Survey Methods - section two: baseline and offset
Tape and offset survey (extended baseline survey and running sizes)
Tape and offset survey can be used to create accurate, scaled plans of both discrete features (such as a single building) and wider areas (such as parkland), although it is best suited to open and uncluttered environments.
This method involves establishing a ‘primary baseline’ parallel to and/or through the features you want to map, and measuring the distance to the features from this known line. The baseline can be a string line and/or a measuring tape laid out taut and anchored at each end. The length of the baseline is determined by the size of the area you want to survey.
When you have determined the scale you will be working at, you can transcribe the line onto your drawing media – you might find it useful to use graph paper under a transparent overlay.
To record points of interest, measure along the baseline (or tape), then take a measurement at right angles from the baseline to the point you wish to record; plot these measurements directly onto your drawing sheet.
The accuracy of a right angle can be checked quite easily by using a set square or Pythagoras’ theorem (3–4–5 triangle).
Example of tape and offset measurement to record external wall plan - or footprint - of townhouse
Geometry enables us to set one line perpendicular (at right angles) to another. If the lengths of the sides of a triangle are in a ratio of 3:4:5, a right angle will be created between the two shorter sides. This is known as Pythagoras’ theorem, or the Pythagorean equation, where the square of the two shorter sides added together (in this example, 3×3 + 4×4 = 9 +16) equals the square of the diagonal side, or the hypotenuse (in this example, 5×5 = 25), as shown below.
Two examples of 3-4-5- ratio triangles used to set up two survey baselines at right angles to each other This applies to other, easily remembered number sets which are straight multiples of the 3:4:5 ratio, such as 6:8:10 and 12:16:20, known as Pythagorean triplets.
While these are easiest to use, the equation works for other numbers too. For example, 4.6 × 8.5 × 9.66 is a right-angled triangle, as shown in the diagram above, as is a triangle measuring 2.5 × 5 × 5.59, but 3:4:5 is the easiest to remember!
Shown below is a simple method for calculating the height of a building using the calculator function on a mobile phone and a basic hand-held laser measuring device. Bear in mind that more sophisticated laser measuring devices have numerous functions available and can perform the calculation onboard. themselves.
In the first of the two examples shown below the angle 54 degrees is arbitrary; it is based on an angle measured with a protractor from the plane table to the top of the building.
In the second diagram (below) the angle 45 degrees is governed by the fixed angle of the set square, requiring the table to be sited in a position that allows the top of the building to be viewed.