LESSON NOTE ON USING PLANE TABLE AND ALIDADE SURVEY

METHODS OF SURVEY:

USING PLANE TABLE AND ALIDADE SURVEY

 

1. This is a method of survey that allows a surveyor to plot and create an accurate scale drawing on site. The idea is to scale down the site to a size that fits onto the drawing board.

2. A plane table is a drawing board which is fixed to a tripod, leveled, and orientated. An alidade is a sighting device used to observe the salient points of the site.

3. Have a good look at the site before you start the survey, time should be taken to walk round and have a good look at the site or building that you are going to record. On a building, the obvious features that you would want to record are the corners, door and window openings, fireplaces. More subtle features that you would look out for are changes in wall direction, straight joints, blocked doors or windows, crack slots, changes in the stonework, changes in floor covering, byre drains.

 

STEP 1:-SELECT A SURVEY STATION

Once you have had a good look at the site, you will select a position for your survey station from which you have a line of site to as many of the main points as possible. It is particularly important to be able to see all the corners and places where a feature has a change in direction. Set up the tripod legs over your selected survey station. Prepare the drawing board; a piece of polyester drafting film is taped onto the drawing board with masking tape. The corners are taped first, with the film pulled tight, and then the edges are taped. The board is fixed onto the tripod.

 

STEP 2:- ORIENT THE DRAWING BOARD

When drawing a building you would orientate the long side of the board so that it is parallel to the long axis of the building. If possible, for a site plan, you would orient the board so that north is up the page. For all plans it is important to check that the whole site is going to fit on the board at your chosen scale. A pin is positioned into the board to give a fixed point from which observations and measurements can be taken. The pin marks the position of your survey station on the board. Once the survey has commenced the plane table should not be moved.

 

STEP 3:- CARRY OUT THE SURVEY

The alidade is placed against the pin and used to observe in turn, selected points. Rays are drawn on the film and measurements between the pin and the points taken with the 30mtape. The measurement is written onto the drawn ray as a record. Subsequently, each point is plotted along the ray at the selected scale, ' with a scale rule. Observations and measurements are taken to as many points as required to allow the shape to be recorded. These plotted points create an accurate framework around which the rest of the plan can be constructed. This method is most useful for plans of single buildings or small siteswithin a radius of 30m from the plane table.

THE 3 MAIN TYPE OF LEVELS / LEVELLING INSTRUMENT

LEVELLING INSTRUMENT

Level is an instrument used in establishing horizontal line. it uses spirit babble tube with the construction of telescope, you can sight or object aid also focus, line of sight is called line of collaboration. Basically there are 3 main types of level or leveling instrument.

Each instrument have a few basic parts the working of which will be firmly under store. Hide are dumping level, tilting level, automatic level. Each is used in sighting different ways.

 

DUMPING LEVEL

This is the most common form of survey level and it consist of able tube attach to the telescope.

The telescope is rightly fixed to the stage which has 3 or 4 foot screw.

The foot screw used to bring the babble to the centre of its run and also to make axis of the babble horizontal. If the line at collimation is parallel to the able axis then the line of collonation is horizontal. The advantage was in the fact that when it is leveled, you can sight all the direct – this instrument is getting out of the ………………….

TILTING LEVEL

Is an improvement of the dumping level, instead of being tilted. When using this instrument it is only necessary to set it roughly leveled. As babble is finally leveled for each reading by means of adjusting screws ad thus greater accuracy is obtained. Since the babbles axis is always mark to be right angle with the axis however the adjustment is done before every reading.

AUTOMATIC LEVEL

Just as the name implies, it is an automatic instrument. the instrument may be out of adjustment if the recticle is mis-position after carrying out the task as described for titling level.

In automatic level, the horizontal hair is brought to the correct readings by MV of recticle.

The instrument has circular spirit babble you don’t have to set the babbles every time. It also gives direct image instead of inverted images.

Along with any of the above 3 types of level we can also use level staff, staff plate or cross-foot especially when we are on soft ground.

