Lesson Note On PROFILE LEVELING

PROFILE LEVELING

DEFINITION OF PROFILE LEVELING

The process of determining the elevations of a series of points at measured intervals along a line such as the centerline of a proposed ditch or road or the centerline of a natural feature such as a stream bed.
Normally we will assign an elevation of 100.00 to the datum rather using the mean sea level elevation.

AN EXTENSION OF DIFFERENTIAL LEVELING

·  Elevations are determined in the same manner.

·  The same definitions define the concepts and terms involved.

·  The same types of mistakes and errors are possible.

·  A page check should always be done.

·  A closure check should be done if the profile line runs between bench marks.

ACCURACY OF ROD READINGS

The backsights, foresights, and elevations of benchmarks and turning points should be recorded to the nearest 0.01 ft. Profile elevations of intermediate points are determined from "ground readings" and thus the foresight readings and subsequent elevations should be recorded to the nearest 0.1 ft.

THEORY

Add rod readings (BS) to benchmark or known turning point elevations to get the elevation of the line of sight (HI).
Subtract rod readings (FS) from the line of sight to establish elevations of unknown points.
Take any number of intermediate FS readings at points along the line until it is necessary to establish a turning point to move the level.
Repeat as required.

LOCATION OF INTERMEDIATE POINTS

A foresight is taken on a bench mark to establish the height of instrument.
A foresight is taken on the stations as required (such as every 100 ft).
Foresights are also taken at breaks in the ground surface and at critical points.
This is repeated until the limit of accurate sighting is reached, at which point a turning point is established and the level is moved.

SCHEMATIC

The level is usually set up off the center line.

PROFILE CROSS SECTIONS

Cross sections are lines of levels or short profiles made perpendicular to the center line of the project. (For example, taking a cross section profile of a stream bed while doing a profile survey of the stream.)
Cross sections are usually taken at regular intervals and at sudden changes in the center-line profile.

CROSS SECTION SCHEMATIC

CROSS SECTIONS

The cross sections must extend a sufficient distance on each side of the center line to provide a view of the surrounding terrain.
Rod readings should be taken at equal intervals on both sides of the center line and at significant changes in the terrain.
Example: for a stream cross section, rod readings could be taken at 15, 30, 45, and 60 ft on each side of the center line as well as the edge of the stream and the top of bank of the stream.

CROSS SECTION FIELD NOTES

·  Field notes for a cross section should include:

an elevation or difference in elevation from the center line
horizontal distance from the center line