Lesson Note On Factors affecting the measuring procedure In Digital Levelling

Factors affecting the measuring procedure
Every operation in a measurement procedure is a possible error source and as such requires careful
consideration in order to assess the effect on the final result.

(1) Pointing and focusing
Obviously the instrument will not work if it is not pointed at the staff. The amount of staff that needs to be read depends on the range of the instrument to the staff. However, there will be a minimum amount necessary at short ranges. It may not be critical to have the staff pointing directly at the instrument.
The precision of the height measurement may be independent of sharpness of image; however, a clear, sharply focused image reduces the time required for the measurement. If the image is too far out of focus then the instrument may not read at all. Some instruments have an auto-focus function to avoid potential focusing problems.
(2) Vibrations and heat shimmer
Vibration of the compensator caused by wind, traffic, etc., has a similar effect on the bar code image as that of heat shimmer. However, as digital levelling does not require a single reading, but instead is dependent on a section of the code, the effects of shimmer and vibration may not be critical.
Similarly, scale errors on the staff are averaged.
(3) Illumination
As the method relies on reflected light from the white intervals of the bar code, illumination of the staff is important. During the day, this illumination will be affected by cloud, sun, twilight and the effects of shadows. Up to a point these variations are catered for by the instrument but under adverse conditions there may be an increase in the measuring time.
(4) Staff coverage
In some conditions part of the bar code section being interrogated by the instrument may be obscured.
Consult the manufacturer’s handbook to ensure that sufficient of the staff is showing to the instrument.
(5) Collimation
The collimation value is set in the instrument but can be checked and changed as required. The method of determining the collimation is based upon one of the two peg methods described earlier. Once the collimation value has been determined it is applied to subsequent readings thereby minimizing its effect.
Note, however, that it can never be completely removed and appropriate procedures according to the
precision required must still be applied.
(6) Physical damage
It is likely that the instrument will be seriously damaged if it is pointed directly at the sun.

LESSON NOTE ON DIGITAL LEVELLING

DIGITAL LEVELLING
The digital level is an instrument that uses electronic image processing to evaluate the staff reading. The observer is in effect replaced by a detector which derives a signal pattern from a bar-code type levelling staff. A correlation procedure within the instrument translates the pattern into the vertical staff reading and the horizontal distance of the instrument from the staff. Staff-reading errors by the observer are thus eliminated.
The basic field data are automatically stored by the instrument thus further eliminating booking errors

Instrumentation
The design of both the staff and instrument are such that it can be used in the conventional way as well as digitally.
(1) The levelling staff
The staff is usually made from a synthetic material, which has a small coefficient of expansion. The staff may be in one or more sections. There are precise invar staves for precise levelling. On one side of the staff is a binary bar code for electronic measurement, and on the other side there are often conventional graduations in metres. The black and white binary code comprises many elements over the staff length. The scale is absolute in that it does not repeat along the staff. As the correlation method is used to evaluate the image, the elements are arranged in a pseudo-random code. The code pattern is such that the correlation procedure can be used over the whole working range of the staff and instrument. Each manufacturer uses a different code on their staffs therefore an instrument will only work with a staff from the same manufacturer.
(2) The digital level
The digital level has the same optical and mechanical components as a normal automatic level. However, for the purpose of electronic staff reading a beam splitter is incorporated which transfers the bar code image to a detector. Light reflected from the white elements of the bar code is divided and sent to the observer and to the detector. The detector is a form of charge couple device (CCD) which turns the black and white staff pattern into a binary code. The angular aperture of the instrument is quite small,

of the order of 1◦–2◦, resulting in a short section of the staff being imaged at the minimum range and
up to the whole staff at the maximum range. The bar code image is compared with a stored reference
code to find the height collimation on the staff. The instrument may not need to see the part of the
staff where the cross-hairs lie. The distance from instrument to staff is dependent on the image scale of the code.
The data processing is carried out within the instrument and the data are displayed in a simple format.
The measurement process is initiated by a very light touch on a measure button. A keypad on the
eyepiece face of the instrument permits the entry of further numerical data and pre-programmed commands.
The data can be stored and transferred to a computer when required. The instrument may have an interface, which permits external control, data transfer and power supply.

