TACHYMETRY TACHYMETRY

LESSON 6LESSON 6STADIA FIELD PRACTICE &STADIA FIELD PRACTICE &

ACCURACY OF STADIA ACCURACY OF STADIA OBSERVATIONOBSERVATION

STADIA FIELD PRACTICESTADIA FIELD PRACTICE

•Stadia tachymetry is mainly used in Stadia tachymetry is mainly used in surveying details in selected areassurveying details in selected areas. .

•Adequate horizontal and vertical Adequate horizontal and vertical control, supplied by traversing and control, supplied by traversing and leveling is required to orientate the leveling is required to orientate the survey and to provide station levels. survey and to provide station levels. It is best suited to open ground It is best suited to open ground where few hard levels are required. where few hard levels are required.

STADIA FIELD WORKSTADIA FIELD WORKFigure below shows an area which needs topographic survey. There are some object illustrated in that figure, such as station (A), building(B), road(C), fence(D) and drainage(E).

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ACCURACY OF STADIA OBSERVATIONACCURACY OF STADIA OBSERVATION

Accuracy of height Accuracy of height measurement :measurement :

Provided that the staff is Provided that the staff is held vertically withheld vertically withreasonable care and reasonable care and angles of sighting are less angles of sighting are less than 10˚, then heights than 10˚, then heights should be accurate to should be accurate to within 0.01 per cent of thewithin 0.01 per cent of the sighting distance.sighting distance.

Accuracy of Accuracy of distance measurement: distance measurement:

Using an ordinary Using an ordinary levelling staff with 10 mm levelling staff with 10 mm divisions practical divisions practical accuracies approximate accuracies approximate to the following:to the following:

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Distance Accuracy

20 m ±100mm

100 m ±200mm

150 m ±300mm

Accuracy of tachymetry workAccuracy of tachymetry work

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• The error of careless staff holding can be resolved by using staff bubbles when implementing field observation.

• Error in reading the stadia intercept, which is immediately multiplied by 100(K1), thereby making it significant. This source of error will increase with the length of sight. The obvious solution is to limit the length of sight to ensure a good resolution of the graduations.

• Error in the determination of the instrument constants K1 and K2, resulting in an error in distance directly proportional to the error in the constant K1 and directly as the error in K2.

• Effect of differential refraction on the stadia intercepts. This is minimized by keeping the lower reading 1 to 1.5m above the ground.

• Random error in the measurement of the vertical angle. This has a negligible effect on the staff intercept and consequently on the horizontal distance.

The main sources of error in elevation are errors in vertical angles and additional errors rising from errors in the computed distance.