Survey Plans, Bearings and Setting-Out

Survey plans, Bearings and Setting-out

Contents Introduction Learning outcomes Survey plans Types of survey plans Imperial to metric conversions Bearings Angles from bearings Why bearings and not angles? Setting-out Locating the first corner Right angles Finish the setting-out

Introduction Survey plans come in many different shapes and sizes. Coupled with all these different shapes and sizes, there is also a great variety of information shown on the plans.

On some plans, the information which is shown is required by law. There is also information which is necessary for cadastral survey work but may not be relevant when it comes to building set out. There are also plans which were prepared many years ago using imperial measurements. These will need to be converted to metric dimensions.

This unit is about the interpretation of the information shown on survey plans, what information is of use and how this information is used for setting out.

We will also look at methods of setting out right angles, applying checks to your work and at the magnitude of errors.

1 Learning outcomes On completion of this unit you will be able to: • interpret the information shown on survey plans • identify the information shown on survey plans which is relevant to building set-out • convert imperial dimensions to metric • calculate angles from bearings • set out right angles and apply checks.

Survey plans Surveyors produce a wide range of plans for such things as the creation of land titles, topographical plans, construction plans for major and minor civil engineering works, and sketch plans shown on survey reports. Here we will only be concerned with two of these types of plans:

• plans for the creation of new land titles • the sketch plans which accompany survey reports.

Because of the detail which is not shown on the sketch plans, these plans are of little use to us when it comes to building set- out. It is the additional information which is shown on the plans for the creation of title which is relevant.

Types of survey plans

Plans for the creation of land titles The various processes which are involved in the creation of a new land title are beyond the scope of this module. Suffice it to say that the basis for a new title is a property boundary survey and preparation of a survey plan. After the local government (council), statutory authorities and legal requirements have been met this survey plan is deposited with the Registrar of Land Titles for the creation of the new titles.

2 Figure 1: Plan of urban subdivision

The detail shown on the plan in Figure 1 is not complete; much information has been omitted for clarity.

You can see there is a wealth of information shown on these plans. All of this detail is required by law, Acts of parliament and parliamentary regulations which require the information to be shown. This information is of great value to surveyors for the re-establishment of property boundaries. These plans are kept in a central register for reference. Should the need arise to fix a property boundary from a subdivision dating back to the 1860s, a surveyor can obtain a copy of the original subdivision plan in order to determine the original intention of the subdivision.

In some states, even the original surveyor’s field notes must be lodged with the survey plan.

Sketch plans attached to survey reports The surveyor’s cadastral work is not restricted to the survey and preparation of plans for the creation of new land titles. There are many occasions when the surveyor carries out property boundary surveys for the identification of a title. In identification of a title the surveyor answers questions such as:

• Do the boundaries of a parcel of land agree with the title description? • Where are the exact corners of a parcel of land?

3 When this type of survey is carried out, the surveyor normally issues a report on the survey. This report will detail the title description, list improvements and detail any encroachments by or upon the land. Attached to the report will be a sketch plan of the site.

Unlike the plan for the creation of a new title, the sketch plan attached to a survey report would appear to be lacking in detail.

Although referred to as a ‘sketch plan’, the plan is not sketched but is drawn accurately to scale. These plans show sufficient detail to illustrate any comments made in the actual report. Two plans are reproduced here illustrating this type of sketch plan. One which is to accompany a peg-out survey, and the other for an identification survey. In the case of the peg-out survey, the owner has requested that the corners of the lot and the two axes of a building to be erected be marked.

Figure 2: Sketch plan accompanying a ‘peg-out’ survey report.

4 Figure 3: Sketch plan accompanying an ‘identification survey’ report

Imperial to metric conversions As I mentioned earlier, some of the old survey plans date back over 150 years. Metric measurements, or metrication to use the correct word, for the building industry only came into use in Australia as recently as 1974! This means that you will have to understand a little of the pre-metric measuring units if you are to deal with plans made before 1974.

