Mada za sehemu hiiSimple Survey And Map MakingMada 5
Plane table surveying is a graphical method of survey in which the field observation and plotting are done simultaneously. It is a graphical construction of straight lines, angles, and triangles for plotting the ground detail points. This method of land surveying is simple and cheaper than theodolite survey and only suitable on small areas.
The plan is drawn by the surveyor in the field while the area to be surveyed is in front of his eyes. Therefore, there is low possibility of omitting the necessary measurements. Hence, the method has very low chances for committing errors. Just as it is for the case of compass and chain surveying, plane table surveying is named after the principal instrument used, the plane table. The earliest mention and account of a plane table and plane table survey dates back to 1551 by Abel Foullon's in his work Usage et description de l'holomètre, which was published in Paris. Prior to 1830, the method was called plane table.
Plane table
Plane table
This is the principal equipment in plane table surveying. It is a board from which rays to the target objects are established, measured and located. Drawing board for plane tabling is made up of well-seasoned wood with its upper surface exactly plane. Plane tables are in a number of sizes. The common dimensions of plane tables are those rectangular in shape with size 75 cm X 60 cm. Plane table comes with wooden or aluminum tripod stand to mount on during the surveying process. They are provided with clamps to fix it in any direction. The table can revolve around its vertical axis and can be clamped in any position when necessary. The plane table is mounted on a tripod stand.
Tripod stand with plane table
Tripod stand
Is provided with three foot screws at its top for levelling and adjustable legs for adjusting the height of the plane table. The height of the tripod stand is normally 120 cm.
Tripod stand
Alidade
Is multi-task purpose equipment useful throughout plane table surveying in assisting as a ruler and a sighting instrument. It is a straight edged ruler, attached with a sighting device. One edge of the ruler is beveled and graduated. Usually, the graduated edge is used for drawing lines of sight to objects from the plane table station. There are two types of alidades available for plane tabling; the
- simple alidade
- telescopic alidade.
Simple alidade
Is used for ordinary work. It consists of gun-metal or wooden ruler with two vertical vanes at the ends. The eye-vane is provided with a narrow slit while the object vane is open and carries a horse hair. Both the slits, provide a definite line of sight which can be made to pass through the object to be sighted. To draw the rays, one of the edges of alidade is beveled and its perfectly smooth working edge is known as the fiducial edge. The fiducial edge is graduated to facilitate the plotting of distances to a scale.
Simple alidade
Telescopic alidade
Is used when it is required to take inclined sights. It essentially consists of a small telescope with a level tube and graduated arc mounted on horizontal axis. It gives higher accuracy and more range of sight.
Telescopic alidade
Trough compass
Is a thin narrow wooden box containing a magnetic needle about 120 mm long. The box is moved around on plane table surface so that when the needle points to the magnetic North, it is parallel to the sides of the box. It is required for drawing the line showing magnetic meridian on the paper. It is used to orient the table to the magnetic meridian.
Trough compass
Spirit levels
Are common and cheaply available for masonries. They are flat based aluminum or woody bar fitted with bubble tube and are used for levelling in brick laying. In plane table surveying, spirit levels are used to level the plane table during surveying or ascertaining if the table is properly leveled. Board's levelling is done by placing the level on the board in two positions at right angles and getting the bubble at the center of bubble tube. To ensure that a plane table is perfectly leveled, a bubble of spirit level must be at the center of the bubble tube and checked with its positions in any two mutually perpendicular directions.
Spirit level
- Errors in leveling — if the plane table is not leveled properly, the readings and drawings will not be accurate.
- Inaccurate centering — if the plane table is not centered correctly over the survey station, the plotted points on the map will be misplaced.
- Parallax error — this happens when the alidade (the instrument used for sighting) is not properly aligned with the survey point.
- Wrong sighting — mistakes in sighting the object or target can lead to incorrect measurements and misaligned drawings.
- Errors in plotting — inaccurate drawing or plotting of points and lines on the paper can lead to errors in the final map.
- Distorted paper — if the paper on the table becomes loose, wrinkled, or stretched due to weather conditions (e.g., heat, rain), the map will be distorted.
- Environmental factors — wind can move the paper or the table slightly, while rain can make the paper wet and unusable.
- Errors in measurement — incorrect measurements of distances or angles during the survey can result in inaccurate plotting.
- Instrumental errors — if the alidade or plane table is faulty or poorly maintained, it may give wrong readings.
- Human errors — mistakes made by the surveyor, such as incorrect readings, poor judgment, or negligence, can lead to errors in the survey.
- Ensure proper leveling and centering of the table.
- Keep the paper tight and secure.
- Avoid working during bad weather conditions.
- Double-check all measurements and sightings.
- Handle instruments carefully and maintain them regularly.
- Work with focus and avoid rushing.
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