LEVELLING

DEPT. OF GEOGRAPHY & RESOURCE DEVELOPMENT GEOG 204

DefinitionThe determination of relative altitude or height of points on the earths surface

Measurement is with reference to a level or horizontal stable surface called a datum

Usual datum is mean sea level (msl) at recognized places (Glasgow, Tema, etc)

The PrincipleA line-of-sight is taken along the horizontal ruler to the vertical ruler at C. Once the bubble tube of the spirit level is appropriately placed, this ensures that the wall is truly horizontal

Levelling InstrumentsSpirit level (bubble tube)Tripod (levelling screws)Levelling Staff

FoldingGraduatedTelescopicField/data sheet/Notebook

Typical Spirit Level

Levelling Instruments - Types

Dumpy levelWatts LevelTilting level

Dumpy level

Watts level

Tilting Level Automatic level Levelling Instruments - Types

Mounted Level

Tripod

Levelling staff

The vertical distance above or below the horizontal surface is read off a Levelling staff.

The levelling staff is a pole usually made of wood or metal.

May be telescopic or folding, extending to a length usually of four meters and graduated (meters/feet) to be easily read in the field of view of the level telescope

These levels are usually mounted on a tripod. Levelling screws provided for centering & adjusting the bubble for levelling the tube and telescope

Levelling Staff

Levelling staff

Together with a level, we use a levelling staff (on which heights are marked in feet or metric system and either inches or decimals of a foot) to measure vertical distance.

Staffs must be held vertical as any leaning of the staff will result in a level reading which is too great.

Some staffs have fitted with circular bubbles to ensure it is vertical.

When a level is set up, it is necessary to adjust it so that the line-of-sight is horizontal to the staff held vertically. Levelling Staff

The PrincipleA line-of-sight is taken along the horizontal ruler to the vertical ruler at C. Once the bubble tube of the spirit level is appropriately placed, this ensures that the wall is truly horizontal

Methods of Levelling Differential levelling

Series levelling Trigonometric levelling

Old technique still widely used today because of its simplicity use of inexpensive instruments.

The method adopts the two basic instruments of levelling the spirit level and a levelling staff.

Minimum number of two people: The surveyor and the staff man

The surveyor begins by setting up the level over a control point (this is the initial starting point) for which the elevation is known (i.e. the datum)

The surveyor then sights on the levelling rod/staff through the telescope of the level and adjusting it for the perfect horizontal sightline, it determines the height of the second point at which the sightline intersects the staff.

This is the principle of the differential levelling which is also called the rise and fall method.

Process can be repeated at desired points.Differential Levelling: ( Rise & Fall Method)

Rise and Fall Method

Series Levelling Procedure The basic differential method (rise and fall method) involving the determination of relative heights between two points can, in principle, be extended in a procedure known as series levelling.

Foresight

Back sight

Inter sight

Series Levelling along a road segment

Table 1: Series Levelling Recording

Backsight IntersightForesightRiseFallReduced LevelDistanceRemarks1.743635.5620.0Spot ht(map A)1.0210.722636.28418B1.1030.082636.20229C1.5230.420635.78230D0.298Change pt2.4542.15634E

Trigonometric Levelling.

Sources of Error in LevellingFive (5) key sources

Instrumental errors

Errors in handling the equipment

Errors due to displacement of the equipment

Errors in reading and booking

Errors due to natural causes

1. Instrumental errorsLine of sight should be parallel to the bubble tube axis (horizontal in automatic levels); if it is not, the error is proportional to the length of the line of sight. Therefore, always maintain equal lengths of backsights and foresights.

The bubble tube may be unsuitable. It should come to rest quickly, but the slightest movement or tilting of the footscrew must alter its position. So always check that the sensitivity of the bubble tube is adequate for the job.

The level must be stable. Examine the tripod for any looseness in the joints or damage to the screw threads caused by over tightening and tighten the tripod joints firmly.

Staff graduations may be in error or the staff may not be properly extended. It is advisable to test the graduations of the staff when extended, particularly on either side of the joints.

2. Errors in handling the equipmentIn tilting levels, the telescope bubble must be central or in coincidence each time a reading is taken. Ensure that the bubble is centred both before and after reading the staff.

Staff should be held vertically or readings will be too great. Always check lateral verticality against cross hairs. Staffman should use a staff bubble or swing the staff slowly towards and away from the instrument, the lowest reading being noted.

3. Displacement of the EquipmentIf the instrument is set up on soft or marshy ground or on thick matted grass it may settle and alter the height of collimation. Also, if the instrument is knocked the H.I will be altered and errors will arise. Always set up on stable ground, forcing the tripod shoes down firmly. Never hold, lean on or trip over the tripod.

Change points must be chosen so that when turning the staff round or when replacing it after removal no alteration of height takes place. Always choose stable change points. On hard ground mark the staff position with chalk, and on soft ground use a change plate.

Sometimes inexperienced operators inadvertently move the level forward while the staffman is also changing position. Never move the staff until a backsight has been taken; never move the level until a foresight has been taken.

4. Errors in Reading & BookingReading the staff upwards instead of downwards.Reading an inverted staff downwards instead of upwards.Reading the staff downwards instead of upwards when using an automatic level.Concentrating on the decimal reading and noting the metres wrongly.Omitting a zero, e.g. recording 3.09 instead of 3.009Entering a reading in the wrong column.Forgetting to book an entry.Noting a reading with numbers interchanged, e.g. 1.501 instead of 1.105Reading against a stadia hair instead of the horizontal cross hair.Noting a wrong distance or point description in the remarks column.

For these reasons, always read the staff, book the observation and then check that the recorded entry agrees with a second reading through the telescope.

5a. Errors due to natural causes - 1

The wind causes vibration of the level and tripod and of the staff, particularly when the staff is fully extended, which can make accurate sighting impossible; in windy weather shelter the instrument and keep sights and the staff short.

The sun can cause an apparent vibration of the staff owing to irregular refraction. It also affects the bubble by causing unequal expansion of the level and tripod and can alter the curve of the bubble tube itself. Sighting is difficult when the sun shines into the object glass. Therefore, in hot sun reduce lengths of the sights, keeping them at least 0.5m above the ground throughout their length. Extend the ray-shade in front of the object glass and shade the instrument with an umbrella.

Rain makes accurate work difficult and unpleasant; raindrops on the objective glass and condensation on the eyepiece make sighting impossible. It is advisable to wait for better weather. However, in rainy weather use the ray-shade and protect the instrument with an umbrella.

Errors arising from refraction and curvature of the earths surface are small and generally negligible in ordinary levelling and are eliminated by adopting the precaution in Instrument error point 1 above. For long sights and in precise work where equality of backsights and foresights cannot be maintained adopt the method of reciprocal levelling5b. Errors due to natural causes - 2

ApplicationsContouring

Civil Works

FieldworkMaterials Levelling staff (number to be determined by the number of students)Resource persons and field assistants/technical staffField/data sheet

Field ActivitiesDetermining of points on a selected undulating field.

ASSIGNMENT

Series Levelling Notebook Recording for site of Proposed factoryExamine the entries critically and provide the necessary corrections indicated as X1-X8.(b) Using an illustrative diagram, comment briefly on the procedure used for the survey

B.SI.SF.SRise Fall R.LRemarks2.39031.517TBM No. 1X20.405X6A1.318X432.589BX11.612X532.295C1.5020.51431.781DX30.08731.868E0.5321.888X8TBM No. 15.7982.4602.4604.1468.8080.80831.517+3.338+3.338+3.338

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