5 So what were the measurements before 1974? 150 years ago surveyors measured distances in units of links and chains. Surveyors did not have tape measures as we have today. The measuring device was a chain made up of 100 links.

Be careful if you should come across a very old plan (or a new copy of a very old plan). Those dimensions are not metres. Even though the distances are shown with a decimal point, the measurements will be in chains and links. Check the scale shown on the plan. It will show the measurement units.

More recently, but still before 1974, the standard units of measurement were feet and inches. There are still some people around who are more comfortable talking about a piece of four by two, rather than 100 x 50. So how do you convert from one to another?

Converting measurements into metric units You may have come across some common approximations (eg. that 4 inches is 100 millimetres). 100 millimetres is an approximation, not an accurate conversion! As the imperial measurements get bigger, the approximations become less accurate. For example, 2400 millimetres is 38 millimetres different from 8 feet!

Now let’s look at converting chains and links into metric measure. You are not likely to come across many plans using these units so I will not spend too much time on them.

One chain is exactly 66 feet. One link is approximately 8 inches. One link is exactly 1½ of 66 feet, which is slightly less than 8 inches. To convert chains and links to feet, simply show links as decimals of a chain and multiply by 66.

For example, a distance of 3 chains 27 links =3.27x66 = 215.82 feet (note that feet are in decimals)

Some of these old terms are still with us. We talk about one- chain roads (roads 66 feet wide) and the distances along roads and pipelines are still called chainages.

Now let’s deal with the conversion of feet and inches to metres or millimetres. The exact conversion of inches to millimetres and metres:

• 1 inch = 25.4 millimetres • 1 foot = 0.3048 metres or 304.8 millimetres.

Do not round off these measures! A foot is 304.8mm, not 300mm.

When we approximate an eight-foot ceiling to a 2400 mm ceiling, the 38.4 millimetres discrepancy makes little difference. However, the difference when converting a 60-foot frontage to an allotment is important. 60 feet approximated at a scale of 300 mm to one foot gives a distance of 18 metres, whereas the exact conversion (60 x 0.3048) gives 18.288 metres. This is a discrepancy of 288 millimetres! On a tight block of land, this could mean the difference between a building complying or not complying with the local council building code.

Converting feet to metres

1. Convert any inches to decimals of one foot (divide inches by 12).

2. Add decimals to feet.

3. Multiply by 0.3048.

Converting inches to millimetres

1. Convert any feet to inches (multiply feet by 12).

2. Add converted feet to inches.

3. Multiply by 25.4.

If all that doesn’t seem too painful, let’s try a few examples.

The standard symbol for feet is’ and for inches is “.

To convert 17’6¼” to metres, first convert the remainder inches to feet: 6¼” = 6.25 inches ÷ 12 = 0.5208 feet add whole feet = 17.5208, and multiply by conversion factor:

7 = 17.5208 x 0.3048

or 5.340 metres (to 3 decimals places or to the nearest mm)

To convert 17’6¼’ to millimetres: l7feet= 17x l2inches =

204 inches

6¼” = 6.25 inches add 6.25” = 6.25 + 204

= 210.25 inches

= 210.25 x 25.4

= 5340 millimetres (to the nearest millimetre)

Remember—If you want to convert chains and links to metric you must first convert them into feet. Then just use the method above to convert the feet into metric units.

Check your progress 1 Convert the following imperial measurements to metric. Show your answer to the nearest millimetre or to 3 decimal places when the units are metres. Check your answers with those given at the end of this unit. 1 50 feet ______2 4’8½” ______3 6 inches ______4 22’11½” ______5 62’9” ______

8 Bearings

Previously we solved many right-angled triangle problems. In each triangle we were given one of the angles of the triangle and this angle was used to solve the triangle.

Now go back to Figure 1 at the start this unit, the plan of a subdivision creating several new allotments.

Looking at the plan in Figure 1, there is not one angle shown but rather angles are shown along each of the boundaries. Only in very rare cases are angles as such shown on a plan. What is shown on the plan and how do we arrive at angles?

