a. Determine the order of accuracy. Refer to Table 4.2 or 4.3. b. Adjust the elevation of BM K110. The length of the level run was 780 m, with setups equally spaced. The elevation of BM 132 is known to be 167.629 m. A series ofhenchmarlee 1 TABLE 4.2 National ocean survey, U.S. coast, and geodetic surveys: Classification, standards of and general specifications for vertical control accuracy, First Order Second Order Classification Class I, Class II Class! Principal uses Basic framework of Secondary control Minimum the national of the national standards; network and of network and of higher metropolitan metropolitan accuracies area control area control may be used Extensive engineering Large engineering for special projects projects Regional crustal Local crustal movement movement and investigations subsidence Determining investigations geopotential values Support for lower-order control Maximum closures* Section: forward 3 mm VK(Class 1) 6 mm VK and backward 4 mm VK(Class 11) Loop or line 4 mm VK(Class 1) 6 mm VK 5 mm VK(Class II) *Check between forward and backward runnings where K is the distance in kilometers. Class 11 Control densification, usually adjusted to the national network Local engineering projects Topographic mapping Studies of rapid subsidence Support for local surveys Third Order Miscellaneous local control; may not be adjusted to the National Network Small engineering projects Small-scale topographic mapping Drainage studies and gradient establishment in mountainous areas 8 mm VK 12 mm VK 8 mm VK 12 mm VK TABLE 4.3 Classification standards of accuracy and general specifications for vertical control-Canada Second Order (First-Order Procedures Classification Special Order First Order Recommended) Third Order Fourth Order Allowable +3 mm VK +4 mm VK +8 mm VK + 24 mm VK +120 mm VK discrepancy +0.012 ft Vm + 0.017 ft Vm +0.035 ft Vm +0.10 ft Vm +0.5 ft Vm between forward and backward levelings Instruments: Self-leveling Equivalent to Equivalent to Equivalent to Equivalent to Equivalent to high-speed 10"/2 mm 10"/2 mm 20"/2 mm sensitivity sensitivity compensator level vial level vial level vial below Level vial 10"/2 mm 10"/2 mm 20"/2 mm 40" to 50"/2 mm 40" to 50/2 mm Telescopic 40X 40X magnification Source: Adapted from "Specifications and Recommendations for Control Surveys and Survey Markers" (Surveys and Mapping Branch, Department of Energy, Mines and Resources, Ottawa, Canada 1973). 40X 4.13 LEVEL LOOP ADJUSTMENTS In Section 4.7, we noted that level surveys had to be closed within acceptable tolerances or the survey would have to be repeated. If a level survey were performed to establish new BMs, it would be desirable to propor- tion any acceptable error throughout the length of the survey. Because the error tolerances shown in Tables 4.2, 4.3, and 4.4 are based on the distances surveyed, adjustments to the level loop are based on the relevant distances, or on the number of instrument setups, which is a factor directly related to the distance surveyed. Example 4.4 A level circuit is shown in Figure 4.28. The survey, needed for local engineering proj- ects, commenced at BM 20. The elevations of new BMS 201, 202, and 203 were determined. Then the level survey was looped back to BM 20, the point of commence- ment (the survey could have terminated at any established BM). 96 CHAPTER FOUR BM 201 0.8 km 1.6 km BM 20 1.7 km BM 202 0.6 km BM 203 Total Distance Around Loop is 4.7 km. FIGURE 4.28 Level loop. TABLE 4.5 Level loop adjustments Loop Distance, Cumulative (km) BM Correction, Cumulative Distance x E = Total Distance (m) Adjusted Elevation Field Elevation (m) 20 201 202 203 20 0.8 2.4 3.0 4.7 186.273 (fixed) 184.242 182.297 184.227 186.258 +0.8/4.7 x 0.015 = +0.003 = +2.4/4.7 X 0.015 = +0.008 = +3.0/4.7 x 0.015 = +0.010 = +4.774.7 X 0.015 = +0.015 = E = 186.273 - 186.258 = -0.015 m 186.273 184.245 182.305 184.237 186.273 SOLUTION According to Table 4.2, the allowable error for a second-order, class II (loca engineering projects) survey is 0.008 VK; thus, 0.008 V4.7 = 0.017 m is the permissible error. The error in the survey of this example was found to be -0.015 m over a total os tance of 4.7 km-in this case, an acceptable error. It only remains for this acceptab error to be distributed over the length of the survey. The error is proportioned accor ing to the fraction of cumulative distance over total distance, as shown in Table 45 More complex adjustments are normally performed by computer, using the adjustmes method of least squares. 