Horizontal Line Definition In Surveying

In case a theodolite has gone through a strong impact it is suggested to test the compensator. We must do this even if the theodolite correctly carries out program adjustment. During the test we should determine the compensator operating range and linearity of its work. We start by placing the theodolite at several meters from the wall so that one of the foot screws is directed to the wall. Now we set the vertical axis into the plumb position using the tubular level.

Then we set the telescope horizontally, rotating it until the vertical angle reading out is equal to 0° or 90°. Then we look up the compensator's operating range in the technical specifications of the theodolite. One of them is horizontal, and other two are 3′ above and below the horizontal line correspondingly. Marking of these lines is fulfilled with the help of the vertical angle readout.

Then we point the theodolite to the horizontal index line on the wall. Now we will turn the foot screw of the tribrach and superpose the horizontal line of the reticle with the upper index line on the wall. This way we incline the theodolite vertical axis at the 3′ level. The allowable difference is ±3″ for high-precision theodolites and it is ±5″ for moderate-precision theodolites.

We test the compensator the same way inclining it in the opposite direction. At this point we superpose the reticle with the lower index line by the means the foot screw. If the deviations exceed the aforementioned values but are still the same at opposite inclinations, we can arrive at the conclusion of a nonrelevant scale factor. In electronic theodolites, the compensator is an independent device that measures slight angular inclinations of the vertical axis. Data from the compensator enters the microprocessor of the theodolite. We are able to give instructions to the microprocessor to take the data into account of angular measuring results.

We can switch off the compensator or put the data into the display for estimating the instruments inclination. There are electronic theodolites that do not have a tubular level. In this case, we can use an electronic level to set the vertical axis into the plumb line. The theodolite preliminary setting is carried out with the circular level.

The inclination along the directionx mostly influences the measurement results. The direction is parallel to the plane of rotation of the telescope. The vertical axis inclination to the x direction directly influences the vertical angle measurement result. So we can see from Equation 2.1that the vertical axis inclination along the y direction has less of an influence on the measurement results.

That is why dual-axial compensators are usually applied in a total station and seldom in a theodolite. The collimation axis of the telescope should be horizontalwhen the vertical circle read out is equal to zero. In order to meet this requirement, we should measure the vertical angle at two positions of the theodolite. The total sum of these readings must be 360° if the theodolite has an ordinary full scale (from 0° to 360°) of the vertical circle. Some low-precision theodolites have an inclination scale of ±90° instead of the full scale. In this case, sightings of the same target should have angles of inclination at both positions of the theodolites and must be equal but have opposite signs.

The difference of the sum from 360° (0° for the instruments with the inclination scale) divided in two is called the vertical circle index error. In order to correct it we should correct the vertical circle read out by means of the vertical tangent screw. Then we superpose the horizontal line of the reticle to the target with the help of the vertical adjusting screws (see Figure 3.13).

We suggest correcting only slight vertical index errors with the help of these screws. If the vertical index value is several minutes, the reticle's horizontal removal or inclination could appear. The horizontal removal of the reticle changes the collimation error value that must be corrected. The vertical index adjustment of low-precision theodolites can be fulfilled only adjusting the reticle screws. A parallel glass plate suspended on elastic strips is set between the vertical circle microscope and a mask. The suspension scheme is similar to a level compensator with a reverse pendulum.

It is balanced by adjusting weights located at the upper part of the compensator pendulum. When the theodolite vertical axis is inclined along the longitudinal direction x the parallel plate glass rotates around its axis keeping its previous balanced state. During this rotation, the vertical limb image is shifted about the mask scale at the required compensation value.

At the moment of theodolite vertical axis inclination, the transversal direction y of compensation does not occur. Therefore, when using optical theodolites it is necessary to closely watch the bubble position in the tubular level. In a moderate precision theodolite the level is always set in a transversal position. If the bubble on the tubular level moves from the center, set it halfway back to the center by the means of leveling screw 3. Now we correct the other halfway by the means of the adjusting screw. We need to be sure that the bubble is in the center by rotating the instrument at 180°.

We need to repeat checking and adjusting until the bubble is in the center at any instrument position. The tubular level scale division ranges from 20″ to 60″ per 2 mm depending on the theodolite's precision. This allows us to establish the vertical axis accuracy from 10″ to 20″. A horizontal plane of sight tangent to level surface at any point is readily established by means of a spirit level or a level vial.

In spirit levelling, a spirit level and a sighting device are combined and vertical distances are measured by observing on graduated rods placed on the points. It is the most precise method of determining elevations and the one most commonly used by engineers. The measuring system of an elementary contemporary optical theodolite is shown in Figure 3.14. The outside light illuminates the vertical limb through the matte window.

Then the light goes through the right-angled prism of the vertical channel and comes to the transparent horizontal limb. The horizontal and vertical scale images do not overlap each other and are parallel if adjustment is correct. As a matter of fact, it is common for both vertical and horizontal channels. This is why after the horizontal channel image adjustment we must confirm the vertical channel image. Having gone through the microscope, the circles' images enter the right-angled prism, which sends the images to the mask. It has two separate transparent windows for the vertical and horizontal channels.

