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close this bookWater and Sanitation Technologies: A Trainer's Manual (Peace Corps, 1985)
close this folderSessions
close this folderSession 32 - Survey and measurement
View the document(introduction...)
View the documentAttachment 32A: Alternative ways of measuring elevations
View the documentAttachment 32B: Profiling

Attachment 32B: Profiling

Accurate knowledge of the ground profile along a pipeline route is often critical to proper pipeline design. Correct profiling depends on correct use of simple equipment. "Eyeball" methods of profiling are sufficient only in the simplest of situations. What follows is a general description of profiling methods. Read through the attachment carefully and work through the examples given as you go along.

Theory of Leveling

1. The line of sight of a properly used level is always at the same elevation, regardless of the direction in which it is pointed.

2. If the elevation at any point on the ground is known, the elevation of the level line of sight may be found by measuring up from the known point. Because of the fact that most work requires the knowledge of relative elevations only, that known point is often assumed to be 100 or
1000.

3. If the elevation of the level line of sight is known, the elevation of any point on the ground may be found by measuring down from the line of sight.

4. By successive use of the above concepts, the elevation of any point may be found.

Equipment

1. Surveying level and tripod or hand level. Levels are surveying instruments that have a telescope and means for orienting the telescope's line of sight in a horizontal plane.

2. A stick marked with distance measurements (feet, meters, etc.). This stick is called a " Rod."

3. Distance measuring equipment, such as a measuring tape, engineer's chain, or optical distance estimating equipment. Pacing is adequate only for flat terrain or short distances.

4. A notebook, properly set up.

Theory of Profiling

1. Profiling involves measurement of elevations (leveling) along a line, together with measurement of horizontal distances.

2. Distances must be measured on a straight line between points for which elevations are taken.

Notekeeping

1. Notekeeping is one of the most critical portions of surveying. Many surveying mistakes can often be traced back to poor notation. A notebook should always be properly set up and the time taken to make notes clear and readable.

2. A site sketch should accompany the measurements. This will help the notetaker remember important surface features of the area. The sketch should show salient features such as houses, streams, hills, trees, etc. along the pipeline route. A North arrow should also be included.

Terminology

1. Sta = Station. This is the point on the profile line at which an elevation was measured. These are normally numbered by hundreds of feet. For example, a station 10 may be 1000 feet from the beginning of the survey. Intermediate distances are indicated by pluses: Sta 10
+ 50 would equal 1050 feet from the beginning.

2. Bm = Benchmark. This is a monument or point of known description which includes elevation.

3. Tbm = Temporary Benchmark. This is an object that is relatively permanent such as large rocks or trees where the elevation has been determined.

4. Bs = Backsight. This is a rod reading at a point of known elevation.

5. HI = Height of Instrument. This is the elevation of the line of sight of the instrument.

6. Fs = Foresight. This is a rod reading at a point of unknown elevation.

7. Elev = Elevation

8. Dist = Distance between points.

9. Tp = Turning Point. This is a point used primarily to serve as a reference elevation to move the instrument, Both a foresight and backsight are taken on the point. The point may be on or off the profile line, but should be a solid, easy to relocate, point.

Profiling Procedure

1. Setup and level instrument.
2. Sight Benchmark (point of known elevation) for Backsight reading.
3. Enter rod reading in Backsight (Bs column 2)
4. Add rod reading (column 2) to Benchmark (column 5) to get Height of Instrument (HI column 3)
5. Sight point to be determined (Foresight) and enter reading in Foresight (Fs column 4)
6. Subtract Foresight (column 4) from Height of Instrument (column 3) to get elevation of Foresight (column 5).

Turning Point

1. Rodman maintains position at Foresight

2. Move setup, and level the instrument at new location (Tp 1)

3. Sight rod at Backsight (last foresight station) and enter reading in column 2

4. Add rod reading (column 2) to elevation of backsight (column 5) to get height of instrument (column 3)

5. Proceed with Foresight (steps 5 and 6 above).

Example I

1

2

3

4

5

6


+


-



Sta

Bs

HI

Fs

Elev

Notes

Pt A

2.5

102.5


1000.0

Assumed Elev.

Pt B



11.5

91.0


Tp 1

4.2

95.2



Pt B

Pt C



12.3

82.9


Table 1

Survey Notation for Figure 1.


Figure 1 - Profiling

The steps used in the example problem are different from those used by professional surveyors. They have been simplified in an attempt to reduce confusion and are more than adequate for the type of surveying that is necessary in small scale piped water systems. When using this method, always remember the following simple calculations:

1. Known elevation + Backsight reading = Height of Instrument.
2. Height of Instrument - Foresight = Next Elevation.

Practical Hints for Surveying

1. Before starting, walk the course to be surveyed and mark the line to be profiled. If the survey is conducted for a piped water system, remember to keep in mind that pipe will have to be laid in trenchs along the course.
Whenever possible, avoid obstacles that will make laying difficult.

2. Mark with a sturdy stake all turning points, foresights, and backsights as work progresses so they will be visible if a recheck is necessary.

3. After you have finished your calculations, redo the survey if unacceptable errors occur. It is much easier to correct a surveying mistake before pipe has been laid in the ground.

4. It is desirable to recheck horizontal distances as well. Approximate methods, such as pacing, will catch major errors with a minimum of effort.

Plotting the Profile

Once the horizontal distances and elevations are surveyed in the field, the data is brought back to the office and plotted on graph paper. This completed profile can be used for sizing of pipelines, locating storage tanks, sir valves, washout points, and so on. Normally, the vertical scale is greater (numerically smaller) than the horizontal scale. For example, the vertical scale may be ten times the horizontal. In the vertical scale, one inch may equal ten feet and in the horizontal, one inch may equal one hundred feet. Other similar ratios may be used.

Example II; Figure 2.

At 0 + 00

100.0 + 0.2 = 100.2 (equals HI)

At 0 + 47

100.2 - 9.7 = 90.5 (equals next elevation)

This is repeated for the next instrument set up

At 0 + 47

90.5 + 1.3 = 91.8 (equals HI)

At 1 + 03

91.8 - 9.9 = 81.9 (equals next elevation)

Many different ground elevations may be found from a single Height of Instrument sight, as shown by the following:

At 1 + 03

81.9 + 4.2 = 86.1 (equals HI)

At 1 + 30

86.1 - 5.5 = 80.6 (equals ground elevation)

At 1 + 37

86.1 - 8.9 = 77.2 (equals ground elevation)

At 1 + 45

86.1 - 5.2 = 80.9 (equals ground elevation)

At 2 + 00

86.1 - 4.2 = 81.9 (equals ground elevation)


Figure 2 - Typical Profile


Survey Sketch And Notation