Topic 5: Leveling, Profiles, And Cross Sections Flashcards
Traditional methods to leveling are:
Differential leveling
Trigonometric leveling
Taping
Barometric
Leveling
Refers to techniques for determining differences in elevations.
results are used for; design highways, sewers, water supply systems. Layout construction projects according to planned elevations. Calculate volumes of earthwork. Investigate drainage characteristics of an area. Develop maps showing general ground configurations.
Vertical Line Definition
A line parallel to the direction of gravity. Determined by plumb bob.
Level Surface Definition
A surface of constant elevation and is perpendicular to the plumb line at every point.
Elevation Definition
The vertical distance above or below a reference level surface. Normally at sea level.
Level Line
A curved line in a level surface, all points of which are of equal elevation.
Horizontal line
A straight line tangent to a level line at one point.
Differential Leveling
Done by using a “level”, which consist of a high-powered telescope and level bubble.
Types of levels: automatic levels, dumpy levels, tilting levels, digital levels, others (theodolites, total stations).
Trigonometric Leveling
Measure horizontal distance and vertical angles to compute elevations. Good for inaccessible points such as mountain peaks, offshore construction.
Barometric Leveling
Measuring elevation by measuring change in air pressure. Surveying altimeters can determine elevations within about 2 feet.
Automatic Level
Standard instrument’s in today’s surveying.
Has a small circular spirit level called bull’s eye and three leveling screws.
Penned to bring bubble in the circle and instrument does fine tuning automatically.
Good in windy conditions or soft grounds since it can re level itself.
Electronic digital level
Automatic instrument.
After the bull’s eye is roughly level, the compensator finishes the leveling.
It displays numerically the rod reading and the distance to the rod.
Theodolites
Primary used for angle measurement, however they can be used for leveling.
Leveling Equipment
The level is setup on a tripod stand and is leveled. Readings are taken from a level rods. A rod may be used to make sure it is vertical.
Bench Mark (BM)
Permanent point of known elevation.
Backsight (BS)
Sight taken to the level rod held at a point of known elevation.
Height of Instrument (HI)
The elevation of the line of sight of the telescope.
Foresight (FS)
A sight taken at any point to determine its elevation.
Turning point (TP)
A temporary point established between requires points.
Leveling procedure
HI = existing elevation + BS
New Elevation = HI - FS
Common Leveling Mistakes
Misreading Rod
Moving turning points: not placing rod back to the same point as before during BS & FS in a TP.
Mistakes with extending the rod
Field notes mistakes
Leveling Errors
Level rod not vertical Settling of rod Mud, snow, or ice accumulation on base of rod. Incorrect rod length BS & FS distance not equal Bubble not center Settling of level Heat waves Wind Instrument out of adjustment Improper focusing of telescope
Suggestions for Good Leveling
Anchor tripod legs firmly
Use straight leg tripods
In sloping ground, two legs of the tripod should be placed on the down hill side.
Check to be sure that the bubble tube is centered before and after.
Take as little time as possible between BS & FS
Use BS and FS distance that are approximately the equal.
Make sure rod is level.
Why keep BS and FS distance approximately equal?
When the horizontal lengths of BS(+) & FS (-) are the same, the systematic error of adjustment of the level is canceled.
Level Loop Adjustments
When we start with a BM and shoot elevations, we have to close by shooting the same BM.
There will always be some errors
Adjustments to the level loop are based on the relevant distance, or on the number of instrument setups, which is a factor directly related to the distance surveyed.
Profile and Cross section leveling
In engineering surveying, a route is considered from three distinct perspectives:
Plane View: looking straight down
Profile: side view or elevation view
Cross section: end view of a section
Cross section leveling
Are lines of levels or short profiles perpendicular to center line of a project.
The provide information necessary for estimating quantities of earthwork.
Route cross sections are normally taken in regular intervals 50 or 100 ft stations and at sudden changes in the center line.