Measuring Procedure
There are two external stages to the measuring procedure; pointing and focusing on the staff and triggering the digital measurement. The whole process takes a few seconds.
Triggering the measurement determines the focus position, from which the distance to the staff is
measured, and initiates monitoring of the compensator.
Acoarse correlation approximately determines the target height and the image scale and a fine correlation using calibration constants produces the final staff reading and instrument to staff distance.
For best results a number of observations are taken automatically and the result averaged. This reduces biases due to oscillations of the compensator and air turbulence within the instrument.
The results may be further processed within the instrument, displayed and recorded. The programs
incorporated will vary from instrument to instrument but typically may include those for:
(1) A single measurement of staff reading and horizontal distance.
(2) The start of a line of levelling and its continuation including intermediate sights. Automatic reduction of data. Setting out of levels.
(3) Calibration and adjustment of the instrument (two-peg test).
(4) Data management.
(5) Recognition of an inverted staff.
(6) Set the parameters of the instrument; a process similar to the initializing procedures used when setting up electronic theodolites.

Lesson Note On Levelling Field procedure

Levelling Field procedure
At the beginning and end of each levelling run a stable and precise benchmark is required. Intermediate points are not observed. To avoid accidental damage or vandalism wall mounted benchmarks can be removed from the wall leaving the barrel, which has been fixed with epoxy resin, capped for protection.
The size of the levelling team depends upon the observing conditions and the equipment available.
In ordinary levelling an observer and staff holder are required. In precise levelling there are two staves and therefore two staff holders are required. If a programmed data logger is available then the observer can also do the booking. If the observations are to be recorded on paper a booker should also be employed.
The booker’s task, other than booking, is to do a series of quality control checks at the end of each set
of observations, before moving to the next levelling bay. Finally, in sunny weather, an umbrella holder is required because it is necessary to shield the instrument and tripod from the heating effects of the sun’s rays.