SURVEYING EQUIPMENT AND LEVEL SET-UP

SURVEYING EQUIPMENT AND LEVEL SET-UP The opposite figure shows a LEICA Level packages. Make sure the surveying equipment you will borrow is reliable and in good working order. To ensure this a two peck test must be carried out each time you borrow a level. Surveying instruments that you need to complete your projects,  can be borrowed from the following persons: Equipment For carrying out your project work you need to borrow the following equipment: Level Tripod Staff (5 metre) Staff bubble Measuring tape (50 metre) Wooden pegs (if needed) Additional equipment is needed for some other projects and the lecturer will inform you what you need to do this exercises. The following information is extracted from 'LEICA NA720 User Manual'. Level types There are different types (Leica, Sokkia, Wild) of levels you can borrow. The level opposite is a Leica NA720. Most level instruments have an automatic horizontal adjustment of the line of sight. Levels are supplied in a case in which the instrument can be shock proof stored. All survey levels operate in a similar way. The function and operation of levels is explained using the Leica 720 which is the most available instrument in the storerooms. Endless drive (both sides) Bubble level (to check horizontal plane) Mirror to view bubble level Base plate (sits on tripod) Footscrew (to adjust the horizontal plane) Eyepiece (focus adjustment Knurled ring (for horizontal circle reading) Objective Course aiming devise Focusing knob (turn until staff reading is clearly visible) Window for digital angle reading (see 7) Setting up the tripod Loosen screws of tripod legs, pull out to required length and tighten screws. In order to guarantee a firm foothold sufficiently press the tripod legs into the ground. When pressing the legs into the ground note that the force must be applied along the legs. Check all screws and bolts for correct fit. When setting up the tripod pay attention to a horizontal position of the tripod plate. Minor inclinations of the tripod can be corrected with the footscrews of the tribrach. Place level onto tripod head. Tighten central fixing screw of tripod. Turn footscrews A and B simultaneously in opposite directions until bubble is in the centre (on the imaginary "T"). Turn the instrument 90° and then turn the foot screw C until bubble is centred. If you want to centre an instrument over a ground point:   1. Attach plumb bob and arrange the tripod in such way that the plummet is over the point.   2. For fine adjustment loosen central fixing screw slightly and shift instrument parallel on tripod until the plummet is exactly over the point.   3. Tighten central fixing screw. Focusing telescope Aim telescope against a bright background (e.g. white paper). Turn eyepiece until reticule is sharp-focused and deep black. Now the eyepiece is adapted to your eye. Aim telescope on staff using the coarse aiming device. Turn focusing knob until image of staff is sharply focused. Levelling staffs (rods) and accessories There are many types of staffs, with names that identify the form of the graduations and other characteristics. Staffs can be one piece, but most of them are sectional and adjust the length by telescoping.. The metric staff has major numbered graduations in meters and tenths of meters (there is a tiny decimal point between the numbers). Our staves have an ''E'' shape mark (or its mirror image) with horizontal spaces between them of 10 mm. When viewed through an instrument's telescope, the observer can easily visually interpolate a 10 mm mark to a quarter of its height, giving a reading accuracy of 2.5 mm. On one side of the rod, the colours of the markings alternate between red and black with each meter of length. The Black arrows indicates where to push to extend the staff to its full length.                                                                  Staff readings The figure below shows three different staff readings: It is easy to read (b) and (c) because the cross-hair is exactly on a mark division. The reading for (a) is between 1.630 and 1.640. To assess the mm reading you have to estimate where the position of the cross-hair is. For (a) the reading is 1.636. The millimeter reading is to be estimated and can very between ± 1 mm. reading (a) is 1.636 (b) is exactly 1.500     and (c) is 1.580 Spirit & line levels There are a wide range of spirit levels to meet the varying requirements of specific jobs. The majority of those used on construction work are made of powder coated aluminum or die cast construction. The length varies from 800 mm to 2000 mm. Spirit levels are very handy for short distance levelling (depending on the spirit level up to 2 metres and with straight edge up to approximate 5 metres). The straight edge is used if the the points to be levelled exceed the length of the spirit level. The line level has been designed and made with two small hooks to hold it on a line as shown in the figure above. A line level is a level designed to hang on a string line. The level must hung in the center of the string and each ''leg'' of the string line extends the levels plane. The line level is a simple surveying instrument which can be used to lay out contours and gradients, and also to assist measuring horizontal distances at slope. Plumb bobs   A plumb-bob or a plummet is a weight with a pointed tip on the bottom