Lesson Note On Total Station Topographic Survey

How to used total station

Total Station Topographic Survey

Description Using a total station and one or more pole-mounted reflecting prisms, plot all topographic features and any additional ground shots that are required to accurately define the terrain. See Figure D.l0.

Equipment:Total station and one, or more, pole-mounted reflecting prisms.

Procedure:

• Set the total station over a known control point (northing, easting, and elevation known).

• Set the program menu to the type of survey (topography) being performed and to the required instrument settings. Select the type of field data to be stored (e.g., N, E, and Z, or E, N, and Z, etc.). Set the temperature and pressure settings-if required.

• Check configuration settings, for example, tilt correction, coordinate format, zenith vertical angle, angle resolution (e.g., 5"), c + r correction (e.g., no.), units (ft/m, degree, mm Hg), and auto power off (say, 20').

• Identify the instrument station from the menu. Insert the date, station number coordinates, elevation, and Hi.

• Backsight to one or more known control point(s) (point number, north and east coordinates, and elevation known). Set the horizontal circle to 0°00'00" or to some assigned reference azimuth for the backsight reference direction. Store or record the data. Measure and store the reflector height.

• Set the initial topography point number in the instrument (e.g., 1,000), and set for automatic point number incrementation.

• Begin taking I.Ss. Most total stations have an automatic mode for topographic surveys, where one button-push will measure and store all the point data.

• Put all or some selected point numbers on the field sketch. These field notes will be of assistance later in the editing process if mistakes have occurred.

• When all required points have been surveyed, check into the control station originally back sighted to ensure that the instrument orientation is still valid.

• Transfer the field data into a properly labeled file in a computer.

• After opening the data processing program, import the field data file and begin the editing process and the graphics generation process.

• Create the TIN (Triangulated Integrated Network) and Contours.

• Either finish the drawing with the working program or finish it on a CAD program.

• Prepare a plot file and then plot the sheet on scale.

Reference: Surveying with Construction Applications Seventh Edition

Barry. F. Kavanagh pages: 616-620 

LESSON NOTE ON DTM

What is a DEM (Digital Elevation Model)?
Digital Elevation Models are data files that contain the elevation of the terrain over a specified area, usually at a fixed grid interval over the surface of the earth. The intervals between each of the grid points will always be referenced to some geographical coordinate system. This is usually either latitude-longitude or UTM (Universal Transverse Mercator) coordinate systems. The closer together the grid points are located, the more detailed the information will be in the file. The details of the peaks and valleys in the terrain will be better modeled with a small grid spacing than when the grid intervals are very large. Elevations other than at the specific grid point locations are not contained in the file. As a result peak points and valley points not coincident with the grid will not be recorded in the file.
The files can be in either ASCII or binary. In order to read the files directly you must know the exact format of the entire file layout. Usually the name of the file gives the reference location to some map corner point in the file. The files usually contain only the z value (elevation value) and do not contain the actual geographical location that is associated with that point. The actual location associated with that elevation data is calculated by software reading the actual DEM file, knowing the precise location of the data value inside the DEM file. In addition, there will be some needed reference information in the header (first part) of the file. When an elevation is calculated at locations other than the actual grid points, some method of interpolation from the known grid points is used. Again, this is done in software that is external to the actual DEM file.
The DEM file also does not contain civil information such as roads or buildings. It is not a scanned image of the paper map (graphic). It is not a bitmap. The DEM does not contain elevation contours, only the specific elevation values at specific grid point locations.

Some companies chose to encrypt their DEMs, thereby prohibiting you from making your own files, converting data from other sources or allowing you access to data files that were provided from anyone other than that software vendor. SoftWright maintains an open architecture on all our data files. Details for all DEM file formats that SoftWright supports are available to any of our customers. 

LESSON NOTE ON CHECK ON CLOSED TRAVERSE

CHECK ON CLOSED TRAVERSE

1. Check on angular measurements

(a) The sum of the measured interior angles should be equal to (2N – 4) x 90⁰ where N is the number of sides of the traverse.

