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. 

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.