Just as with ordinary levelling, a two-peg test is required to confirm that the instrumental collimation is acceptable. Precise levelling procedures are designed to minimize the effect of collimation, but even so, only a well-adjusted instrument should be used.
Precise level lines should follow communication routes where possible because they generally avoid
steep gradients; they are accessible and have hard surfaces. However, there may be vibration caused by traffic, especially if using an automatic level.
The following procedures should be adhered to when carrying out precise levelling:
(1) Precise levelling can be manpower intensive, and therefore expensive to undertake. It is important
to carry out a full reconnaissance of the proposed levelling route prior to observations being taken to
ensure that the best possible route has been chosen.
(2) End and intermediate benchmarks should be constructed well before levelling starts to prevent settling during levelling operations.
(3) Steep slopes are to be avoided because of the unequal and uncertain refraction effects on the tops and bottoms of staves.
(4) Long lines should be split into workable sections, usually each section will not be more than about 3 km, because that is about as much as a team can do in one day. There must be a benchmark at
each end of the line to open and close on. The length of each line will depend upon terrain, transport,
accommodation and other logistical considerations.
(5) Each section is to be treated as a separate line of levelling and is checked by forward and backward levelling. This will isolate errors and reduce the amount of re-levelling required in the case of an unacceptable misclosure.
(6) On each section, if the forward levelling takes place in the morning of day 1, then the backward
levelling should take place in the afternoon or evening of day 2. This will ensure that increasing
refraction on one part of the line in one direction will be replaced by decreasing refraction when
working in the other direction. This will help to compensate for errors due to changing refraction
effects.
(7) On bright or sunny days an observing umbrella should be held over the instrument and tripod to avoid differential heating of the level and of the tripod legs.
(8) Take the greatest care with the base plate of the staff. Keep it clean. Place it carefully onto the change plate and do not drop the staff. This will avoid any change in zero error of the staff. When the staff is not being used, it should be rested upon the staff-man’s clean boot.
(9) The distances of foresight and backsight must be as nearly equal as possible so as to limit the effect of the Earth’s curvature, refraction and bad instrumental collimation. This will also avoid the need to re-focus the level between sightings.
(10) Take care when levelling along roads or railways. Stop levelling when traffic or vibrations are heavy.
When the staff is not being used, it should be rested upon the staff-man’s clean boot. Vibration may
damage the staff base plate and so change its zero error.
(11) On tarmac and soft ground the instrument or staff may rise after it has been set up. This may be
apparent to the observer but not by the staff person.
(12) In gusty or windy conditions stop levelling because there will be uncertainty in the readings. In variable weather conditions consider levelling at night.
(13) The bottom 0.5 m of the staff should not to be used because of unknown and variable refraction
effects near the ground.
(14) If a precise automatic level is to be used, it should be lightly tapped and rotated before each reading to ensure that the compensator is freely operative. This will reduce errors by ensuring that the
compensator always comes from the same direction. Some automatic levels have a press button for
this purpose.
(15) The rounded centre on the change plate should be kept polished and smooth to ensure that the same staff position is taken up each time it is used.
(16) The change plate must be firmly placed and not knocked or kicked between foresight and backsight readings. Remember there is no check on the movement of a change plate between these observations.
The staff holder should stand clear between observations.
(17) The observation to the back staff must be followed immediately by an observation to the forward
staff, both on one scale. This is to ensure that refraction remains constant during the forward and
back observations of one bay. Then, an observation to the forward staff is followed immediately by
an observation to the back staff on the other scale. This procedure helps to compensate for unknown
changes in refraction, by balancing the errors. Using two double scale rods the sequence of observation would be:
(1) BS left-hand scale on staff A
(2) FS left-hand scale on staff B
(3) FS right-hand scale on staff B
(4) BS right-hand scale on staff A
Then (1)−(2) = H1 and (4)−(3) = H2; if these differences agree within the tolerances specified,
the mean is accepted. Staff A is now leapfrogged to the next position and the above procedure repeated starting with staff A again
(18) If the back staff is observed first at one set-up, then the forward staff is observed first at the
next set-up. This ensures that changing refraction will affect each successive bay in an equal and
opposite manner. The order of observations this time will be:
(1) FS left-hand scale on staff A
(2) BS left-hand scale on staff B
(3) BS right-hand scale on staff B
(4) FS right-hand scale on staff A
Note that in each case the first observation of a bay is to the same staff, which is alternately the back
and then the forward staff.
(19) The same staff that was used for the opening backsight must also be used for the closing foresight.
This will eliminate the effect of different zero errors on the two staves. This means that there must
always be an even number of set-ups on any line.
(20) Levelling should always be carried out in both directions, forward and back. If, on the forward
levelling, the A staff was used to open and close the line, then the B staff should be used to open and
close the line on the backward levelling. This will equalize the number of readings on each staff.
(21) Lines of sight should not exceed 50 m, especially in haze, or on sloping ground. This will minimize the effects of refraction, curvature of the Earth and difficulty of reading the staff. A good average length of sight is 35 m.
(22) Use the procedure already outlined for levelling the circular bubble on automatic levels. This will happen as a matter of course if the telescope is aimed at staff A each time when centring the circular bubble.

LESSON NOTE ON PRECISE LEVELLING

PRECISE LEVELLING
Precise levelling may be required in certain instances in construction such as in deformation monitoring, the provision of precise height control for large engineering projects such as long-span bridges, dams and hydroelectric schemes and in mining subsidence measurements. For example, a dam that has been in place for many years is unlikely to be moving. However, should the dam fail the results would be catastrophic for those on the downstream side. Being under the pressure of water when full, the dam may be liable to distortion. The behaviour of the dam must therefore be monitored. One way of monitoring any vertical movement along the dam is by levelling. Since early warning of small movement is required, and since conclusions about movement must be made with statistical confidence, the levelling must be very precise.
There is more to precise levelling than precise levels. High quality equipment is very important, but so is the method by which it is used. Indeed the two components of precise levelling are precise equipment and precise procedures. Precise levelling uses the same principles as ordinary levelling but with:
(1) Higher quality instruments and more accurate staves
(2) More rigorous observing techniques
(3) Restricted climatic and environmental conditions
(4) Refined booking and reduction
(5) Least squares adjustment for a levelling net

Lesson Note On Civil 3D Building a Surface

Building a Surface

To build a surface

1. Create a new surface if needed. For more information, 

2. Add surface data to the surface folders if you haven't already.

3. From the Terrain menu, choose Terrain Model Explorer.

4. Right-click on the surface folder, for example to display the shortcut menu.

5. Click Build to display the Build Surface dialog box.

6. Click the Surface tab if it is not already active.

7. In the Description box, you can type a description for the surface. The surface description can be up to 255 characters.