(b) The sum of the measured exterior angles should be equal to (2N + 4) x 90⁰.

(c) The algebraic sum of the deflection angles should be equal to 360⁰.

Right-hand deflection is considered positive and left-hand deflection negative.

2. Check on linear measurement

(a) The lines should be measurement once each on two different days (along opposite directions). Both measurements should tally.

(b) Linear measurements  should also be taken by the stadia method. The measurements by chaining and by the stadia method should tally.

LESSON NOTE ON CHECK ON OPEN TRAVERSE

CHECK ON OPEN TRAVERSE

In open traverse, the measurements cannot be checked directly. But some field measurements can be taken to check the accuracy of the work. The methods are discussed below.

1. Taking cut-off lines Cut-off lines are taken between some intermediate stations of the open traverse. Suppose ABCDEF represents an open traverse. Let AD and DG be the cut-off lines. The lengths and magnetic bearings of the cut-off lines are measured accurately. After plotting the traverse, the distances and bearings are noted from the map. These distances and bearings should tally with the actual records from the field

2. Taking an auxiliary point Suppose ABCDEF is an open traverse. A permanent point P is selected on one side of it. The magnetic bearings of this point are taken from the traverse stations A,B,C,D, etc. If the survey is carried out accurately and so is the plotting, all the measured bearings of P when plotted should meet at the point P. The permanent point P is known as the ‘auxiliary point’

LESSON NOTE ON MEHODS OF TRAVERSING

MEHODS OF TRAVERSING

Traverse survey may be conducted by the following methods :

1.     Chain traversing (by chain angle)

2.     Compass traversing (by free needle)

3.     Theodolite traversing (by fast needle) and

4.     Plane table traversing (by plane table)

1.Chain traversing Chain traversing is mainly conducted when it is not possible  to adopt triangulation. In this method, the angles between adjacent sides are fixed by chain angles. The entire survey is conducted by chain and tape only and no angular measurements are taken. When it is not possible to form triangles, as, for example, in a pond, chain traversing is conducted,

The formation of chain angles is

(a) First Method Suppose a chain angle is to be formed to fix the directions of  sides  AB and AD. Tie stations T₁ and T₂ are fixed on lines AB and AD. The distances AT₁, AT₂ and T₁T₂ are measured. Then the angle T1AT2 is said to be the chain angle. So, the chain angle is fixed by the tie line T1T2.

(b) Second Method Sometimes the chain angle is fixed by chord. Suppose the angle between the lines AB and AC is to be fixed. Taking A as the centre and a radius equal to one tape length (15 m), an arc intersecting the lines AB and AC at points P and  Q, respectively, is drawn. The chord PQ is measured and bisected at R.

The angle θ can be calculated from the above equation, and the chain angle  BAC can be determined accordingly.

2. Compass traversing In this method, the fore and back bearings of the traverse legs are measured by prismatic compass and the sides of the traverse by chain or tape. Then the observed bearings are verified and necessary  corrections for local attraction are applied. In this method, closing error may occur when the traverse is plotted. This error is adjusted graphically by using ‘Bowditch’s rule’ (which is described later on).

3. Theodolite traversing In such traversing, the horizontal angles between the traverse legs are measured by theodolite. The lengths of the legs are measured by chain or by employing the stadia method. The magnetic bearing of the starting leg is measured by theodolite. Then the magnetic bearings of the other sides are calculated. The independent coordinates of all the traverse stations are then found out. This method is very accurate.

4. Plane table traversing In this method, a plane table is set at every traverse station in the clockwise or anticlockwise direction, and the circuit is finally closed. During traversing, the sides of the traverse are plotted according to any suitable scale. At the end of the work, any closing error which may occur is adjusted graphically.

LESSON NOTE ON CROSS-STAFF AND OPTICAL SQUARE

CROSS-STAFF AND OPTICAL SQUARE

A. Cross-staff

The cross-staff is a simple instrument for setting out right angles. There are three types of cross-staves.