that is suspended from a string and used as a vertical reference line. This instrument has been used since the time of the ancient Egyptians by bricklayers, masons, and carpenters to ensure that their constructions are "plumb", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible. Small plumb bobs are often included in the kits of various instruments such as levels and theodolites. They are used to set the instrument exactly over a fixed datum marker, prior to taking fresh readings. In conjunction with the line-level the can also be used to assist the horizontal measurement on slopes. Water level An old device but a simple instrument for measuring the level differences of two points. This level, is illustrated in the opposite figure. The two levelling staffs are of the same length with a graduated tape attached to each stave. The tube is filled with water. The ends of the tube are fitted with rubber stoppers to prevent loss of water. The total length of tube defines the range of the instrument. Straight edge A 'straight edge' in conjunction with a spirit level and tape measure can be used to establish a gradient. The straight edge is usually 3 to 5 metres long and set horizontally with the aid of a spirit level. This method should be used for the measurement of gradients which continue only for short distances, e.g. to calculate the horizontal distance shown in plan-views. The figure below shows how a gradient for the ground profile is found. Measurements along a slope Distinguish between a horizontal distance and a slope distance. All distances should be measured 'horizontally'. Do not measure along slopes. Sag (tape are not supported for its length will sag under the influence of gravity) and to a lesser extent temperature may have an effect on the distance measurement also. To reduce the sag break tape measurement into shorter lengths. The sum of horizontal lengths (L1 & L2) equals the horizontal distance of the slope from A to C. Remember the horizontal distance is always shorter than the measurement on the slope. For an accurate measurement, the tape should be held horizontal and straight with a specified tension applied to it. Slopes - Gradient calculations A slope is the steepness, incline, gradient, or grade of a straight line, and it is defined as the ratio of the "rise" divided by the "run" between two points on a line. The gradient of a straight line shows how steep a straight line is. The slope of a line in the plane containing the y (rise) and x (run) axes may be represented as: (a) gradient      (1:50;  1:2) (b) percentage  ( 2%; 50%) (c) decimal fraction  (0.02;      0.5) (d) angle                  (1.146°;  26.565°)  Here are the calculation example of the figures shown in the "Straight edge" section (a) Gradient     200 : 3000 = 1 : x     x = 15     Gradient = 1 :15 (b) Decimal fraction 200/3000 = 0.066666     Fraction = 0.0667 (c) Percentage multiply decimal fraction by 100                      0.06666 × 100 = 0.06666            Percentage = 6.667% (d) Angle in degree inverse tan-function rise over run (tan-1 function on calculator)          tan a = 200/3000 = 0.06666                     a = 3.814° Distance measurement methods For measuring a distance we use steel or fibre glass tapes as shown in the opposite figure. They are available in 30 metre and 50 metre length. A more sophisticated method is to use the Electronic Distance Measuring (EDM). We will not use this method. EDM devices use electromagnetic waves, infrared waves, or lasers to measure distances precisely. Approximate (fairly accurate) distance measurement method is 'pacing' or using the 'stadia lines' on the reticle of the level Pacing Don't try to pace out one metre with every step. Walk casually over 50 or better 100 metre counting the number of steps. Work out the length of a casual step and use this instead. The longer the walking distance the more accurate is the step measurement. Example If it takes you 65 steps to walk 50 metres; then your step is 50/65 = 0.77 metre. If you would waked 39 steps, then the distance is 39 x 0.77 = 30 m. Stadia lines The stadia lines on the reticle can be used for simple distance measurement. The distances intercepted on the vertically-held rod between two stadia hairs seen in the eyepiece gives the distance. Just multiply the difference on the rod between the top and bottom stadia lines by 100* as shown in the figure below. In the example above the distance between the top and bottom stadia hair is 62 mm. Therefore, the distance to the staff is 62 × 100 = 6200 mm or 6,2 metres. * The 100 figure should be checked before beginning any survey by measuring the known distance with a tape. Surveyors usually use total stations for land surveying. A total stations is a combination of an electronic theodolite (transit), and electronic distance measuring device (EDM). Other accessories Optical square Range Pole Opticle Square and Range Poles are used in surveying course No W5939 Change Plate - a small flat metal plate with a raised point in the centre that is used to support the staff on the ground. The plate is firmly embedded into the ground to stop the staff from subsiding. There are various types of staff bubbles available to assist in keeping the staff in an upright position. You will need pegs or stakes and hammer for Project 4 (Grid points for RL's) All can be borrowed from the surveying department.