Elementary microscopes have index-marked windows (see Figure 3.15). The microscopes of moderate precision theodolites have scaled windows. If you are well experienced in adjusting surveying instruments, you could try to adjust a monoaxial compensator by yourself. First, we set the vertical axis into the plumb position.

Then we loosen the compensator slightly from the fastening screws. Then we put the telescope into the horizontal position and point to the horizontal index line on the wall. Now carefully turn the foot screw until the vertical angle readout stops changing. For the next step, we turn the foot screw to the opposite direction and mark the opposite point at which the compensator ceases operation. We test the horizontal axis inclination in the following way.

Direct the telescope to the upper ending of the wire at one of the circle positions. Then unfasten the vertical clamp and direct the telescope to the lower ending of the wire . The vertical line of the reticle may coincide slightly with the center of the wire.

This is natural because some inclination of the vertical axis may occur. Then we find out the deviation by the means of the reticle bisector or the horizontal circle of the theodolite. If we apply the second method, we should do the ruler reading. Then we turn the theodolite to another position and direct it again to the upper target. The vertical direction deviation from the lower target at both positions of the theodolite should not be more than 10″ for moderate- and high-precision theodolites.

The 30″ difference is allowable for low-precision theodolites. In case we try the second way, the rulers' read out difference should not exceed 0.2 mm (0.6 mm for low-precision theodolites). If the limits are exceeded, we should correct the horizontal axis inclination with the aid of accommodation described earlier or the vertical axis flange fastening screws.

Vertical distances are measured along the vertical axis to determine the difference in height between points. Vertical angles are measured in the vertical plane either above or below the horizontal plane of the instrument. In optical theodolites significant values of limb eccentricities may be visible. We could see limb images shifting about the mask edges while rotating the theodolite. It is advised to test limb eccentricity influence in the lab.

In the center of a room with a stable floor we set up our tested theodolite. In order to test the horizontal circle eccentricity, we put from six to twelve marks at the same angular interval on the walls of the room. The marks must be set up at the same horizontal line and it is advisable that they be at the same distance from the theodolite.

Then we carry out angular measurements pointing to these marks at both positions of the theodolite. Now we calculate collimating errors of every direction. Then we draw a diagram illustrating collimation error dependence of the horizontal limb position (Figure 3.21).

We take a typical limb of an 80 mm diameter and then superpose it with the axis of rotation. According to this formula we evaluate the maximal value angular error as from 5″ to 10″! Now we realize that we need not only high accuracy of theodolite adjustment, but also the highest quality of axial systems and bearings. The eccentricity influence could be minimized methodically by measuring the angle at two positions of the theodolite (see Figure 3.7).

Two diametrically opposed sensors are set in high- and moderate-precision electronic theodolites to minimize this error. Some of leading manufacturers apply mathematical correction methods in electronic theodolites. After assemblage, the instrument is tested on an angle-measuring stand. According to testing, angular and linear components of this error are determined. Then they are written into the permanent storage of the microprocessor, which calculates eccentricity correction data and inserts them into every angular reading out.

It is an important requirement for the vertical axis to remain highly stable. When the instrument is new there is usually little worry about this, even with low-precision theodolites. However, after a shock or unskilled repair the tight vertical axis may develop some gaps or internal dents made by the bearing balls.

The first sign of a problem is usually inadequate tubular level reactions during adjustment. In order to verify this malfunction we should direct our theodolite to a very clear target at a distance of about 10 m. Beforehand, we should set the instrument very carefully to the vertical position using a tubular level. Then, we unfasten the horizontal circle clamping screw and rotate the instrument several times to one direction and contrariwise. Before changing the rotation direction we should make sure that the horizontal line of the reticle and the target coincide. In the case of visible noncoincidence at any change of direction, and also attended with the bubble deviation, this indicates vertical axis instability.

The problem is resolved by changing out the axial pair in a specialized workshop. In the past there were multitrack code limbs in angular measuring surveying instruments. Because of CCD line technology development, nowadays only barcode solutions are used in absolute electronic theodolites. Such a limb has an infinite barcode stripe circumferentially spaced. An absolute angular sensor consists of a light-emitting diode and the CCD line where the barcode stripe images are projected. The CCD signal is processed the same way as it was described in Chapter 2about digital levels.

The only difference is that a digital rod is coded in linear values, whereas a barcode limb is coded in angular values. This is the same way we find out the exact part of the angular value according to the phase shift of the barcode support grid. This is how we find millimeters and their fractions in a digital level. For example, Topcon applies the same phase-measuring method to code leveling rods and their theodolites limbs.

Other leading manufacturers use their technical groundwork both in digital theodolites and levels. Before collimation error corrections, we should be sure that reticle inclination has not occurred. It is convenient to use a suspended vertical wire (see Figure 3.12). First, we should properly set the vertical axis of the theodolite into the vertical position.

There is another suggested method for reticle inclination adjustment. We start by superposing the reticle vertical line with the target. Then we remove the target image to the lower edge of the reticle by means of the vertical tangent screw. In the case that the image removes more than the size line thickness, adjusting is needed. The collimation error's influence on horizontal angle readings could be excluded the following way.