1. Open
2. French
3. Adjustable

The open cross-staff is commonly used.

Open cross-staff

The open cross-staff consists of four metal arms with vertical slits. The two pairs of arms (AB and BC) are at right angles to each other. The vertical slits are meant for sighting the object and the ranging rods. The crossstaff is mounted on a wooden pole of length 1.5m and diameter 2.5 cm. The pole is fitted with an iron shoe.

For setting out a perpendicular on a chain line, the cross-staff is held vertically at the approximate position. Suppose slits A and B are directed to the ranging rods (R, R₁) fixed at the end stations. Slits C and D are directed to the object (O). Looking through slits A and B, the ranging rods are bisected. At the same time, looking through slits C and D, the object O is also bisected. To bisect the object and the ranging rods simultaneously, the cross staff may be moved forward or backward along the chain line

B. Optical Square

An optical square is also used for setting out right angles. It consist of a small circular metal box of diameter 5 cm and depth 1.25 cm. It has a metal cover which slides round the box to cover the slits. The following are the internal arrangements of the optical square.

1. A horizon glass H is fixed at the bottom of the metal box. The lower half of the glass is unsilvered and the upper half is silvered.

2. A index glass I is also fixed at the bottom of the box which is completely silvered.

3. The angle between the index glass and horizon glass is maintained at 45⁰.

4. The opening ‘e’ is a pinhole for eye E, ‘b’ is a small rectangular hole for ranging rod B, ‘P’ is a large rectangular hole for object P.

5. The line EB is known as horizon sight and IP as index sight.

6. The horizon glass is placed at an angle of 120⁰ with the horizon sight. The index glass is placed at an angle of 105⁰ with the index sight.

7. The ray of light  from P is first reflected from I, then it is further reflected from H, after which it ultimately reaches the eye E

Principle

According to the principle of reflecting surfaces, the angle between the first incident ray and the last reflected ray is twice the angle between the mirrors. In this case, the angle between the mirrors is fixed at 45⁰. So, the angle between the horizon sight and index sight will be 90⁰.

Setting up the perpendicular by optical square

1.     The observer should stand on the chain line and approximately at the position where the perpendicular is to be set up.

2.     The optical square is held by the arm at the eye level. The ranging rod at the forward station B is observed through the unsilvered portion on the lower part of the horizon glass.

3.     Then the observer looks through the upper silvered portion of the horizon glass to see the image of the object P.

4.     Suppose the observer finds that the ranging rod B and the image of object P do not coincide. The he should move forward or backward along the chain line until the ranging rod B and the image of P exactly coincide

5.     At this position the observer marks a point on the ground to locate the foot of the perpendicular.

LESSON NOTE ON PROCEDURE OF PLOTTING

PROCEDURE OF PLOTTING

1. A suitable scale is chosen so that the area can be accommodated in the space available on the map.

2. A margin of about 2 cm from the edge of the sheet is drawn around the sheet.

3. The title block is prepared on the right hand bottom corner.

4. The north line is marked on the right-hand top corner, and should preferably be vertical. When it is not convenient to have a vertical north line, it may be inclined to accommodate the whole area within the map. 
5. A suitable position for the base line is selected on the sheet so that the whole area along with all the objects it contains can be drawn within the space available in the map.

6. The framework is completed with all survey lines, check lines and tie lines. If there is some plotting error which exceeds the permissible limit, the incorrect lines should be resurveyed.

7. Until the framework is completed in proper form, the offsets should not be plotted.

8. The plotting of offsets should be continued according to the sequence maintained in the field book.

9. The main stations, substations, chain line, objects, etc. should be shown as per standard symbols

10. The conventional symbols used in the map should be shown on the right-hand side.

11. The scale of the map is drawn below the heading or in some suitable space. The heading should be written on the top of the map.

12. Unnecessary lines, objects etc. should be erased.

13. The map should not contain any dimensions.