontal distances and
  vertical angles are measured and used to compute
  elevation differences
• This method can be used for Inaccessible points
  such as mountain peaks, offshore construction,
  and so on. For purposes of mapping, rough
  leveling or preliminary surveys
• Example the stadia method, is a variation of
  trigonometric leveling

10
Barometric leveling

• Involves the determination of elevations by
  measuring changes in air pressure. Although air
  pressures can be measured with mercurial
  barometers, these instruments are cumbersome and
  fragile and are impractical for surveying
  purposes
• Instead, the light and sturdy but less precise
  aneroid barometers commonly called altimeters are
  used.
• The altimeters are manufactured with a 2 ft
  margin for error. Such precision is sufficient
  only for preliminary work

11
Spirit leveling

• Called direct leveling, is the usual method of
  leveling. Vertical distances are measured in
  relation to a horizontal line, and these values
  are used to compute the differences in elevations
  between various points
• A spirit level is used to fix the line of sight
  of the telescope, this line of sight is the
  assumed horizontal line with respect to which
  vertical distances are measured

12
Types of Levels

• Dumpy Level
• Wye Level
• Automatic or Self-leveling Level
• Electronic Digital Level
• Tilting Level
• Laser Level
• Transits or Theodolites Used as Levels
• Total Stations Used as Levels

13
The Level
14
Level Rods

• Level rods are usually made of wood, fiberglass,
  metal, or combinations of both
• They usually are graduated from zero at the
  bottom. Level rods are usually read directly
  through the telescope by the instrumentman and
  are commonly called self-reading rods

15
Level Rods

• The rodman sets the target at the desired
  position and then makes the reading directly on
  the rod
• These rods are commonly called target rods,
  although such a designation is technically
  incorrect, as the target is only an accessory to
  the regular self-reading level rod
• Level rods are frequently named for cities or
  states. For instance, there are the Philadelphia
  rod, the Chicago rod, the San Francisco rod, the
  Florida rod, and so on

16
Level Rods

• The Philadelphia rod has one major advantage over
  these other rods
• The Philadelphia rod is distinctly divided into
  feet, tenths, and hundredths by means of
  alternating black and white spaces painted on the
  rod
• The scales, or verniers, shown on the targets
  will be discussed later
• The telescopes of many levels are equipped with
  stadia hairs, or three horizontal cross hairs
  that are used to make the readings

17
Care of equipment

• Surveying equipment are very precisely and
  delicately manufactured, they can be very durable
  when properly used and maintained
• In fact, these instruments may last a lifetime in
  the hands of a careful surveyor, but a few
  seconds of careless treatment can result in
  severe or irreparable damage

18
Care of equipment (cont.)

• Using sometimes antique levels and transits is
  not a disadvantage because a good old instrument
  will still be quite precise
• Used transits and levels decline very little in
  value as the years go by
• As a matter of fact, their selling prices in
  dollars often remain stable for several decades
• However, finding parts for older instruments is
  sometimes a problem

19
Care of equipment (cont.)

• After the instrument is taken from its box, it
  should be handled by its base when it is not on
  the tripod
• It should be carefully attached to the tripod
• The surveyor must not permit anything to
  interrupt him or her until this task is completed
• A level should never be left unattended unless it
  is in a protected location. If it is turned over
  by wind, cattle, children, or cars, the results
  will probably be disastrous

20
Care of equipment (cont.)

• The most common Injury to levels by the novice
  surveyor is caused by applying too much pressure
  to the leveling screws

21
The Level Rod

• The level rod should never be dragged on the
  ground or through water, grass, or mud, and its
  metal base should never be allowed to strike
  rocks, pavement, or other hard objects
• such use will cause leveling errors due to the
  change in length of the rod itself
• Furthermore it must not be used to knock down
  weeds or brush
• The Philadelphia rod must not be carried over the
  shoulder when it is fully extended, or leaned
  against trees or walls, it is just too flexible
• It should be placed flat on the ground with the
  numbers facing upward