LINEAR MEASUREMENT IN SURVEYING

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In construction projects, there is erection of different structural and non structural members (components) to their adequate position to form a required structure.

The positioning of structure components into their accurate and precise position is done by through surveying by considering both vertical and horizontal references.

In either vertical or horizontal referencing there is either linear measurement or angular measurement sometimes both.

Linear measurement in surveying refers to horizontal or vertical measurement of linear distances on earth’s surface so as to allocate different features either man made or natural.  

Units conversions in linear measurements  

In linear measurements, there are two most preferable, acceptable and used systems of measurement which are metric and imperial system of measurements.

The two systems units can be converted as follows.

Conversion of distance measuring units


Instruments used for linear measurement

Various instruments are used with different accuracy for different works of varying qualities and accuracies.

Tapes

Tapes

Odometer

This instrument measures distance between two points by converting wheel revolutions into known and standard distance units, this instrument is used in less accurate works. 

Odometer

Tacheometer

This instrument is used to determine horizontal and vertical distances between two points through reading tacheometric stadia which has top hair, middle hair and bottom hair.

EDM

EDM is the abbreviation of words Electromagnetic Distance Measuring equipment.
EDM examples

This determine distance between two points through electromagnetic waves  emission to a reflector, were the ware is reflected back to instrument so distance is calculated by equipment on basis of time spent by wave to travel to reflector and back to EDM.

Methods of distance measurement 

Different methods are applied in linear distance measurement some are old and new methods.

Direct method by using tape

In this method, linear distance is measured through laying tape between two points of required distance; this is direct method as it does not require any calculation to determine the distance.

Distance measuring by taping method.

In this method error accumulation is significant due to different conditions, so correction is required to a measured length.

Tacheometric method or optical method

This method can be used to determine the horizontal and vertical distances between two points; horizontal distance can be obtained through calculations but vertical distance through reading on vertical staff.

Tacheometric measurement of distances

The line of sight of tacheometer its stadia has three hairs with following function.

Middle hair is read to have elevation (vertical distance) between two points, so elevation is directly measured from tachometer.

Top and bottom hair are used in calculation of horizontal distance between two points.

Horizontal distance = (Top hair reading – Bottom hair reading) × 100

Electromagnetic Distance Measurement method

This is indirect method, where by distance is obtained through electromagnetic waves transmission from equipment to reflector at a point where distance is required, then waves are reflected back to equipment. 

How EDM works





 


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