A right angle contains 90°. There are four right angles in a complete circle (360°). This is very basic it is mentioned because angular measurements in surveying are measured around the full circle.

Figure 4 shows a ‘full-circle’ protractor. Note that on this protractor there are two sets of angles, one reading to the left of zero and the other reading to the right of zero or clockwise from zero.

The range turning to the left is called the mathematicians circle. This circle is of ho interest to us except to create confusion.

9 Figure 4: A full-circle protractor.

All surveying angular measurements are clockwise. The lines dividing the circle into four parts are called cardinal lines (see Figure 5). These are the north, south, east and west directions. NORTH

WEST EAST

Figure 5: The cardinal lines SOUTH

10 Here is the clue to our angle problem. I referred to the cardinal lines as directions.

In NSW there is a little hamlet called Millbank (Postcode 2440). Where is it? About 60 kilometres from Kempsey. Not much help if you were to try to find it! How about another place called Seldom Seen? Yes, it really does exist. It is just a couple of hundred kilometres from Melbourne.

These distances mean nothing—a couple of hundred kilometres from Melbourne, but which way? South would put you in the middle of Bass Strait, west well on the way to Adelaide, north almost into NSW. But if you went east to Morwell, then south on the Midland Highway (watch out for the turn-off to the east on the Grand Ridge Road), then travelled east along the Grand Ridge Road (Grand in name only, you will need a four- wheel drive!) and watch out for the signpost ‘Seldom Seen’—you would find it. (As for Milibank, it is west of Kempsey.)

Directions are important. On survey plans each line has a direction related to north, not an angle off another line. East has a direction of 90o, south 180o, west 270o and back to north 360o. Note that north has two angles, 0° and 360°. When we have got back to 360°, we have completed the full circle and are ready to start at zero again.

These directions are called bearings. The definition of a bearing is a direction measured clockwise from north.

Imagine that you are standing in the centre of a giant full-circle protractor, looking north. Someone else is standing on the edge of the protractor at the north point looking towards you. You are both on the same line. Your direction is 0°; the other person’s direction is 180°.

The line has two bearings, one north and one south, depending on which way you are looking. The south to north line you are standing on continues behind you in the south direction.

The same applies to the east and west or to any line. The difference in each direction is exactly 180°. The direction in which you are looking is called the forward bearing. The direction behind you at your back is called the back bearing or the reverse bearing.

11 Angles from bearings The basics The plan from Figure 1 is reproduced on the next page so that you can see the bearings on each line.

Figure 6: Plan of subdivision

Because each boundary can have two bearings, the forward bearing and the back bearing, you need to be very careful about directions.

Look at the bearing of the common boundary between lots 4 and 5. If you were to start walking the boundaries of lot 4 commencing at the north- western corner, when you get to this corner you will be facing west. The bearing will show this. Due west is 270o. The bearing of this boundary is shown on the plan as 290o. This is close to the west direction.

Now start around lot 5 commencing at the north-west corner. The direction of the common boundary is now easterly, even though the actual value shown on the plan is westerly. This is not an error. Only one bearing or direction is shown on any line. You need to be aware of the direction of the line and make adjustment if necessary by adding 180°.

It helps when you come across a plan of survey to actually indicate the direction shown by the bearings. Figure 7 is a plan of a four-sided allotment. On this plan there are arrows drawn to indicate the directions indicated by the bearings. Remember that the directions (bearings) are related to north.

12 Figure 7: Bearing directions

Check your progress 2

1. On the subdivision plan shown in Figure 6, draw direction arrows for all the bearings.

2. How do we make use of the bearings? The easiest way to show this is to use an example.

3. What is the angle at the south east corner of lot 2? A detail of this corner is shown in Figure 8.

Tip Figure 8: South-east corner of lot 2

13 Some more detail on bearings The rule to find an angle is to imagine that you are standing on the intersection of the two lines forming the angle. These two lines are going away from you in the direction you are facing. Subtract the left-hand bearing from the right-hand bearing to obtain the angle.

