By: Scott Hunt and Jonathan Funtanilla Traversing WHAT IS A TRAVERSE? A traverse a series of consecutive connected lines of known lengths related to one another by known angles. The lengths of the lines are determined by direct measurement of horizontal distances and by slope measurement. These line courses run between a series of points called traverse stations. The angles at the traverse stations are measured by tape, transit, theodolite, compass or total stations. These angles can be interior angles, deflection angles, or angles to the right and the lengths and azimuths or bearings of each line of the traverse are estimated through field measurements TYPES OF TRAVERSING OPEN TRAVERSING When it comes to traversing there are two distinct types to look at. The First method is referred to as open traversing. Open traversing starts with a known pre-determined point already determined with respect to a horizontal datum, and end at an unknown horizontal position further down the line. Thus open traverses end without closing the loop and are geometrically and mathematically open. Hubs (steel or wooden stakes) are used in open traversing and set at each traverse station where a change in direction will occur. Spikes, Nails and scratched crosses are used when the change of direction occurs on pavement, while or painted marks are made on concrete. Open traverses should be avoided if at all possible because they offer no means of checking for observational errors and mistakes. If they must be used, observations should be repeated as well as distance and angles measurements, forward and backward, and then the averaged to carefully guard against surveying errors. Open traverses are typically used for plotting a strip of land which will be used to plan a route in road construction. CLOSED TRAVERSING The second and more common traversing method is known as closed traversing. There are two distinct categories for closed traversing which include the polygon and link method. In the polygon traverse, the lines return to the starting point, thus forming a closed figure that is both geometrically and mathematically closed as shown below. The second method of closed traversing is referred to as link traversing. Link Traversing finish upon another horizontal station, which is defined by its geographic latitude and longitude, Y- and X coordinates on a grid system, or by its location in relation to a fixed boundary. Link Traversing should have a positional accuracy that is equal to or greater than that of the starting original starting point. USES OF TRAVERSING Traverses are used to find accurate positions of a small number of marked stations. From these stations, less precise measurements can be made to features to be located without accumulating accidental errors. Thus, traverses usually serve as control surveys. When construction plans are created, stations can be used as beginning points from which to lay out work. When construction takes place, a system of traverse stations in the area must be established and surveyed. These traverse surveys are used for a variety of different purposes which include: Determining the positions of exiting boundary markers. Establishing the positions of boundary lines. Finding the area encompassed within a boundary. Determining positions of arbitrary points from which data can be obtained for. (Example establishing control points for map creation). To establish ground control for photographic mapping. To establish control points for gathering data and locating railroad, highway, utility work. METHODS OF TRAVERSING There are a verity of methods used in observing angles and directions of traverse lines which include: interior angles, angles to the right, deflection angles, and Azimuths. Traversing by Interior Angles (Most Common) Interior angle traversing is used in a variety of different types of work, but are especially convenient for property surveying. Interior angles can be observed in either a Clock-Wise or Counter-Clockwise direction but in order to reduce mistakes in reading, recording, and computing, they should always be turned clockwise from the backsight station to the foresight station. Precision of interior angles can be improved by averaging equal numbers of direct and reversed readings. Traversing by Angles to the Right Angles observed clockwise from a backsight on the rear traverse station towards the foresight on the forward traverse station are referred to as angles to the right. This is normally done by numbering consecutive traverse stations so that they increase in the forward direction. Depending on the direction of the traversing, angles to the right may be interior or exterior angles within a polygon traverse. If the direction of traversing is counter clockwise, then clockwise interior angles will be observed. If the direction of traversing is clockwise, then exterior angles will be observed. By averaging equal numbers of direct and reversed readings, observed angles