PLANE SURVEYING ON CIVIL ENGINEERING PROJECTS BASIC INFORMATION AND TUTORIALS


What Is Plane Surveying? How Plane Surveying Works?

In the past, most survey work depended on triangulation from known fixed points using a theodolite and this may still be a suitable method for smaller sites. Again it is necessary to ensure the instrument is in good condition and that its base is truly horizontal.

Readings taken on both faces of the instrument may reduce residual errors. Setting out by taping along a line given by the theodolite may also still be the clearest way of providing centre lines or points, particularly for regular structure layouts such as building columns.

The appropriate time for this is when blinding concrete has been placed to column and wall foundations. The base line, which is either the centre line of the building, or a line parallel to it but clear of the building, should have been set out previously by end pegs sited well clear of the work.

It is usual to work from co-ordinates along this base line from some fixed zero point, and measuring right angle distances out from them. In this way lines of walls and column centres can be marked on the blinding concrete.

Distances may be measured by steel or fibreglass tape pulled horizontally, so it is a great convenience if the site is level. If not a plumb bob has to be used to transfer distances. Distance co-ordinates along the base line from the zero peg are set out, using the steel tape and marking a pencil line across the peg.

The theodolite is set out over the pencil line, and its position is adjusted laterally so that it transits accurately on the two outermost base line marks. The plumb bob on the theodolite gives the mark for the co-ordinate point, a round headed nail being inserted on this point.

Distances at right angles to the base line are then set out with theodolite and steel tape. The advantage of this method is that the theodolite can sight down into column bases which are usually set deeper than the general formation level. For the assistance of bricklayers and formwork carpenters, sight boards can be provided, with the cross-arm fixed at a given level above formation level and with saw cuts exactly on the lines of sight to be used.

A builder’s line can then be fixed through such saw cuts. An alternative to the foregoing is to set out two base lines at right angles to each other and use theodolite right angle settings from these to give centres for such column bases, etc.

The introduction of EDM equipment has, however, meant that accurate distance and angle measurements can now be made from a single point set up. The instruments work by measuring the time of a wave in travelling from the transmitter to a reflector and back.

Readings may be automatically repeated to improve accuracy. Built-in or add-on equipment allows for automatic data logging, reduction of distances to horizontal and vertical components and for downloading to a computer.

Accuracy over short distances is good. Over longer distances corrections may need to be made for atmospheric conditions which vary from the manufacturers’ setting. The improved accuracy available has meant that setting out on site or general survey work is often done by some form of traversing. By this method the position of two known points is extended by noting the angle to a third point and its distance from the instrument set up over one of the points. Extended traverses should be closed onto another known point to check for errors.

Even with EDM equipment, setting out of regular structures is probably best done using a marked baseline as described above. The equipment also has major advantages in ground surveying since the location and elevation of any point in the area to be surveyed can usually be determined directly from just one or two positions of the instrument.

Data from the instrument can then be downloaded into a computer and with the use of appropriate software, contoured plans of the area can be produced for design or for earthworks measurement purposes.

Acertain amount of planning is necessary to produce the best results by ensuring a regular grid of locations is used for targeting and that any individual feature, such as sharp changes in slope are picked up. As an alternative ranging poles can be used to set out a rough grid and readings at say 20 m intervals between these should give sufficient coverage for accurate plotting.

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