You need to be very careful about the directions. Compare the bearing (direction) of the line with the north point, which is always shown on the plan.

Remember, if in doubt draw direction arrows on the plan. I have shown these arrows on the diagram in Figure 8.

The left-hand line in Figure 2 is going away from the corner, or in the direction you are looking. The right-hand line is towards the corner. The bearing of this line must be reversed.

Right-hand line: 191°02’OO” + 1800 = 371°02’OO”

At this stage, do not worry about the bearing being greater than 3600.

Angle: 371°02’OO” — 270003’20” = 100°58’40”

When carrying out mathematical operations on angles, remember that there are 60 seconds in one minute and 60 minutes in one degree.

Now it is your turn to work out some angles from bearings. Refer to the parcel of land shown in Figure 7. Determine the four angles made by the bearings of the sides.

Why bearings and not angles?

Why not angles? The answer to that is fairly easy. Bearings relate directions to a common origin, north. An angle is isolated and refers to just two directions only. Because all bearings are related we can readily relate several directions to each other.

14 Now to the application of bearings. Go back to where we were calculating right angled triangles and look at Figure 14. In this diagram is shown a right-angled triangle where we were given the angle and the hypotenuse (lot frontage). Figure 9 below is the same diagram as in Unit 1 but with bearings shown on the three sides. This is the more usual occurrence. It is from these bearings that we determine the angle from which to calculate the square width.

Figure 9: Lot boundary bearings

To calculate the square width of the lot we need to determine the angle at the northern corner of the lot.

The bearing of the side boundary is 102°52’25”.

A line square to this is this bearing plus 900.

102°52’25” + 90° = 192°52’25” Angle: 199°25’l0” — 192°52’25” = 6°32’45” (note reverse direction of given bearing) Having determined the angle, it is an easy step to complete the calculation of the triangle.

In most of the survey calculations which you will encounter, the routines through which we have just worked will apply. These are in order:

15 1. Establish the right-angled triangle. 2. Determine one of the angles from the bearings of the sides. 3. Solve the triangle for the missing values.

Setting-out Up to this point the more practical examples have been in determining the square width of allotments. In practice, the architect or building designer should have made sure that the building will fit on the land. It does help if you can also make your own independent check.

Locating the first corner

‘So, why all the calculations?’ you ask.

To answer this let me go back to the same block of land which was shown in Figure 9. You have checked the square width and told the architect that the building width (with eave overhang) will not comply with the local authority’s requirements. That is OK because special approval has been given for reduced standards.

Go ahead and set out the building. Figure 10 shows one more requirement with which the building has to comply. The nearest corner of the building must be 6 metres from the street.

In this example, the lot has been surveyed and there are two pegs, one at the front corner and the other at the back. You can string a line between these pegs.

16 Figure 10: Street building alignment

Where do you place the front corner of the building, the first peg? In Figure 11, you can see the corner. The diagram has been exaggerated to illustrate the problem and also how to solve the problem using right- angled triangles.

Figure 11: Corner of building related to lot corner

17 In Figure 11, we have labelled the corners of each triangle so that you can follow through each step.

Step 1 Solution of triangle cde

Bearing of side boundary: 102°52’25” Bearing of line dc: 19°25’lO” + 900 = 109° 25’ 10’ Angle of triangle at C: 289°25’l0” — 282°52’25” = 6°32’45”

You may have noticed that it was not necessary in this instance, to reverse both bearings. As 180° was added to both bearings, we could have simply worked with the original bearings. We have reversed the bearings to conform to the rule:

The two lines are going away from you in the direction you are facing.

Side cd: 6 metres Side ec: 6 ÷ cos (6°32’45”) = 6.039 Side ed: 6 x tan (6°32’45”) = 0.688

Check = √(6.0392 — 0.6882) = 6 (actually 5.99968)

Do not forget this last step—to check your calculations. In the check calculation, I have shown the answer as 6 even though the actual value is 5.99968.1 think you will agree that one quarter of one millimetre is dose enough! The reason why the answer did not come out exactly 6.0 is because I have rounded off the length of each side calculated to the nearest millimetre. Add a couple more decimal places and you will get the exact answer in the check calculation.

