Cleveland State University

Transcription

1 CVE Surveying Lab #4 LOCATION SURVEY EQUIPMENT Right angle mirror (pentaprism) Surveyor s tape Plumb bobs Range poles INTRODUCTION Many times the surveyor is called upon to catalog details of a project prior to construction and after completion of the improvements. Prior to construction information concerning the location of details in the vicinity of a construction project is included on plans created by the engineer for the project. In a similar fashion, details of the project area after construction are placed on plans that are appropriately called as-built. Important information can include (but is not limited to) the following: a) trees, shrubs and hedges - type and size b) buildings - type and plan outline c) edge of pavements - asphalt, concrete, gravel, sidewalks, etc. d) catch basins e) concrete culverts d) corrugated steel pipe e) hydrants f) manholes g) fences h) utility poles - light, electricity, telephone, etc. The above list is by no means complete. Before a location survey takes place, a clear understanding is needed in regards to the precision necessary to locating details. If the survey is required to locate details that will later appear on small scale maps, then low order precision is called for. However, for most urban and suburban location surveys, cataloging and locating existing details can aid the engineer during the design phase of a project. In addition, the surveyor can minimize litigation after a project is completed if the contractor must restore facilities to their prior condition. Here the order of precision is determined by the economics of designing around existing features and/or replacing existing features. Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 9/26/2006 Page 1

2 There are several ways to locate existing details mentioned above. These methods include: a) station and right angle offset (rectangular coordinates). b) station, angle, and distance (polar coordinates). c) angles from two known points (usually project stations or project bench marks). d) distances from two known points (usually project stations or project bench marks) For higher order precision the objective is to locate existing details with at least one measurement more (i.e., a check measurement) than is actually needed to plot a detail in plan view. The competing economic objective is minimizing the time necessary to achieve the first objective. Many types of surveys use the right-angle offset technique to locate details. With this method existing details are located by measuring the perpendicular distance from a base line (or traverse leg) to the detail. In addition, the distance along the base line (traverse leg) to the point of perpendicularity (see the figure below) must be recorded. The base line (traverse leg) is established in the field with stakes, PK nails in asphalt, or drill holes in concrete pavement. Appropriate intervals, usually ft are established along the base line. If the terrain is smooth a tape can be placed on the ground between stations. This will permit the student surveyor to move along the tape (preferably in the direction of the forward station), making field sketches and recording information relative to the stations, and offset dimensions, of details on either sides of the base line. A sketch is entered in the field book before the measurements for each detail commences. The right angle for each location tie can be established using a pentaprism (a right angle mirror) or the arm method (an approximate method to establish a right angle in the field). With the arm method the student surveyor stands on the base line facing a point on the detail being located. One arm is pointed down the base line in the direction of one bench mark, and the other arm is pointed down the baseline towards another bench mark in the opposite direction. After checking both arms for proper alignment by alternatively sighting down either index finger, the student surveyor closes his or her eyes while swinging the arms together in front. Presumably, the student surveyor should be pointing at the requisite point on the detail. If the student surveyor is not pointing at the appropriate spot on the detail, the student surveyor moves slightly along the baseline and repeats the procedure until the detail has been correctly sighted. The station is then read off the tape and recorded in the field book. This approximate method is used a great deal in route surveys and municipal surveys. This technique provides good results over short offset distances (50 ft or 15 m). For longer offset distances or for important details, a pentaprism is used to determine the station along the base line. Once all pertinent details have been located along the base line and recorded, the student surveyor must then measure the offsets left and right of the baseline.. Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 9/26/2006 Page 2

3 The figure below illustrates topographic field notes that have been recorded using a single base line. In this figure the offsets are shown on the dimension lines and the stations are shown opposite the dimension line, or as close as possible to the actual tie point on the base line. As previously indicated, at least one check tie or check dimension should be taken on each object. These checks are omitted in the figure above so that the various methods may be shown clearly Catch Basin 79.2 ft Station BM A 31.7 ft Station 2+55 Residence 223 Elm Drive 10 ft Station ft Station ft Station 2+00 Ash Tree 18 in diam. BM B Edge of Pavement Elm Drive - 75 ROW NTS When field drawings become too cluttered the reference base line in a drawing can be split. Here two lines are drawn representing the base line. leaving a space (representing zero dimension) between the base lines for the inclusion of stations. The split base line technique is particularly valuable in densely detailed areas where single baseline notes would tend to become crowded and difficult to decipher (note the area in the general vicinity of BM B in the previous figure). The earliest topographic surveyors in North America used the split base line method of note keeping. Methods (b), (c) and (d) listed above can be implemented using an EDMI. Although the EDMI will not be utilized until a later lab, the student surveyor should recognize that the polar coordinate approach illustrated in the figure below is more efficient than the right angle approach if existing field details are readily visible. Efficiency requires that the student surveyor use as Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 9/26/2006 Page 3

4 few instrument setups as possible when mapping an area. If the EDMI is used in a location survey then the instrument setup point should be a bench mark within the traverse. The most efficient instrument setup point would be located at a place where as many existing details as possible within the project area are visible. In taking the field measurement, the student surveyor must constantly keep in mind the way in which these measurements will be used in plotting the data. SFD Wharehouse ft Traverse Line BM B BM A When using an instrument, the setup point is located nearby a horizontal control point that is visible from the instrument. With the instrument in position over the setup point, the telescope is sighted on the control point. The back azimuth of the instrument setup point to the nearby control point is set into the instrument s horizontal circle and the lower motion is clamped. This requires prior knowledge of the back azimuth of the instrument setup point and the control point Thus when the EDMI is sighted on an existing detail in the field, the horizontal circle will read the azimuth to the point. This procedure is called the azimuth transfer method. Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 9/26/2006 Page 4

5 PROCEDURE Each student survey crew will locate details along one course of the traverse utilized in Lab #1. The instructor will assign the student survey crews to a specific course. The two courses utilized in this lab will be the north-south course between Stilwell Hall and Woodling Gym as well as the east-west course that parallels Chester Avenue. The student survey crew will locate as many details as directed by the lab assistant. In order to locate details the student survey crew will: 1. Begin at one bench mark within the traverse and lay the surveyor s tape on the ground on line (use range poles) with the next bench mark in the traverse. 2. Mark 50 foot stations. 3. Utilizing the right angle prism locate the position of details relative to the tape. Mark the position at the tape with keel. 4. After the position of specified points on a detail has been determined along the surveyor s tape, measure the perpendicular distance from the tape to the each point on the detail with a cloth tape. 5. Repeat steps 3 and 4 and locate all details mentioned earlier along the assigned traverse course. Note that this effort will be repeated with the EDMI in a later lab. The student will notice the increase in productivity (i.e., more details will be located) when azimuths and distances are determined with the EDMI. Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 9/26/2006 Page 5

6 FIELD NOTES Take careful notes. Record sizes and descriptions. A data collector will not have the descriptive capacity that a field book has, so record detailed information in this lab. Use the following format in your field book: Station Dist. from Description Size ID # 9/6/00 J.Salem Baseline 66 F J. Palko E. Baker BM A ' north Catch Basin 4 sq ft ' north Parking Meter 1" diam 1101 Sketch Here - See Previous Figures ' south Oak Tree 4" diam ' south NW Corn. Bldg Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 9/26/2006 Page 6