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INTRODUCTION:A total station is an optical instrument used a lot in modern surveying and archaeology and, in a minor way, as well as by police, crime scene investigators, private accident re-constructionists and insurance companies to take measurements of scenes. It is a combination of an electronic theodolite (transit), an electronic distance meter (EDM) and software running on an external computer known as a data collector.
With a total station one may determine angles and distances from the instrument to points to be surveyed. With the aid of trigonometry and triangulation, the angles and distances may be used to calculate the coordinates of actual positions (X, Y, and Z or Northing, easting and elevation) of surveyed points, or the position of the instrument from known points, in absolute terms.
Some total stations also have a GPS interface which combines these two technologies to make use of the advantages of both (GPS line of sight not required between measured points; Traditional Total Station - high precision measurement especially in the vertical axis compared with GPS) and reduce the consequences of each technology's disadvantages (GPS- poor accuracy in
Station - requires line of sight observations and must be set up over a known
point or within line of sight of 2 or more known points).
Most modern total station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes atched on rotating glass cylinders or discs within the instrument. The best quality total stations are capable of measuring angles down to 0.5 arc-second. Inexpensive"construction grade" total stations can generally measure angles to 5 or 10 arc-seconds.
Measurement of distance is accomplished with a modulated microwave or infrared carrier signal, generated by a small solid-state emitter within the instrument's optical path, and bounced off of the object to be measured. The modulation pattern in the returning signal is read and interpreted by the onboard computer in the total station. The distance is determined by emitting and receiving multiple frequencies, and determining the integer number of wavelengths to the target for each frequency. Most total stations use a purpose-built glass Porro prism as the reflector for the EDM signal, and can measure distances out to a few kilometers, but some instruments are"reflectorless", and can measure distances to any object that is- reasonable light in color, out to a few hundred meters. The typical Total Station EDM can measure distances acurate to about 3 millimeters or 1/100th of a foot.
The basic principle of Total Station is that the distance between any two points can be known once the time light takes to travel the distance and back and the velocity of light is known. Then the following relation, which is already programmed in the memory of the instrument along with other correction factors, calculates the required horizontal distance and is displayed on the LCD screen.
1. Place tripod approximately over a known point locking legs at a convenient height so machine will be at or lower than eye level and the legs are at equal distances from each other. Eyeball the head
tripod so it is as close to level as possible.
*Be sure the legs of the tripod are firmly planted into the ground. * For smooth surfaces (such as concrete, asphalt, or tile), use folding metal tripod footing to secure the legs.
2. Remove instrument carefully from casing with both hands. Place on top (supporting with top handle) of tripod and tighten centering screw below platform into instrument, aligning the three corners of machine and platform. Use sight tangent screw on back side of LCD display to
center the instrument over the exact known point to be surveyed.
POWER AND PREPARATION1. Attach one of the batteries to the side of instrument with the clamp side up. Press any one of the five buttons below the display to turn on machine. It shall beep and the display should indicate
the instrument is not leveled and indexed (prècisely level internal components).
* To switch power off, hold ESC button and press indicated button that corresponds to OFF on the display. * If the battery is at a low level, the following will be displayed "battery is low!"-switch batteries and charge the drained one using provided jack.
2. Locate the horizontal level bubble above the LCD display. Rotate instrument by loosening the horizontal clamp and align the display with any two of the leveling screws. Tighten or loosen the left screw so bubble is in center. Rotate instrument clockwise to the next two screws and again use the left one to center bubble. Rotate to the final two pair of screws and center bubble. Check stationary leveling bubble to see if it is center. If not, repeat previous leveling process.* if the error message "Tilt out of range" is displayed, it is indicating the instrument is off-level. Relevel the instrument
3. To index the vertical circles, loosen the vertical clamp, and manually rotate the telescope either way twice. The beep should be heard and the zenith angle (ZA vertical angle) will appear on the LCD display.
4. Loosen the horizontal clamp and rotate the instrument clockwise twice to index the horizontal circles. The beep is heard again and the horizontal angle (HAR) is displayed.
* Vertical and horizontal indexing has now been completed.
5. Note the menus displayed. Each option shown on the home page (reached by pressing ESC) opens a section which contains several (up to 3) pages. To scroll through these pages to reach other options, press button left of the yellow ESC button that reads PX.
