Lesson Note On Carrying out a level traverse

Carrying out a level traverse To determine the difference in level between points on the surface of the ground a 'series' of levels will need to be carried out; this is called a level traverse or level run. There are two method of levelling: Rise & Fall method and Height of collimation (height of instrument) methods Click the link to see the animated rise&fall methods then click next for the height of collimation method. Please note when the shifting of the staff or level can be done using the rise&fall method Leveling or Field Procedures The leveling or field procedure that should be followed is shown in Figure 1 below.. Figure 1 Procedure: Set up the leveling instrument at Level position 1. Hold the staff on the Datum (RL+50 m) and take a reading. This will be a backsight, because it is the first staff reading after the leveling instrument has been set up. Move the staff to A and take a reading. This will be an intermediate sight. Move the staff to B and take a reading. This also will be an intermediate sight. Move the staff to C and take a reading. This will be another intermediate sight. Move the staff to D and take a reading. This will be a foresight; because after this reading the level will be moved. (A changeplate should be placed on the ground to maintain the same level.) The distance between the stations should be measured and recorded in the field book (see Table 1) Set up the level at Level position 2 and leave the staff at D on the changeplate. Turn the staff so that it faces the level and take a reading. This will be a backsight. Move the staff to E and take a reading. This will be an intermediate sight. Move the staff to F and take a reading. This will be a foresight; because after taking this reading the level will be moved. Now move the level to Leveling position 3 and leave the staff at F on the changeplate. Now repeat the steps describe 8 to 10 until you finished at point J. Field procedures for leveling All staff readings should be recorded in the field book. To eliminate errors resulting from any line of sight (or collimation) backsights and foresights should be equal in distance. Length of sight should be kept less than 100 metres. Always commence and finish a level run on a known datum or benchmark and close the level traverse; this enables the level run to be checked. Booking levels There are two main methods of booking levels: rise and fall method height of collimation method Table 1   Rise & Fall Method Back- sight Inter- mediate Fore- sight Rise Fall Reduced level Distance Remarks 2.554 50.00 0 Datum RL+50 m 1.783 0.771 50.771 14.990 A 0.926 0.857 51.628 29.105 B 1.963 1.037 50591 48.490 C 1.305 3.587 1.624 48.967 63.540 D / change point 1 1.432 0.127 48.840 87.665 E 3.250 0.573 0.859 49.699 102.050 F / change point 2 1.925 1.325 51.024 113.285 G 3.015 0.496 1.429 52.453 128.345 H / change point 3 0.780 2.235 54.688 150.460 J 10.124 5.436 7.476 2.788 54.688 Sum of B-sight & F-sight, Sum of Rise & Fall -5.436 -2.788 -50.000 Take smaller from greater 4.688 4.688   4.688 Difference should be equal The millimeter reading may be taken by estimation to an accuracy of 0.005 metres or even less. Backsight, intermediate sight and forsight readings are entered in the appropriate columns on different lines. However, as shown in the table above backsights and foresights are place on the same line if you change the level instrument. The first reduced level is the height of the datum, benchmark or R.L. If an intermediate sight or foresight is smaller than the immediately preceding staff reading then the difference between the two readings is place in the rise column. If an intermediate sight or foresight is larger than the immediately preceding staff reading then the difference between the two readings is place in the fall column. A rise is added to the preceding reduced level (RL) and a fall is subtracted from the preceding RL Arithmetic checks While all arithmetic calculations can be checked there is no assurance that errors in the field procedure will be picked up. The arithmetic check proves only that the rise and fall is correctly recorded in the appropriate rise & fall columns. To check the field procedure for errors the level traverse must be closed. It is prudent to let another student check your reading to avoid a repetition of the level run. If the arithmetic calculation are correct, the the difference between the sum of the backsights and the sum of the foresights will equal: the difference between the sum of the rises and the sum of the falls, and the difference between the first and the final R.L. or vice versa. (there are no arithmetic checks made on the intermediate sight calculations. Make sure you read them carefully) Table 2  Height of collimation method (height of instrument)  Back- sight Inter- mediate Fore- sight Height of collimation Reduced level Distance Remarks 2.554 52.554 50.00 0 Datum RL+50 m 1.783 50.771 14.990 A 0.926 51.628 29.105 B 1.963 50591 48.490 C 1.305 3.587 50.272 48.967 63.540 D / change point 1 1.432 48.840 87.665 E 3.250 0.573 52.949 49.699 102.050 F / change point 2 1.925 51.024 113.285 G 3.015 0.496 55.468 52.453 128.345 H / change point 3 0.780 54.688 150.460 J 10.124 5.436 54.688 Sum of B-sight & F-sight, Difference between RL's -5.436 -50.000 Take smaller from greater 4.688   4.688 Difference should be equal Booking is the same as the rise and fall method for back-, intermediate- and foresights. There are no rise or fall columns, but instead a height of collimation column. The first backsight reading (staff on datum, benchmark or RL) is added to the first RL giving the height of collimation. The next staff reading is entered in the appropriate column but on a new line. The RL for the station is found by subtracting the staff reading from the height of collimation The height of collimation changes only when the level is moved to a new position. The new height of collimation is found by adding the backsight to the RL at the change point. Please note there is no check on the accuracy of intermediate RL's and errors could go undetected. The rise and fall method may take a bit longer to complete, but a check on entries in all columns is carried out. The RL's are easier to calculate with the height of collimation method, but errors of intermediate RL's can go undetected. For this reason students should use the rise and fall method for all leveling exercises. Closed and open traverse Always commence and finish a level run on a datum, benchmark or known RL. This is what is known as a closed level traverse, and will enable you to check the level run. Closed level traverse Series of level runs from a known Datum or RL to a known Datum or RL. Misclosure in millimeter   24 x √km Closed loop level traverse Series of level runs from a known Datum or RL back to the known Datum or RL. Misclosure in millimeter   24 x √km Open level traverse Series of level runs from a known Datum or RL. This must be avoided because there are no checks on misreading