4.14 SUGGESTIONS FOR ROD WORK The following list provides some reminders when performing rod work: 1. The rod should be extended and clamped properly. Ensure that the bottom of the plate does not become dirty, which could result in mistaken readings. If a rod targe being used, ensure that it is positioned properly and that it cannot slip. a. Determine the order of accuracy. Refer to Table 4.2 or 4.3. b. Adjust the elevation of BM K110. The length of the level run was 780 m, with setups equally spaced. The elevation of BM 132 is known to be 167.629 m. A series ofhenchmarlee 1 TABLE 4.2 National ocean survey, U.S. coast, and geodetic surveys: Classification, standards of and general specifications for vertical control accuracy, First Order Second Order Classification Class I, Class II Class! Principal uses Basic framework of Secondary control Minimum the national of the national standards; network and of network and of higher metropolitan metropolitan accuracies area control area control may be used Extensive engineering Large engineering for special projects projects Regional crustal Local crustal movement movement and investigations subsidence Determining investigations geopotential values Support for lower-order control Maximum closures* Section: forward 3 mm VK(Class 1) 6 mm VK and backward 4 mm VK(Class 11) Loop or line 4 mm VK(Class 1) 6 mm VK 5 mm VK(Class II) *Check between forward and backward runnings where K is the distance in kilometers. Class 11 Control densification, usually adjusted to the national network Local engineering projects Topographic mapping Studies of rapid subsidence Support for local surveys Third Order Miscellaneous local control; may not be adjusted to the National Network Small engineering projects Small-scale topographic mapping Drainage studies and gradient establishment in mountainous areas 8 mm VK 12 mm VK 8 mm VK 12 mm VK TABLE 4.3 Classification standards of accuracy and general specifications for vertical control-Canada Second Order (First-Order Procedures Classification Special Order First Order Recommended) Third Order Fourth Order Allowable +3 mm VK +4 mm VK +8 mm VK + 24 mm VK +120 mm VK discrepancy +0.012 ft Vm + 0.017 ft Vm +0.035 ft Vm +0.10 ft Vm +0.5 ft Vm between forward and backward levelings Instruments: Self-leveling Equivalent to Equivalent to Equivalent to Equivalent to Equivalent to high-speed 10"/2 mm 10"/2 mm 20"/2 mm sensitivity sensitivity compensator level vial level vial level vial below Level vial 10"/2 mm 10"/2 mm 20"/2 mm 40" to 50"/2 mm 40" to 50/2 mm Telescopic 40X 40X magnification Source: Adapted from "Specifications and Recommendations for Control Surveys and Survey Markers" (Surveys and Mapping Branch, Department of Energy, Mines and Resources, Ottawa, Canada 1973). 40X 4.13 LEVEL LOOP ADJUSTMENTS In Section 4.7, we noted that level surveys had to be closed within acceptable tolerances or the survey would have to be repeated. If a level survey were performed to establish new BMs, it would be desirable to propor- tion any acceptable error throughout the length of the survey. Because the error tolerances shown in Tables 4.2, 4.3, and 4.4 are based on the distances surveyed, adjustments to the level loop are based on the relevant distances, or on the number of instrument setups, which is a factor directly related to the distance surveyed. Example 4.4 A level circuit is shown in Figure 4.28. The survey, needed for local engineering proj- ects, commenced at BM 20. The elevations of new BMS 201, 202, and 203 were determined. Then the level survey was looped back to BM 20, the point of commence- ment (the survey could have terminated at any established BM). 96 CHAPTER FOUR BM 201 0.8 km 1.6 km BM 20 1.7 km BM 202 0.6 km BM 203 Total Distance Around Loop is 4.7 km. FIGURE 4.28 Level loop. TABLE 4.5 Level loop adjustments Loop Distance, Cumulative (km) BM Correction, Cumulative Distance x E = Total Distance (m) Adjusted Elevation Field Elevation (m) 20 201 202 203 20 0.8 2.4 3.0 4.7 186.273 (fixed) 184.242 182.297 184.227 186.258 +0.8/4.7 x 0.015 = +0.003 = +2.4/4.7 X 0.015 = +0.008 = +3.0/4.7 x 0.015 = +0.010 = +4.774.7 X 0.015 = +0.015 = E = 186.273 - 186.258 = -0.015 m 186.273 184.245 182.305 184.237 186.273 SOLUTION According to Table 4.2, the allowable error for a second-order, class II (loca engineering projects) survey is 0.008 VK; thus, 0.008 V4.7 = 0.017 m is the permissible error. The error in the survey of this example was found to be -0.015 m over a total os tance of 4.7 km-in this case, an acceptable error. It only remains for this acceptab error to be distributed over the length of the survey. The error is proportioned accor ing to the fraction of cumulative distance over total distance, as shown in Table 45 More complex adjustments are normally performed by computer, using the adjustmes method of least squares. 4.14 SUGGESTIONS FOR ROD WORK The following list provides some reminders when performing rod work: 1. The rod should be extended and clamped properly. Ensure that the bottom of the plate does not become dirty, which could result in mistaken readings. If a rod targe being used, ensure that it is positioned properly and that it cannot slip