Step 2 The next step is to calculate the triangle abc.

Bearing of line ac: 19°25’l0” Bearing of line ab: 102°52’25” — 90° = 12°52’25” Angle at a: 19°25’l0” — 12°52’25” = 6°32’45”

You may have already worked out that in this instance the two triangles are similar triangles. There was no need to go through the calculation for the angle at ‘a’.

Here is another golden rule: If in doubt, work it out.

18 If you have the slightest doubt about the triangles being similar, then work out the angles in the second triangle from the bearings. If in doubt do not say ‘It will be OK’. It may not be, and you will set out the house incorrectly.

Side ab: 1 metre Side ac: 1 ÷ cos (6°32’45”) = 1.007 Side bc: 1 x tan (6°32’45”) = 0.115 Check = √(l 0072-0.1152) = 1.0

Now we can determine the distance eb.

Distance eb: = 6.039 — 0.115 = 5.924

The peg marking the corner of the building can now be placed. Place a mark in the ground 5.925 along the stringline from the corner peg. This is the point ‘b’ in Figure 11. Lay in point ‘a’ one metre square off this point (how to lay in square off is explained later).

Is this point in the correct position? Are you sure it is correct? Would you be happy to walk away and say ‘It is right!’? This is the first peg from which the rest of the setting out will be based. It is important that this first peg is correct.

Always check your work. Do not check by repeating the same process. If you have made a mistake then by repeating the same procedure, it is an odds-on bet that you will make the same mistake again. Use a different method.

One way of checking the work in this example would be to fix point c, and measure the distance from ‘a’ to ‘c’. Not such a good check because the distances are small but a check nevertheless. Figure 12 shows a much better check—a distance onto the other corner of the lot.

19 Figure 12: Tie to second corner

Note that this diagram is also drawn out of proportion. This is to show the triangles clearly.

The triangle is another right-angled triangle, with the right angle f. In this triangle we do not have an angle. The angle at g is not the same as the angle at c. Without an angle, we need two sides. Side af we know is 6 metres. What is the length of side gf? To find this we turn back to the calculation of the two triangles abc and cde. From these we have the length of ed and ac.

Side fe = dc—ac = 0.688 — 1.007 = —0.319

20 Note the negative value. Because the diagram in Figure 12 is out of scale, the point f appears outside the line from g to e. In fact, f is between g and e.

Solution of triangle agf Side fg: 15.34— 0.319 = 15.021 Side ga: √(15.0212 + 62) = 16.175

After the peg marking the corner of the building at a has been placed, check-measure the distance from a to g.

These calculations have been very straightforward. Unfortunately, no two building sites are the same unless they are adjoining lots. I cannot give you an example how to calculate setting out for every possible site. This is just an example how to go about determining the position of the first corner mark where the site is not square. For each case which you encounter, draw a diagram of the lot and draw the right- angled triangles to enable this corner to be located.

Now you see the value of being very familiar with right-angled triangle solutions

Go back to the subdivision shown in Figure 1, this time to lot 4. Figure 13 shows the location of a dwelling on lot 4. The building is to be 6 metres from Johns Street, built parallel to the southern boundary of the lot and one metre from this boundary.

Carry out all the necessary calculations required to be able to set a mark at the corner of the proposed building shown ‘A’ on the plan.

21 Take particular care in determining which corner is the closest corner to Johns Street.

I Figure 13: Site plan for proposed building

Right angles

A few words about theodolites and accuracy.

Surveyors use a theodolite for setting out angles of any magnitude. Although the use of theodolites is beyond the scope of this subject, the accuracy with which an angle can be observed is worth mentioning.

There are several classifications of theodolite ranging from fourth-order to first-order. The ‘builder’s’ theodolite is within the classification of a fourth-order instrument. The instrument used by surveyors for everyday work is classified as a third- order instrument. Precise instruments are of higher orders.

