Fenn College of Engineering Cleveland State University Civil Engineering Department 216-687-2400 Cleveland, Ohio 44115 CVE 212 - Surveying Lab #7 MEASURING HORIZONTAL ANGLES WITH A THEODOLITE EQUIPMENT Transit with tripod Magnifying glass Two plumb bob's (one in transit box) Range pole Plumb bob target INTRODUCTION A theodolite is an instrument designed to measure horizontal and vertical angles. It consists of a telescope mounted so as to rotate vertically on a horizontal axis supported by a pair of vertical standards attached to a revolvable circular plate containing a graduated circle for reading horizontal angles. Another graduated arc is attached to one standard so that vertical angles can be observed. Strictly speaking, "theodolite" is the correct word to be used in describing such an instrument. However, through common usage in the United States, the term "transit" has come to be applied to the conventional, double center instrument. The engineers transit is used to measure horizontal angles, vertical angles, and is usually equipped with a compass for determining magnetic bearings. Following are some guidelines for use of the equipment. To place the transit on the tripod, first adjust the friction of the tripod legs at the tripod head. The legs should fall freely of their own weight from a horizontal position. Set up the tripod with the legs well spread and pressed firmly into the ground. If the surface is hard, each tripod shoe should be placed in an indentation on the surface. Remove the transit from the case and lifting it by the base immediately screw it firmly on the tripod. Remove the dust cap from the objective lens and replace it with the sunshade. The sunshade should always be used. When carrying the transit, hold it on the shoulder in a horizontal position, instrument to rear. Balance it to carry the weight on the arm. If there are any overhead obstructions, carry the instrument under the arm. _________________________________________________________________ Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 10/29/2006 Page 1 216-687-3874 Stilwell Hall 114 Fenn College of Engineering Cleveland State University Civil Engineering Department 216-687-2400 Cleveland, Ohio 44115 To set up the transit, seize two legs and place the third leg on the ground about two feet from the point. pull the other two legs outward and backward and place them on the ground so that the foot plate is almost level. This, may take some practice. Attach the plumb bob and center the instrument on the foot plate. Move the transit body without moving the legs so that the plumb bob hangs within about 2 inches of the point. Adjust the legs, with the plumb bob moved to about one inch above the point to insure that they are firmly set with the plumb bob still within about 2 inches of the point. The points of the tripod shoes should be placed in cracks or indentations to prevent slipping if the instrument is set up on a paved or masonry surface. Relive any residual friction in the tripod leg hinge by loosening and retightening the tripod hinge thumbscrews. Loosen two adjacent leveling screws. Level the transit head roughly without tightening the screws. Turn the transit until the [plate levels are in line with pairs of opposite leveling screws. Turn the screws following the rule: "Thumbs in, thumbs out, the bubble follows the left thumb." Slide the head until the plumb bob is over the point. Level the instrument. Leave the leveling screws firm but not bound. When the bubbles have been centered turn the instrument 180o in azimuth. If the bubbles do not center, the level is not in adjustment. Bring the bubbles halfway toward the center with the leveling screws and the instrument is now level. Once set up, do not touch it or allow it to touch anything except for transit operations. PROCEDURE To set an angle or to point the instrument, first choose the proper motion to use. Loosen the clamp, bring the instrument approximately into position, clamp it, and make a fine adjustment with the tangent screw. It is good practice to bring the instrument on with a final clockwise motion of the tangent screws. 1. Set up the transit over the station/stations give for your squad. Center and level over the said station/stations by plumb bob and two pairs of leveling screws. Make sure that the telescope is at a convenient height for sighting. 2. To measure an angle (interior and exterior at each station), set Vernier A at zero and then read Vernier B. Record the readings of both Verniers. Make a pointing on the object that marks _________________________________________________________________ Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 10/29/2006 Page 2 216-687-3874 Stilwell Hall 114 Fenn College of Engineering Cleveland State University Civil Engineering Department 216-687-2400 Cleveland, Ohio 44115 the left hand side of the angle with a spindle clamp (lower motion) unlocked; use slow motion screw (tangent screw) for fine adjustment. Check the Vernier readings after locking the lower motion (LM). This corresponds to OD (zero direct) reading sighting on the back sight. During this operation, the plate clamp (upper motion) is locked. 3. Unlock or release the plate clamp (upper motion) and make a pointing on the object that marks the right hand side of the angle. Lock upper motion and make fine adjustment on the sighting with the tangent screw. Read Verniers A and B. This corresponds to 1D (one time direct reading) reading. 4. Repeat the complete procedure again for measuring the exterior angle and thus close the horizon at each station. Make sure that the summation of interior and exterior angles lie with + 1o of 360o. 5. Balance the summation of the interior and exterior angles so that the sum is exactly 360o 00' 00''. 6. While set up at one station, determine the magnetic bearing of one line of the traverse using the transit compass. Calculate the true bearing of that line. The magnetic declination in the Cleveland area is approximately 5oW. Starting at that station, calculate the true bearings of all lines on the traverse as an office calculation. FIELD NOTES Following are typical field notes for angle measurements made with the transit in closing the horizon. Direct Plunged Average Direct Total Cloudy Chris H. Interior Interior Interior Exterior Int.+Ext. 43 F Denis Z. Sta. Reading Reading Reading Reading Reading 264-04-40 A 259-35-00 IS @ G G 93-55-10 100-25-20 BS - F N <R to A 100-25-20 200-50-40 100-25-20 <L to A 259-35-00 360-00-20 B F IS @ A BS - G <R to B 93-55-35 187-51-10 93-55-10 C <L to B 266-04-40 D 359-59-50 E _________________________________________________________________ Prof. S.F. Duffy PhD, PE, F. ASCE Revised: 10/29/2006 Page 3 216-687-3874 Stilwell Hall 114 .