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1、1Project 2: EDM CalibrationCalibrate an EDM (total station) on an arbitrary (unknown) baseline and determine the error constant (zero error correction) and the scale (ppm) error associated with changes in temperature and pressure. Objectives: - Determine the zero error and first velocity corrections
2、 for an EDM in a total station - Learn techniques for calibrating EDMs - Learn sources and magnitude of errors in EDMsEquipment Requirements: 30 m tape, tripods for each point, total station (with fully charged batteries and prism), PK Nail and iron pins, flagging, field book, thermometer.Location:
3、Open area just north of football stadium.Background: There are three types of errors that may occur in EDM measurement. Constant or zero error related to misalignments in the EDM and prism First velocity correction related to meteorological conditions Scale error due to oscillator frequency changesM
4、easurement System:OperatorAtmosphere (temp, press, humiditytripodtribrachEDMtripodtribrachPrismThe constant or zero error is an accumulation of errors from three sources (a) eccentricities in the EDM (electronic and geometric), (b) differences between the electronic center of the EDM and the plumb l
5、ine, (c) differences between the optical center of the prism and the plumb line. The error is specific to a particular EDM/prism combination, making it essential to note the prism being used and to calibrate each combination of EDM/prism separately.The first velocity correction is similar to the typ
6、e of corrections applied to taping. The EDM is standardized at a specific reference refractive index. Refractive index is a function of temperature, atmospheric pressure and humidity. The reference refractive index is usually defined as:2Temperature = 120C; Pressure = 1013.25 millibars (mb) or 760 m
7、mHg. This means that any deviation from that standard atmospheric condition will create small errors (much like they do in a steel tape) that are proportional to scale (larger for longer distances). As a rule of thumb, every 10C deviation from standard temperature and every 3.5mb deviation from the
8、standard pressure will produce errors of 1 ppm respectively.1Scale errors due to oscillator variations arise from differences between the true oscillator frequency and the frequency of the oscillator at the time of observation. Large variations in temperature (from the standard temperature) can affe
9、ct oscillator frequency and in older instruments this frequency can drift. For very precise surveying work the oscillator frequency can be read directly, but in most cases this is just determined by calibrating the EDM over a known baseline composed of points set at different distances apart. We wil
10、l not do this as we do not know the distances to a very high degree of precision.Procedure (Field):1. Locate the three points that have been set out on the calibration site. Station Third is approximately one third of the distance on-line between West and East. 2. Set up tripods and tribrachs at eac
11、h of the three points. These tribrachs will not be moved again until you have completed the entire calibration. 3. Insert the total station into the tribrach already set on the tripods and use the total station to center the instrument (and tribrach) over the first point. 4. Remove the total station
12、 (NOT THE TRIBRACH) and insert it into the tribrach at the next point and center the instrument over that point. 5. Remove the total station (NOT THE TRIBRACH) and repeat the centering (step 4) this technique is called “forced centering.” 6. With a thermometer measure the air temperature at both end
13、s of the line (if the line passes through shade and sunshine, test for any relative differences in temperature and record both). You should do this at the start of each set up. 7. Set up the total station over West, the starting point for the calibration and set a prism over Station East (using the
14、tripod and tribrach already set up). Do not try and re- center the prism. Make a note of which total station and prism you are using (prisms are numbered) 8. Set the ppm on the total station to 0. 9. Measure the slope distance from West to East. Record the distance in meters and feet, and also recor
15、d the zenith angle. Plunge the telescope and repeat the 1 dn = (-1.00 dt + 0.28 dp 0.04 de) 10-6STA: WestSTA: ThirdSTA: East3measurement. Repeat this until 10 measurements have been recorded for Direct and 10 for Reverse (total of 20 observations). 10. Check for index error in the zenith angles and
16、correct zenith angles for this error. 11. Move the prism to station Third and insert it into the tribrach set up at that point do not re-center. Repeat steps 6-10 measuring the distance West-Third. 12. Move the total station to Third, insert it into the tribrach at that point. DO NOT RE- LEVEL THE TRIBRACH. Measure the distance Third-East. 13. Using a book or similar object in place of the reflector (in reflectorless mode)2 measure t
