L3: Levelling Flashcards
List the general uses of levelling and its results
To design highways, railroads, canals, water supply systems
To lay out construction projects according to planned elevations
To calculate volumes of earthwork and other materials
To investigate drainage characteristics of an area
To develop maps showing general ground configurations
To study earth subsidence and crustal motion
Define levelling
The processing of finding elevations of points or their differences in elelvations
What is the reference datum universally employed in surveying for levelling?
Mean sea level (MSL): the average height for the surface of the seas for all stages of tide (over a 19-year period as defined by the National Geodetic Vertical Datum of 1929)
Define vertical line
A line that follows the local direction of gravity as indicated by a plumb line
Define local plumb line
A line that follows the direction in which gravity acts
Define level surface
A curved surface that at every point is perpendicular to the local plumb line
Define level line
A line in a level surface (it is therefore curved)
Define horizontal plane
A plane perpendicular to the LOCAL direction of gravity (it is NOT curved)
Define horizontal line
A line which is normal to the LOCAL direction of gravity at a particular point
They are tangential to level lines at those individual points
Define vertical datum
Any level surface to which elevations are referenced
This is the surface that is arbitrarily assigned an elevation of zero
Define elevation
The distance measured along a vertical line from a vertical datum to a point/object
Define vertical control
A series of benchmarks or other points of known elevation established throughout an area
Also termed: basic control, level control
What are five principle methods of levelling (briefly describing each)?
BAROMETRIC LEVELLING: using the fact that air pressure is inversely proportional to altitude
GRAVIMETRY: by measuring the gravitational force between different points
TRIGONOMETRIC LEVELLING: by using a theodolite to measure the vertical angle and horizontal distance to then use trig
SATELLITE POSITIONING: using satellites
DIFFERENTIAL LEVELLING: by stationing a level in between two points
What are the two types of vertical angles and where is each measured from?
Zenith angles: measured from the vertical
Altitude angles: measured from the horizontal
What is the formula for a difference in elevation between points A and B using trigonometric leveling with a theodolite?
ΔElev = HI + S*sinα - r
Where:
ΔElev - change in elevation
HI - height of instrument
S - slope distance
α - altitude angle
r - height of rod (from theodolite crosshair)
What are the formulas for calculating the departure of a horizontal line from the level line?
C (ft) = 0.667M² = 0.0239F²
C (m) = 0.0785K²
Where:
C - departure from level line
M - distance in miles
F - distance in thousands of feet
K - distance in kilometers
Define refraction and explain its relevance
Light rays passing through Earth’s atmosphere are bent towards the Earth’s surface
A theoretically horizontal line of sight is actually bent
What are the formulas for calculating the effects of refraction?
R (ft) = 0.093M² = 0.0033F²
R (m) = 0.011K²
Where:
R - error from refraction
M - distance in miles
F - distance in thousands of feet
K - distance in kilometers
NOTE: the effect of refraction is 1/7 of that of curvature but in the opposite direction
What is the formula for the combined effect of curvature and refraction?
h (ft) = 0.574M² = 0.0206F²
h (m) = 0.0675K²
Where:
h - combined error
M - distance in miles
F - distance in thousands of feet
K - distance in kilometers
List the four types of benchmarks in decreasing accuracy (briefly describing each)
GREAT TRIGONMETRICAL SURVEY (GTS) benchmarks:
very accurate from high precision surveys; take MSL as datum; established by higher survey authorities of a particular country; usually a very permanent object
PERMANENT benchmarks: established with reference to GTS benchmarks; established by local state government agencies
TEMPORARY benchmarks: created by surveyors in the field to mark the point in the field; easier to continue survey from that point after time gap
ARBITRARY benchmarks: when strictly the difference in elevation is necessary, can pick some arbitrary fixed point to be a benchmark and assign to it an arbitrary benchmark value
What are the four categories of levels in increasing level of accuracy (and some characteristics of each)?
DUMPY levels: use level vials to orient lines of site
TILTING levels: also called optical levels; slightly more complex and accurate than dumpy levels; need rounded head tripod
AUTOMATIC levels: telescope with compensator; self-leveling feature to ensure horizontal line of sight
DIGITAL levels: set up same way as optical level but have rod with barcode so that measurement is done electronically; data stored automatically
What are the three main knobs on an automatic level and what do they do?
TURNING KNOBS: fine turn with high precision
FOCUSING KNOB: focus the lens
CROSSHAIRS KNOB: fine tune crosshairs, remove parallax
What are the steps for using an automatic level?
1) Set tripod in position for initial readings
2) Firmly push tripod legs into the ground
3) Ensure top is levelled by eye
4) Attach level to the tripod with clamp and use three foot screws to centralize circular bubble (fine adjustments made by compensator)
To what precision can foot rods be read? What about metric rods?
Foot rods: 0.01 ft
Metric rods: 0.01m with mm being estimated