to the right can also be checked and their accuracy improved. Traversing by Deflection Angles Route surveys are commonly run using deflection angles observed to the right or left from the lines extended. Deflection angles cannot exceed 180° and can be obtained by subtracting 180° from angles to the right. Positive values should be denoted as right and negative to left. Traversing by Azimuths With help of a total station, traverses can be calculated using azimuths. This process permits reading azimuths of all lines directly, which eliminates the need to calculate them. Since Azimuths are observed clockwise from the north end of the meridian. The instrument is oriented at each setup by sighting on the previous station with either the back azimuth on the circle or the azimuth (if deflection angles are turned), followed by the forward station. Thus the resulting reading on the horizontal circle will be the forward line’s azimuth. TRAVERSE LENGTHS The length of each traverse line must be calculated while preforming a traverse. The easiest and most economical method of accomplishing this involves the use of a total station. Total stations are used due to their high levels of accuracy and speed at completing the task. One such distinct advantage of traversing with total station instrument is that both angles and distances can be calculated at each station setup. Traverse lengths on a construction sites depend on the allowable limits of closure as well as on the use and extent of the traverse. In closed traverses, each course is observed and recorded as a separate distance, and on long link traverses such as highways and railroads, distances are carried along continuously from the starting point using stationing. SELECTING TRAVERSE STATIONS AND REFERENCING Selecting positions for setting traverse stations can vary with the type of survey being conducted. A few guidelines to consider when choosing stations include: accuracy utility efficiency (number of setups) Number of instrument setups. However, utility may override using very long lines because intermediate hubs, or stations at strategic locations, may be needed to complete the survey’s objectives. Once these stations are positioned they must be referenced for future checks. This is accomplished by referencing benchmarks in the area or by citing features in the area such as trees and buildings. ANGLE MISCLOSURE FOR TRAVERSING The angular misclosure for interior-angle traverse is the difference between the sum of the observed angles and the geometrically correct total for the polygon. The sum, of the interior angles of a closed polygon should equal: ∑ = (n - 2) x 1800 n= number of sides If the direction of the traverse is clockwise when observing angles to the right, exterior angles will be observed. In this case, the sum of the exterior angles will equal: ∑ = (n + 2) x 1800 n = number of sides Permissible misclosure can be computed by the formula C= k√N , n = number of angles, K= constant related to accuracy. AREA OF A CLOSED TRAVERSE When a closed traverse has been completed the area within can then be calculated. This is done using the methods of calculation. 1) ABC =1/2 acsin(alfa) when you have three sides a,b,c (referred to as the triangle method) When the area of the traverse has greater then three sides eg: 6 sides ,you can split the area in half into two triangles and take the sum. ERRORS IN TRAVERSING SYSTEMATIC ERRORS: A systematic error is an error that always has the same magnitude and sign under the same conditions. These errors are caused by physical and natural conditions that vary depending on the circumstance. Some examples include: Incorrect prism setting on total station. Curvature and refraction Incorrect length of a level rod. RANDOM ERRORS: A random error is an error that does not follow any fixed relation to the circumstances of the observation. These types of errors are produced by irregular causes beyond the control of the observer. HUMAN ERRORS: Human errors are caused by errors the observer makes during surveying process. INSTRUMENT ERRORS: Instrument errors are caused by imperfections in the design and construction of the surveying equipment. Some of these errors include: Calibration of the total station. Laser leveling error. CALCULATION ERRORS: Calculation errors occur during the adjustment process of surveying. These errors include: Misclosure calculation errors (angular and linear) References http://www.engr.mun.ca/~sitotaw/Site/Fall2007_files/TP_Manual_CloseTra vers-ENGI3703.pdf http://engineeringtraining.tpub.com/14069/css/14069_463.htm http://www.firefightermath.org/index.php?option=com_content&view=artic le&id=58&Itemid=72 http://www.globalsecurity.org/military/library/policy/army/fm/3-34331/ch6.htm http://classof1.com/homework_answers/civil_engineering/traverse_surveyin g/ www.globalsecurity.org http://www.floridageomatics.com/publications/gfl/chapter-three.htm http://www.oc.nps.edu/oc2902w/geodesy/geolay/gfl84b_a.htm