8. Select one or more of the following options to control how the surface is built:

• Log Errors to file: Select this check box to create a <surface name>.err file in the following folder:

c:\Land Projects <Version Number>\<project name>\dtm\<surface name>

This log file records the time it takes to build the surface, and records each step that the Build Surface command performs, such as adding point files or point groups to the surface.

• Build Watershed: Select this check box to build a watershed at the same time the surface is built. If you select this option, then be sure to click the Watershed tab and set up the watershed options.
• Compute Extended Statistics: Select this check box to generate extended surface statistics. These statistics are displayed when you click the surface name, for example in the Terrain Model Explorer.

9. Select any of the following surface data options to control how surface data is processed:

• Use point file data: Select this check box to build the surface using the data in the surface's folder. If you clear this check box, then the surface is built without the point file data.
• Use point group data: Select this check box to build the surface using the data in the surface's folder. If you clear this check box, then the surface is built without the point group data.
• Use DEM File data: Select this check box to build the surface using the DEM file data in the surface's folder. If you clear this check box, the surface is built without the DEM file data.

NOTE Building a surface using DEM files that contain large numbers of points can use significant system resources, especially if the "Build Watershed" check box is selected.

• Use breakline data: Select this check box to build the surface using the data in the surface's folder. If you clear this check box, then the surface is built without the breakline data.
• Convert proximity breaklines to standard: Select this check box to convert proximity breaklines to standard breaklines when the surface is built. Proximity breaklines obtain their exact point location and elevation by snapping to the nearest point on the surface. If you convert proximity breaklines to standard breaklines when building the surface, the breaklines are saved with fixed locations and elevations. Therefore, if any of the surface point data that the proximity breaklines were snapping to is subsequently changed, the breaklines are not updated with these changes.

When a proximity breakline is converted to a standard breakline, one or more standard breaklines are added to the breakline file in the Terrain Model Explorer, each with the description of the breakline from which it was converted.

Clear this check box if you want to preserve proximity breaklines when building the surface.

• Use contour data: Select this check box to build the surface using the data in the surface's folder. If you clear this check box, then the surface is built without the contour data.
• Minimize flat triangles resulting from contour data: When building a surface, select this check box to check each contour in the surface for any triangles that have three points at the same elevation. The program attempts to remove any such triangle by flipping faces. 
• Apply boundaries: Select this check box to build the surface using the data in the surface's folder. If you clear this check box, then the surface is built without the boundary data.
• Apply Edit History: Select this check box to apply the Edit History to the surface after it is built. The Edit History records all the surface editing that you have performed. For example, if you built a surface and edited it, but need to build it again, you do not have to make all the edits that you made previously. Just select the Apply Edit History check box and the edits repeat automatically.
• Don't add data with elevation less than: Select this check box to exclude any surface data that has an elevation less than the elevation you type in the box.
• Don't add data with elevation greater than: Select this check box to exclude any surface data that has an elevation greater than the elevation you type in the box.

10. If you selected the Build Watershed check box, then click the Watershed tab.

11. In the Minimum Depression Depth box, type the minimum depth at which a depression in the surface is to be considered a watershed. This setting prevents minor depression depths from being defined as watershed subareas.

12. In the Minimum Depression Area box, type the minimum area at which a depression in the surface is to be considered a watershed. This setting prevents minor depression areas from being defined as watershed subareas.

13. Select or clear the Must Exceed Both Minimum Area And Minimum Depth check box:

• Select this check box to create watershed subareas of only those depressions that exceed both the minimum area and the minimum depth.
• Clear this check box to create watershed subareas of those depressions that exceed either the minimum area or the minimum depth.

