A. General Description: The engineer's transit is an instrument designed for measuring horizontal and vertical angles (A section of the transit is shown in Fig. 9). Two spindles, or centers, one inside the other, are attached to a horizontal circular plate, the outer spindle being attached to the lower plate and the inner one to the upper plate. The lower plate carries the graduated circle and the upper plate carries the verniers for reading the angles on this circle. On this upper plate are two uprights or “standards” supporting a horizontal axis. At the center of the horizontal axis is a telescope attached at a right angle to it. The length of the telescope and the height of the standards are commonly such as to allow the telescope to make a complete rotation on its horizontal axis. The motion of the telescope and the horizontal axis is controlled by means of a clamp and a slow-motion screw called a “tangent screw”. This screw acts against an opposing spring.
For leveling the instrument, there are two spirit levels on the upper plate, one parallel and the other at right angles to the horizontal axis. The spirit level that is parallel to the axis is the most important because it controls the position of the horizontal axis of the telescope. In the transit, the leveling is done by means of four (sometimes three) leveling screws.
The upper plate usually carries a compass-box with a magnetic needle and a circle graduated to half degrees, so that the transit may be used also as a compass. The lower spindle fits into a socket in the leveling base. This base is provided with a ball-and-socket joint and four leveling screws. Both the upper and lower plates are provided with clamps to hold them in any desired position and tangent screws for setting the telescope or the circles in an exact position.
Under the center of the ball-and-socket joint hangs a short chain to which the plumb-line is attached. The entire head of the instrument can be shifted a fraction of a centimeter laterally with reference to the tripod and thus can be readily placed exactly over a point on the ground.
The horizontal circle is usually graduated either to half-degrees or to 20-minute spaces. The graduations are usually numbered from 0° to 360° by two rows of figures running in opposite directions. In some older transits they are numbered from 0° to 360° in a right-hand direction and, by a second row of figures, from 0° each way to 180°. The transit is usually provided with opposite pairs of verniers.
The normal or direct position of the transit is with the upper clamp and its tangent screw nearest the observer and the focusing screw of the telescope on top of the telescope. When the instrument is turned 180° in azimuth from the direct position and the telescope is inverted (turned about its horizontal axis) it is said to be in the reversed position. This is often spoken of as “plunging” the telescope.
If the telescope is provided with a long level tube and a vertical circle, or arc it is called an “Engineer's Transit”, or “Surveyor's Transit”. If it does not have these attachment, it is called a “Plain Transit”.
B. Setting Up the Transit Over a Point. The following are the steps in setting up the transit over a point:
Set instrument on ground near the point, with the legs spread at such an angle as will insure stability. Without reference to the point, move the legs so that the plates are nearly level. The tripod's leg screws should be firm but not binding.
Center the instrument over the point with the use of a plumb-bob suspended by a string from the hook under, and in the center, of the instrument. Pick up the instrument bodily, without disturbing relative position of the legs and head, until the plumb-bob is within about an inch of the point.
Press each leg firmly into the ground, watching the position of the bob as it moves closer to the point. While doing this, care should be taken to see that the level head remains nearly horizontal thereby requiring little turning of the leveling screws to make the plates level.
Bring bob exactly over the point or tack by loosening two adjacent leveling screws so that the head will shift a little bit. When the plumb-bob is over the point, the leveling screws are tightened just enough so that they have a firm bearing.
When setting the transit on sloping ground, the usual practice is to place one leg of the tripod up, and the other two legs down, the slope.
C. Leveling the Transit. To level the plates, turn the instrument about its vertical axis until the axis of one spirit bubble is in parallel position with a pair of opposite leveling screws. The second spirit bubble will then be parallel to the other pair of opposite leveling screws. Grasp a pair of opposite screws between the thumbs and forefingers and turn them uniformly either toward or away from each other. This process is actually tightening one screw by the same amount that the other is loosened. Note that the leveling head tilts slowly as the screws are being turned and the bubble moves from one direction of the spirit level to another. Continue turning the screws until the bubble is situated at the center of the spirit level. Similarly, the other bubble is centered by means of the other pair of screws. Check if the first bubble is still at the center of the spirit level. Alternately repeat this process until both bubbles are in the center. Then observe the position of the plumb-bob; if it has moved off the point, reset by means of the shifting head and again level. Should one of the screws bind, the other should be turned faster, and if this does not release the tight screw, one of the screws of the other pair should be loosened slightly. When using the leveling screws it is convenient to note the bubble moves in the direction of the left thumb.
After both bubbles have been centered, in one position, turn the plate about the vertical axis through an angle of 180°. If the bubbles do not remain central, it indicates that the adjustment of the spirit level is imperfect. If this happens, move again the leveling screws until each bubble moves halfway back to the center of its tube. If this is done correctly, the plate will be truly horizontal and the bubble will then stay in the same place in the tube even if the plate is turned slowly around the vertical axis.
D. Measuring Horizontal Angles. When a horizontal angle is to be measured both upper and lower clamps are loosened and the zero of the vernier is set close to 0 on the circle. The upper clamp is then tightened and by means of the upper slow motion screws, the index of the vernier is set exactly opposite 0 on the circle. The telescope is now directed at one of the objects to be sighted and when the object is in the field of view and near the vertical hair, the lower clamp is tightened. The vertical hair is set exactly on the object by means of the lower slow motion screw. The telescope should be focused carefully.
The upper clamp is now loosened and the telescope is sighted on the second object. The upper clamp is tightened, and the slow motion screw is used to bring the object on the vertical crosshair. The angle can now be read by adding to the circle reading the minutes read on the vernier. To eliminate instrument errors, the angle can be measured again with the telescope inverted and the mean of the measured angles are taken. Another procedure is by measuring the angle by repetition, once with the telescope direct and once reversed and the mean of the measured angles is taken. The vernier is read in the direction the angle is turned.
E. Measuring Vertical Angles. To measure a vertical angle, the instrument is carefully leveled and the telescope is directed to the object. When the object is observed in the telescope, the vertical motion is clamped and by means of the vertical tangent screw the middle horizontal crosshair is set exactly on the point. The reading on the vertical arc is the vertical angle. When the point is above the horizontal plane, the angle is called a positive angle or angle of elevation; when the point is below the horizontal plane, the angle is called a negative angle or angle of depression. In the survey notes, they are designated by a + sign and a - sign, respectively.
If the instrument's line of sight and the axis of the telescope bubble are not in adjustment, it is impossible to obtain a correct vertical angle with the vertical arc only. If the instrument has a full vertical circle, the error can be eliminated by reading the vertical angle first with the telescope direct and then reversed and the average of the two readings is determined.
