MEASUREMENT OF DISTANCE

• Methods of determining distance depends on the following:

  • PURPOSE FROM WHICH THE MEASUREMENT IS INTENDED

  • REQUIRED PRECISION

  • OTHER CONDITIONS

  • COST
• COMMONLY EMPLOYED METHODS OF LINEAR MEASUREMENTS IS BY:

  • PACING

  • PHOTOGRAMMETRY

  • TAPING

  • TACHYMETRY

  • MECHANICAL DEVICES

  • GRAPHICAL METHOD
• PACING -is counting the number of steps or paces in a required distance

  -it is suitable in determining approximate distance where a low precision of measurement is sufficient
• PACE -defined as the length of a step in walking
• TAPING -the most common method of measuring or laying out horizontal distance

  -is stretching of calibrated tape between two points and reading the distance indicated in the tape
• TACHYMETRY -it is based on the optical geometry of the instruments employed and is an indirect method of measurement. -distances are computed by trigonometry
• STADIA METHOD - This method provides a rapid means of determining horizontal distance. It was introduce by James Watt of Scotland and referred to at the time as micrometer for measuring distance.
• GRAPHICAL METHOD -unknown distance may be determined through their relationship with known distance geometrically. This method are widely used in plane table surveys and in triangulation work. Determining distances by scaling from maps or aerial photographs could also provide sufficiently accurate results.
• MECHANICAL DEVICES - are only applicable for low precision surveys or where quick measurement are desired
• COMMONLY USED MECHANICAL DEVICES:

  • ODOMETER

  • MEASURING WHEEL

  • OPTICAL RANGEFINDER
• ODOMETER -a simple device that can be attached to a wheel for the purpose of roughly measuring surface distance

  -the wheel is rolled over the distance to be measured and the number of revolutions of the wheel is directly registered by the device. -only give surface distance
• MEASURING WHEEL -is similar in operation to an odometer except that it is a more portable and self-contained measuring device.

  -it consists of a small wheel attached to a rod and handle, and can be pushed by an operator

  -suitable for measuring distances along irregular or curved line boundaries
• OPTICAL RANGEFINDER - This device is handheld or mounted on a small tripod, can be used to determine the distance approximately by focusing

  -the object is brought to sharp focus by focusing knob
• PHOTOGRAMMETRY -distance are measured with the aid of photographs taken from the aircraft with the axis of the camera pointed vertically towards the terrain pictured
• STADIA CONSTANT - which is the distance from the center of the instrument to the principal focus
• STADIA INTERVAL - the difference between upper hair reading and lower hair reading
• SUBTENSE BAR METHOD - is a convenient and practical device used for quick and accurate measurement of horizontal distances. The bar is 2 meters long, consists of a rounded tube through which runs a thin invar rod

  -it is mounted horizontally on a tripod and placed perpendicular to the line of sight - is often used in obtaining distances over very rough or inaccessible terrain
• HORIZONTAL DISTANCE - is the distance between two points regardless of the relative elevation.