Survey Control

Survey Control

   The primary purpose of surveying is to locate points of precise position on the earth’s surface. To achieve this, a network of primary survey control stations (or points) have been established throughout Australia. From these primary control points further secondary survey control stations have been located.

Survey Control Stations

   A Survey Control Station (also known as a Standard Survey Mark - SSM) is a permanent point of known Reduced Level (height) and Horizontal Position (Latitude and Longitude or Eastings and Northings). A Standard Survey Mark is normally a brass plaque set in concrete, such as the one below, although metal spikes and metal rods set in concrete or driven long distances into the ground are also used.

   These stations are usually positioned below ground level to protect them from being accidentally or purposely destroyed. An identification plaque is located nearby so that it can be easily found.

 

Where the height and the position of the point is known, or blue for a survey control point where only the height is known (this is known as a bench mark).

   There are other plaques, such as a white one that indicates the centre line of a road – control used by Main Roads WA.

   A Reduced Level is a known height above or below Mean Sea Level. This height is determined by a survey method known as Levelling, which will be discussed in

later chapters.

   A Bench Mark is a point of known Reduced Level. Only the vertical position of a Bench Mark is known, whereas both the horizontal and vertical positions of a

Standard Survey Mark are known. In the past a Bench Mark was made by cutting a bench out of the base of the trunk of a well grown tree, as shown in the diagram below. A blaze was made above the bench mark with the details of the bench mark cut into the blaze.

Because of the nature of trees, the section near the bottom of the tree does not grow. Because most of these were located along highways, they were easily damaged by motor vehicles, and therefore the practice of using trees was discontinued. Most Bench Marks today are brass plaques, the same as Standard Survey Marks, as shown below.

Fig 2.6

   From the primary Survey Control Stations, a surveyor will need to extend the control onto the work site in the form of Secondary Control. They will do this by performing a Bearing and Distance traverse commencing at a primary survey control station to obtain the horizontal control of the secondary control point and a level traverse to obtain the Reduced Level of the point.

   Most secondary control points that are established for a specific task are marked with a survey  peg, usually made of red gum or hardwood measuring about 50mm x 50mm square and about 250mm long.

   These pegs are also used to mark the corners of buildings, as well as to mark the corner points of property boundaries

   In the centre of a Survey Peg there is usually a flat-headed nail. It is to this nail that surveyors will make their measurements. Often a surveyor will use a nail driven into the road or a mark on the road, usually near a street corner as a survey mark.

   The nail may have a small aluminum plate around it, making the mark more visual and can be reused by other surveyors.

 

   More often than not a surveyor will make a temporary mark in the road, such as a chip taken out of the concrete gutter marked with spray paint or an unmarked nail in the road. Such a survey mark is a Temporary Survey Mark (TSM) that can be destroyed or forgotten after the particular task has been completed

Protecting Survey Control Stations

   All construction projects are set out from survey control points of some sort, either primary survey control points or from secondary survey control points.

   For this reason, it is very important that these points are protected from either accidental

damage or from purposeful destruction.

   Most standard survey marks consist of a brass plaque set in precast concrete. These are buried in the ground to a distance of about 150-200mm. Above that is a precast metal or concrete cover and lid.

  The metal cover and lid are totally detached from the survey mark, so that if the cover is run over or knocked, the survey mark will not be disturbed. In areas where there is little or no chance of the standard survey mark being disturbed, the mark consists of the same brass plaque set in precast concrete.

  This time the mark is set above ground by about 75mm, with a metal pole a short distance away so that the mark can be easily found, as shown in the diagrams below.

 

   Most Standard Survey Marks are well protected from the time that they are

established. Secondary control points are not so well protected because most are close to the

Construction site. Surveyors try to protect secondary marks by a variety of ways.

These include:

Recovery Marks

   After the control point has been placed in the ground, four further pegs or metal rods are placed so that there is a peg on each side of the control point. If a string is placed across each of the opposing recovery marks, they will intersect directly over the nail in the survey peg.

If the control point is destroyed, it can be replaced by using the recovery marks.

 

Survey Stakes are wooden stakes made from red gum or hardwood, each measuring about 25mm x 25mm and about 1m long. These stakes are used to indicate where a peg has been placed in the ground, so that the peg can easily be found and any machine operators working in the area can easily see the location of each peg,

   To protect a survey peg, survey stakes are placed around the location of the peg and bright coloured flagging (25mm wide plastic tape) is either wrapped around each of the stakes or wound from one stake to another.

The bright coloured flagging ensures that the stakes can be easily seen by anyone working in the area.

 

   The third method commonly used to protect Survey Pegs is by enclosing the peg inside a wooden fence, similar to the method above.

   The fence must be built far enough back so that the surveyor can set up the instruments inside

the fence.

 

   The fourth method of protecting Survey Pegs is to place the peg well out of the way of any possible construction work. This is not always possible as the secondary control points must be placed in such locations that as much of the construction site can be seen as possible.