6.3　Straight Line Location Survey

6.3.1　The straight line location survey shall comply with the following basic technical requirements:

1　Available routing survey methods include direct routing with total station or theodolite，indirect routing with total station，routing with the combination of GPS and total station，and RTK GPS routing.

2　When a total station and GPS are alternatively used for routing，two straight-line stakes or turning stakes at the ends of intersection which are determined with total station shall be surveyed with GPS.

3　The straight-line stakes shall be arranged at places where the surveying of stake spacing，elevation differences，plans and sections，crossing-over，location survey，and check survey can be easily performed and the stakes can be durably preserved.The stake spacing should not be larger than 500m.Where constrained by the large-span landform in mountainous areas，the spacing of straight-line stakes shall not be larger than the span.

4　The straight-line stakes and angle stakes shall be numbered in sequence separately.Repeated numbers shall be avoided.The permanent stakes may be buried based on the specific conditions of the project.The specifications of stakes and burying dimensions may follow the provisions in Appendix D of this Code.

5　When GPS survey is used for arrangement of straight-line stakes，the requirements on plan and section survey，cross-over survey as well as check survey shall be satisfied，and clear intervisibility shall be maintained between the straight-line stakes and at least one adjacent straight-line stake.

6.3.2　For straight line location survey，a total station or theodolite with the distance or angle centerparted method may be used for direct routing.The midpoint of theodolite telescope in normal and reversed direction at different locations shall be taken as the front sight point location of distance center-parted method.While for the angle center-parted method，the bisecting point of the two horizontal angles of theodolite telescope in normal and reversed direction shall be taken as the front sight point location.Where the telescope in reversed direction is impossible for an electronic theodolite，one-half observation set shall be carried out on the horizontal angle.

6.3.3　Where the coordinates of turning angle points and positioning stakes surveyed with GPS during the route selection stage are used to inversely calculate the azimuth for routing survey，the following provisions shall be observed:

1　Using the GPS coordinates of three stake points（the angle stake and positioning stake at the beginning end，and the angle stake at the terminal end）to inversely calculate the azimuth of the angle stake at the beginning end to the positioning stake，and taking it as the basic line of sight for routing survey with total station or theodolite.

2　When measuring the value ofΔd（transverse offset distance between the end GPS angle stake and the marked line），the following provisions shall be observed:

1）IfΔd is less than or equal to 0.1m，the horizontal positions of the nearest one or two straight-line stakes can be adjusted to annex the line to the GPS angle stake on the premise that 180°±1 is satisfied.

2）IfΔd is larger than 0.1m，the survey shall be made based on the new turning angle marked through straight line location survey，and the coordinates of the new turning angle can be measured based on the original turning angle and the auxiliary stake.

6.3.4　Station-by-station observation or skip-stop observation may be used for direct routing.When skipstop observation is used，the distance from the farthest point to the observation station should not exceed 800m in flat areas and not exceed 1200m in mountainous areas.The added straight-line stakes should be spaced evenly.

6.3.5　The precision of direct routing survey shall conform to the provisions in Table 6.3.5.

6.3.6　The foresight and backsight targets collimated during direct routing shall be vertical and the lower parts of targets should be collimated.If no obstacle exists within 100m from the collimated target on the ground，a thin and small mark shall be located at the stake pin.If the distance to the collimated target is less than 40m，the point location of stake or the lower part of the slender and straight target shall be collimated.

6.3.7　In case of direct routing survey，the technical requirements for surveying the horizontal angle of a turning angle shall conform to the provisions in Table 6.3.7.

6.3.8　If obstacles exist and the terrain is flat，a rectangle or isosceles triangle may be arranged，which is used for indirect routing with the combination of total station or steel tape distance measurement and theodolite.The technical requirements for measurement of angle and distance shall comply with the provisions in Table 6.3.8-1 and Table 6.3.8-2.

6.3.9　When the traverse method is used for indirect routing survey，the following provisions shall be observed:

1　When the traverse method is used for indirect routing，the photoelectric distance measurement instruments used for distance measurement shall not be lower than Class II，and two observation sets in opposite directions shall be made.The relative RMSE of legs shall not be less than 1/14，000.

2　The horizontal angle of traverses shall be surveyed using an instrument with the precision no lower than DJ2 with each observation set for left and right angle，and the average value shall be taken.The misclosure of circumferential angle shall not be larger than±20"，and the RMSE of angle observation shall not be larger than±10".

3　The coordinate system of traverses should have the straight-line stake at the beginning as its origin，the direction of the beginning side（i.e.，the route line direction）as its X axis direction and the direction of the line that passes through the origin and is perpendicular to the route line as its Y axis direction.

4　The coordinates and azimuth of traverse point shall be calculated on site，with the angle being accurate to seconds and side length and coordinates accurate to millimeters.

5　The two straight-line stake points at the terminal end shall be marked with two steps:setting out and location survey.For the calculation results of location survey after setting out，the transversal offset distance value when regressed onto the line shall be smaller than 5mm.

6　The number of intermediate traverse points should not exceed five，and the length of traverse should not exceed 2km.

7　When the traverse method is used for indirect routing survey，electro-optical distance measurement trigonometric leveling shall be used，and the survey precision shall satisfy the requirements in Table 6.5.6-1 on Class II electro-optical distance measurement trigonometric leveling.

6.3.10　The RTK GPS method may be used for routing survey，during which the actually surveyed coordinates of turning angles（selected in the field）are taken as the basis of direct routing.The coordinates of turning angles shall be checked before direct routing，the checking discrepancy shall not be greater than 7cm and elevation discrepancy shall not exceed 10cm.

6.3.11　When using RTK GPS for surveying straight-line stakes，the surveyors may firstly arrange the straight-line stakes on maps.Each straight-line stake should have good intervisibility in at least one direction and the stake spacing should not be less than 100m（larger spacing is allowed for mountainous areas according to the topography）.

6.3.12　When RTK GPS is used for surveying straight-line stakes，one GPS receiver is taken as the reference station and one or more GPS receivers as mobile stations to carry out surveying activities.Doublefrequency GPS receivers should be used.The horizontal and elevation precision of straight-line stakes shall conform to the provisions in 3.1.5.The work with mobile stations shall comply with the provisions in 6.1.5.