4.2　Leveling Survey

4.2.1　The main technical requirements for leveling survey shall be as specified in Table 4.2.1.

4.2.2　The series and basic technical parameters of levels shall be as specified in Appendix J.

4.2.3　The instruments and level rods used for leveling surveys shall comply with the following requirements:

1　The included angle between collimation axis and leveling tube axis shall not exceed 10"for DS05 level，not exceed 15"for DS1，and not exceed 20"for DS3.

2　The compensation error of self-leveling levels，Δα，shall not exceed 0.2"for second-order leveling，and not exceed 0.5"for third-order leveling.

3　The difference between average length and nominal length of a meter on rod shall not exceed 0.15mm for Invar rod，not exceed 0.10mm for bar code rod，and not exceed 0.5mm for double-faced wooden rod.

4.2.4　The arrangement and monumentation of benchmarks shall comply with the following requirements:

1　The benchmarks shall be placed on rigid，compact and stable ground，or on stable buildings（or structures）in such a manner that they can be easily spotted，conserved and referenced.When electronic levels are used for surveying，the leveling route shall be clear of the interference from electromagnetic fields.

2　The benchmarks should be monumented，or may be embedded into walls.The sizes and burial of markers，monuments and benchmarks shall be as specified in Appendix K.

3　When the benchmarks are monumented outside the surveying range，the descriptions of stations or descriptions of positions shall be developed.

4.2.5　The leveling observations shall be conducted after the monuments become stabilized.The main technical requirements for leveling observations of various orders shall be as specified in Table 4.2.5.

4.2.6　The leveling survey may be conducted using the observation method and sequence as specified in the current national standards GB/T 12897，Specifications for the First and Second Order Leveling and GB 12898，Specifications for the Third and Forth Order Leveling.

4.2.7　Leveling surveys shall comply with the following requirements:

1　For levels with oblique leveling screws，the leveling zero point of the leveling screw shall be found and marked prior to observation.For self-leveling levels，they shall be leveled prior to observation.

2　Never place the rod pedestals besides ditches or in trenches.

3　At each survey station，the instrument and the foresight and backsight rods shall be arranged linearly as practical as possible，except where the route is deflected.

4　During observation at the same station，the instrument shall not be refocused.When rotating oblique or micrometric screws，ensure that their final rotations are in screw-in direction.

5　The number of stations requiring bidirectional observation in each survey segment shall be even in both direct and reverse orders.Otherwise，the correction of zero point differentials between rods shall be considered.When observation transits from direct order to reverse order，the two rods used shall be transposed，and the instrument shall be reset and leveled.

6　When digital levels are used for observation，the bar code surfaces of rods shall be illuminated evenly，and imaged legibly.

4.2.8　If the discrepancy between the elevation differences obtained from direct and reverse orders exceeds the specified limit，the survey segment in question shall be resurveyed.For second-order leveling，the resurveyed bidirectional observations with permitted discrepancy shall be taken as the final results.For other orders，the resurveyed observations shall be compared with the original observations，and the average of three observations with permitted discrepancies shall be taken.

4.2.9　The intermission should occur at benchmarks.If not so，the surveys should suspend at two fixed points that are solid，reliable，and convenient for placing rods which shall be marked.The surveys shall proceed after intermission.If the checking results do not exceed the limits specified in Table 4.2.5，the subsequent surveys can start with such points.Otherwise，the surveys shall start with the preceding benchmark.

4.2.10　Leveling routes across obstacles shall comply with the following requirements:

1　Obstacles，such as rivers，lakes，ponds，swamps，wide trenches or valleys should be avoided in leveling routes of various orders.Where it is unavoidable to cross obstacles，the segment spanning obstacles can be surveyed uni-directionally using ordinary observation method，but the instrument height shall be reset at the station for twice observations，provided that the sight distance is not more than 200m.The discrepancy between such two elevation differences shall not exceed 7mm，and the elevation differences shall be averaged as the final result.

2　Where sight distance exceeds 200m，the leveling methods including direct reading，micromotion vane，theodolite tilt angle，and trigonometric leveling may be used as appropriate in accordance with GB/T 12897，Specifications for the First and Second Order Leveling and GB 12898，Specifications for the Third and Forth Order Leveling based on the width of obstacle and the availability of instruments and equipment.

4.2.11　Leveling readings and records shall be rounded in accordance with the following principles:

1　Where DS1 level is used in combination with Invar rods，their observations shall be read and recorded to the nearest 0.05mm or 0.1mm；

2　Where DS3 level is used in combination with wooden rods，their observations shall be read and recorded to the nearest 1mm；

3　Where electronic records are used，the provisions in sub-clauses 1 and 2 of 4.2.11 shall apply，and the original data and various tolerances in the electronic documents shall be printed out.

4.2.12　Leveling data shall be processed in accordance with the following requirements:

1　The discrepancy between the elevation differences observed in direct and reverse orders in each survey segment of leveling network shall be calculated，and shall meet the requirements defined in Table 4.2.1.The accidental RMSE in leveling survey per kilometer shall be calculated using the following formula:

Where:

MΔ——accidental RMSE of elevation difference（mm）；

Δ——discrepancy between elevation differences of a survey segment observed in direct and reverse orders（mm）；

L——length of survey segment（km）；

n——number of survey segments.

2　The absolute value of MΔshall not exceed 1/2 of the total RMSE of elevation difference per kilometer as specified in Table 4.2.1.

3　The total RMSE of elevation difference per kilometer shall be calculated using the formula below，and shall meet the requirements defined in Table 4.2.1.

Where:

Mw——total RMSE of elevation difference（mm）；

W——misclosure of connecting or loop route（mm）；

L——length of leveling route used for calculating W（km）；

N——total number of connecting or loop routes.

4　Computer programs shall be used for adjustment calculations.Only for fifth order leveling，simple distribution adjustment is allowed.The software used for adjustment computations shall meet the requirements of 1.0.4.Prior to adjustment，the original data and remarks entered into computer shall be verified.After adjustment，the total RMSE of elevation difference per kilometer shall meet the requirements defined in Table 4.2.1.

5　Where weights are determined based on the number of stations，the total RMSE of elevation difference per kilometer shall be obtained by multiplying the RMSE of each station computed by program by the square root of average number of stations per kilometer.

6　The leveling results shall be rounded to 0.1mm for second order，and 1mm for third，fourth，and fifth orders.