Lesson Note On Topographic Map

Reading Topographic Maps

 Interpreting the colored lines, areas, and other symbols is the first step in using topographic maps. Features are shown as points, lines, or areas, depending on their size and extent. For example, individual houses may be shown as small black squares. For larger buildings, the actual shapes are mapped. In densely built-up areas, most individual buildings are omitted and an area tint is shown. On some maps, post offices, churches, city halls, and other landmark buildings are shown within the tinted area.

The first features usually noticed on a topographic map are the area features, such as vegetation (green), water (blue), and densely built-up areas (gray or red).

Many features are shown by lines that may be straight, curved, solid, dashed, dotted, or in any combination. The colors of the lines usually indicate similar classes of information: topographic contours (brown); lakes, streams, irrigation ditches, and other hydrographic features (blue); land grids and important roads (red); and other roads and trails, railroads, boundaries, and other cultural features (black). At one time, purple was used as a revision color to show all feature changes. Currently, purple is not used in our revision program, but purple features are still present on many existing maps.

Various point symbols are used to depict features such as buildings, campgrounds, springs, water tanks, mines, survey control points, and wells. Names of places and features are shown in a color corresponding to the type of feature. Many features are identified by labels, such as “Substation” or “Golf Course.”

Topographic contours are shown in brown by lines of different widths. Each contour is a line of equal elevation; therefore, contours never cross. They show the general shape of the terrain. To help the user determine elevations, index contours are wider. Elevation values are printed in several places along these lines. The narrower intermediate and supplementary contours found between the index contours help to show more details of the land surface shape. Contours that are very close together represent steep slopes. Widely spaced contours or an absence of contours means that the ground slope is relatively level. The elevation difference between adjacent contour lines, called the contour interval, is selected to best show the general shape of the terrain. A map of a relatively flat area may have a contour interval of 10 feet or less. Maps in mountainous areas may have contour intervals of 100 feet or more. The contour interval is printed in the margin of each U.S. Geological Survey (USGS) map.

Bathymetric contours are shown in blue or black, depending on their location. They show the shape and slope of the ocean bottom surface. The bathymetric contour interval may vary on each map and is explained in the map margin.

Lesson Note On 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. 

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 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 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 

What is a DEM (Digital Elevation Model

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. 

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