6. Set the target and instrument height by pressing Ht. in S-O mode. Measure the target height by reading the measurement on the reflector pole at the clamp (set at any arbitrary height suitable for job). Measure the-instrument height by taping the distance from the black point on side of instrument (level with center of telescope) to the known point on ground.
* Be sure to note the units used (currently default set at feet and decimal fractions of feet, see manual to change to metric units) and height of instrument and target in the field book.
* When using two reflecting poles, be sure to set each at same height.
1. Sight the first point (focus with eye piece and align centre hairs with center of reflector) using the horizontal clamp and fine motion screw. Set the angle to zero by pressing OSET in THEO mode. Sight the second target and read the HAR on the display.
*If you wish to read the angle by rotating the instrument to left, press R/L in THEO mode (display will read HAL or HAR for left or right respectively).
2. For higher accuracy, the average of a number of readings can be using repetition. Sight the first target and press REP in THEO mode. Press BS (back sight) then sight the second target. Press FS (fore sight) and the angle between the two will be displayed. Sight the first target again,
presses BS, and site the second target again and press FS. The average of the two readings will be displayed. Repeat up to 10 times for higher accuracy.
3. The slope of the line being shot can be displayed as a percentage by pressing ZA% in THEO mode. This is read as VA and gives the percentage grade of the line. Press it again to return to the ZA reading.
*VA% will be displayed when the parameter is set to"HORIZONTAL 0" instead of "Zenith O" but performs the same function.
DISTANCE AND ANGLE MEASUREMENTThis is the most useful and suggested method.The working procedure is described as follow
1. Sight target and select for slope, horizontal, or height(SHV) measurement. Press Sdist to start the measurement and STOP to end. The distance, vertical, and horizontal angle are displayed. Press SHV to view the other measurements (Horizontal distance or Height Difference).
2. To measure the horizontal distance several times and display the average, sight the target and press Hdist in THEO mode. Three measurements are taken and the average (H-A) is displayed after a few seconds.
* The most recently taken data can be recalled and displayed by pressing RCL in the EDM mode.
1. In order to begin the coordinate measure, set the initial coordinates of the station. This is done by pressing the S-O button at the main menu. Then press the Stn-P button on the second page of the S-O menu. Choose the Input button, then set the initial coordinates and press ENTER.
2. Sight the target and press COORD in S-O mode, then press STOP to end the measurement. The coordinates of the target are given with respect to the initial starting position (0,0,0) and designated direction to be North.
MEASURING THE DISTANCE BETWEEN 2 POINTS:1. Sight the first position and press either Sdist, Hdist, or Vdist in EDM mode to start the measurement. Stop the measurement by pressing the STOP and sight the next point. Press MLM on the same page to start the measurement, the press STOP to stop the measurement. The slope, horizontal, and height difference between the two points is displayed. This can be repeated as many times as necessary
2. The slope may be read as a percentage by pressing S96 in the same mode
after the missing line measurement has finished. This displays the
percent grade between the two points.
DISTANCE SETTING-OUT MEASUREMENT:
1. To find the direction and distance of a point set out a wanted distance from the instrument station, sight the reference direction and press OSET in THEO mode to set the HAR at 0. Turn theodolite until the required angle is displayed and locks the horizontal movement.
2. Press ESC to go to basic mode and go to S-O mode. Go to S-O_D for the data and input the desired distance to set out. Set the reflecting prism in the sighting line and press SO_Hd to start the distance measurement. The difference between the desired distance and the measured distance is displayed on the 1st line.
3. Move the reflecting prism towards or away from the instrument until H distance becomes Om to determine the point at the desired distance.
*If there is negative (-) data: Move prism away from Instr. * If positive (+) data: Move prism towards Instr.
* Press STOP to end the measurement.
COORDINATES SETTING-OUT MEASUREMENT:1. Set the station coordinates and initial azimuth angle. Press S-O_P in S-o mode and input the desired coordinates for N and E and press YES to store the data. Press SO_HA in S-0 mode to start the angle measurement. The setting-out horizontal angle, dHA is displayed. Use the horizontal clamp and fine motion screw to turn theodolite until dHA reads 0° 00' 00" and lock the clamp.
2.Sight the reflecting prism on the sighting line and press so_HD and move reflecting prism until H reads Om as in part 3 of the distance setting-out measurement.