14. Click OK to build the surface.

A message dialog box is displayed, informing you that the program has finished building the surface.

15. Click OK.

16. If you want to view and edit the surface triangulation, then Import 3D Lines into the drawing. 

Lesson Note On Civil 3D Creating Contours From a Surface

Creating Contours From a Surface

To create contours from a surface

1. Build a surface. 

2. From the Terrain menu, choose Create Contours to display the Create Contours dialog box.

3. From the Surface list, select the surface that you want to create contours for. If the surface name is not displayed in the list, then click Browse to search for it. Surfaces have the file extension .tin.

4. Under Elevation Range, define the range of the surface's elevation for which to create contours by entering values in the From and To boxes. The low and high elevations of the surface are displayed as defaults.

TIP If you change the Elevation Range, then you can return to the default range by clicking the Reset Elevations button.

To exaggerate the elevational changes of the contours when you look at them in 3D, enter a value other than 1 in the Vertical Scale box.

NOTE If you exaggerate the vertical scale, the contours are drawn at an exaggerated elevation and are therefore incorrect when labeling or as a basis for future TIN creation.

5. Under Intervals, select one of the following options:

• Both Minor and Major
• Minor Only
• Major Only

6. Define the contour intervals by entering values in the Minor Interval and Major Interval boxes. For example, if you enter a minor interval of 2, and your drawing units are meters, then a minor contour is created every place there is a 2-meter change in elevation.

7. Specify the layers for the major and minor contours. By placing the minor and major contours on different layers, you can easily control the contour colors and linetypes. You can select a layer or type in a new layer name.

8. Under Properties, select one of the following options:

• Contour Objects: To create contour objects. 
• Polylines: To create polyline contours.

NOTE If you select the Polylines option, then you cannot select a contour style to use.

9. From the Contour Style list, select the contour style to use for the contours.

TIP Click the Preview button to see a preview of the contour style.

10. If you need to load a contour style, edit a style, or create a new style, then click the Style Manager button to display the Contour Style Manager dialog box.

11. Click OK to generate the contours.

The following prompt is displayed:

Erase old contours (Yes/No) <Yes>:

12. Type Yes or No:

• Type Yes to erase any existing contours that may be present on the contour layers.
• Type No to preserve existing contours.

WARNING! If you type Yes to erase the old contours, then existing contours on both the major and minor contour layers are erased. When you develop grading plans, pay attention to the layers that the Create Contours command uses so that your existing ground contours are not erased. 

Lesson Note On Theodolite/EDM Topographic Survey

How To Used Theodolite/EDM Equipment

 Theodolite/EDM Topographic Survey

Description: Using EDM instruments and optical or electronic theodolites, locate the positions and elevations of all topographic detail and a sufficient number of additional elevations to enable a representative contour drawing of the selected areas. See sample field notes in Figure D.9.

Equipment: Theodolite, EDM, and one or more pole-mounted reflecting prisms.

Procedure:

• Set the theodolite at a control station (northing. easting, and elevation known), and backlight on another known control station.

• Set an appropriate reference angle (or azimuth) on the horizontal circle (e.g., 0°00'00" or some assigned azimuth).

• Set the height of the reflecting prisms (HR) on the pole equal to the height of the optical center of the theodolite/EDM (Hi).

• Prepare a sketch of the area to be surveyed.

• Begin taking readings on the appropriate points. Entering the data in the field notes (Figure D.9) and entering the shot number in the appropriate spot on the accompanying field-note sketch. Keep shot numbers sequential, perhaps beginning with 1,000. Work is expedited if two prisms are employed. While one prism-holder is walking to the next shot location. The instrument operators can be taking a reading on the other prism-holder.

• When all field shots (horizontal and vertical angles and horizontal distances) have been taken, sight the reference backsight control station again to verify the angle setting; also, verify that the height of the prism is unchanged.

• Reduce the field notes to determine station elevations and course distances, if required.

• Plot the topographic features and elevations at scales.

• Draw contours over the surveyed areas.