A fourth-order instrument has a reading scale which allows observations to one minute of arc accuracy. Modem third-order ‘surveyor’s’ theodolites read to 5 seconds of arc. This still does not mean a great deal. When I refer to ‘one minute of arc’, this means a variation in an angle of one minute. The same with ‘5 seconds of arc’.

22 One minute (‘/ooth of a degree) makes an angle of 3 millimetres in 10 metres. Surveyors refer to a ‘swing’ of 0.030 (30 mm) in 100 metres. 5 seconds makes a swing of 2 millimetres in 100 metres. This type of accuracy is possible using an ordinary theodolite. Precise instruments can do a lot better!

Figure 14: Angular swing in 100 metres

The reason why I am mentioning the extent of swing at this point is because some builder’s and surveyor’s levels have a ‘circle’ or protractor built into the base of the instrument. This circle is to allow you to turn angles. Note that the circle is graduated in degrees.

In some instruments the smallest graduation may be half degrees (or 30 minutes). Using this circle it is possible to measure ‘by guesstimation’ to one quarter of one degree or 15 minutes. Refer to Figure 14 and you will see that a swing of 15 minutes is over 400 millimetres in 100 metres. Across a normal residential building of, say, 13 metres width, this could give an error of 57 millimetres!

23 Do not discard the use of an instrument with this type of circle. The last paragraph is only to serve a warning. If you use a level with a circle for setting out a right angle (or any other angle), always check the angle by some other means, preferably by two separate methods.

Setting out right angles

There are two simple requirements when setting out a right angle—the most obvious of which is to have a baseline. This baseline can be a taut string line on or parallel to the boundary or side of the building. On the baseline is a mark from which the right angle is to be set out.

The builder’s square

The steps to use this square are:

1 Align the corner of the square over the point from which the right angle is to be set out.

2 Align one side of the square alone the baseline. This side of the square must be horizontal.

3 Sight along the other arm of the square and place a mark on the line of this arm. This mark does not have to be at the correct distance. Distance can be laid in later.

4 Turn the square over and repeat the process from the opposite direction. This should give two marks ‘square off’.

5 Mean the two marks and this should be exactly square. Hopefully, of course, you will only have one mark from using both sides of the square.

24 Figure 15: The builder’s square

The folding square

When using this square, a peg is driven in at the point from which the right angle is to be set out. The actual point from which square off is to be set out is marked on the peg. The steps then are:

1. Fix the hole in the folding arm over the point on the peg.

2. Swing one end of the fixed arm along the baseline aligning the hole in this arm on the baseline until an angle of approximately 45° is formed.

3. The hole in the other end of the arm is then at right angles to the baseline. Place a mark under this hole.

4. Repeat the process using the other side of the baseline. As with the builders square, this will give two marks, the mean of which will be the right angle.

25 Figure 16: The folding square

Setting out by measurement

If you can be in three places at once, or have two assistants, you can use two tapes. Usually you wifi either be on your own or will have only one assistant and will have only one tape measure.

The method is described as using two tapes because it involves two measurements. If you are working on your own, two measurements can involve several measurements and a lot or running between marks!

The method makes use of the classic 3:4:5 triangle. As you know, a triangle with sides of these lengths is a right angled triangle. The right angle being opposite the side of length 5 units. The sides do not have to be 3,4 and 5 units. As long as the three sides are in the ratio of 3:4:5, the angle will be a right angle. For example, a triangle with sides of 6,8 and 10 is a right-angled triangle, as is a triangle with sides of 1.5,2 and 2.5.

26 Go back to the previous topic and look at the solution of right angled triangles using Pythagoras’s theorem. Check the check your progress problems and see how many of the triangles have sides of ratio 3:4:5!

You will note that when discussing the 3:4:5 triangle I have not mentioned metres but used ‘units’. The lengths of the sides do not have to be metres. You can use feet, miles (using a very long tape measure), cubits (Egyptian), li (Chinese), or whatever takes your fancy.

1. Measure along the baseline a distance of 4 units 2. Measure approximately square off 3 units. 3. Check the hypotenuse for 5 units. 4. Swing the 3 unit measurement until the hypotenuse measures exactly 5 units. 5. The line through the 3 unit mark is at right angles. 6. Repeat the process from the other side of the baseline. With two tape measures, it is simply a matter of holding the end of each tape on the marks on the baseline. Running the tapes out until you have 3 on one tape and 5 on the other.

Figure 17: A right angle using two tapes and the 3:4:5 triangle

There is another method using two tapes (or two measurements). This method makes use of an isosceles triangle. An isosceles triangle is one in which two sides are equal in length. A line joining the mid-point of the base of this triangle to the apex makes a right angle with the base.

27 To use the isosceles triangle for setting out a right angle:

1. Mark two points on the baseline and equal distance on each side of the point from which the right angle is to be set out.

2. From each of these points measure a distance longer than the distance forming the base of the triangle. Note the distance from each point must be the same.

3. Where these two distances intersect is the point at right angles from the baseline point.

Figure 18: Setting out using an isosceles triangle

There are several other devices on the market which have been specifically designed for setting out right angles, such as the site square and the optical square. If you come across one of these instruments, read the instructions which came with the instrument.

Familiarise yourself thoroughly with how the instrument works, and then after using the instrument to set out a square— check your work.

Checks

Remember the old carpenter’s adage—’measure twice and cut once’. The same rule applies when setting out. Measure only once and order in the jack hammers to break up the incorrect footings!

28 Always check your work. Do not use the same method to check. Use an independent method. I have been stressing this point throughout this unit and also in the previous unit. Check your work and then to be sure to double-check.

So you have checked your calculations. Checked the first corner by measurements to both the lot corners. Checked your right angle by repeating the set out from both left- arid right-hand sides of the baseline. What else is there to check?

Finish the setting-out

Proceed to mark each corner

This is only the start of the setting-out process. You need to proceed around the building marking each corner and angle. When you get to the last corner—check! Do not assume that because you have arrived at the last corner that everything will automatically be OK.

Measure the final distance back to the starting corner. To use a surveying term, this measurement should ‘close’—that is, it should agree exactly with the plan measurement. You have come back to the starting point. This is the first check. You may miss by two or three millimetres. How close can the brickies work? This is your tolerance.

The final proof

Now for the final proof. Firstly you need to calculate the length of each diagonal using Pythagoras’s theorem (assuming that the building is comprised of right angles). Now measure each diagonal across the site to the points which you have just placed. If they agree with your calculations, everything should be all right.

29 There is one final point to remember when setting out. When you use a line of, say, 1 metre length (for example, one side of a builder’s square), to lay in a much longer line, be aware of the errors which can occur. If you turn off a line 15 metres long and you have a one millimetre error in the first metre (the side of the square), that one millimetre becomes 15 millimetres across the building. You might be able to tolerate 15 millimetres on one side of the building but the error can be cumulative. Before you know where you are you could have 50 to 70 millimetres error. To overcome this problem—take care and always check your work.

Figure 19 shows the dimensions of a building. Calculate the length of the principal diagonals across the building. That is, determine the lengths of the lines from a to c and from b to d.

Figure 19: Building dimensions

30 Assignment: Unit 2

1. For this question you will have to refer to Figures 4 and 5 below

Figure 4

The building shown in Figure 5 is to be erected on Lot 3 in the subdivision. The criteria for the site are setbacks of 6 metres on both road frontages. The building is to be parallel to the western side boundary of the lot. These are shown in Figure 6

31 Figure 5

32 Figure 6: Building on Lot 3

Calculate distances from:

(a) the north-eastern corner of the building to the two adjacent pegs (b) the north-western corner of the building to the north-western corner peg.

Note that the side boundary is not at right angles to the front. Although the diagram shows the 6-metre setback along the front of the building, you should check which corner will be at 6 metres.