5.2　Mapping Control Survey

5.2.1　The position RMSE of mapping control point with respect to starting mapping control point shall be as specified in Table 5.2.1.The elevation RMSE shall not exceed 1/10 of basic contour interval.

5.2.2　The density of mapping control points and high accuracy control points shall be tailored to the requirements for surveying.See Table 5.2.2 for the minimum number of control points in each map sheet.

Ⅰ　Horizontal Mapping Control

5.2.3　For the purpose of horizontal mapping control，the use of GPS-RTK，polar coordinate，intersection and mapping traversing techniques shall be permitted.Densified mapping control points between control points should be established by traversing，but the connecting traverses should not exceed two.In areas where it is difficult to arrange connecting traverses，open traverses may be allowed.The main technical requirements for horizontal mapping control are as follows:

1　The main technical requirements for mapping control traversing shall be as specified in Table 5.2.3-1.

Each leg of traverse shall be measured uni-directionally with distance measurement instrument for an observation set and readings shall be made twice.Such two readings shall have a discrepancy less than 10mm.The measured leg length shall be corrected for meteorological conditions and instrument constants.

For 1:500 or 1:1000 scale mapping，the length of connecting traverse specified in Table 5.2.3-1 may be increased by 50%；the number of traverse legs should not exceed 15；azimuth misclosure shall not exceed±；and the absolute misclosure shall not exceed 0.5mm on map.Where the traverse length is less than 1/3 of the length specified in Table 5.2.3-1，the absolute misclosure shall not exceed 0.3mm on map.

Open traverses are generally used in areas where it is difficult to arrange connecting or loop traverses.Open traverses shall have a length not more than 1/2 of the connecting traverse length specified in Table 5.2.3-1 and the number of legs should not exceed three.Horizontal angles shall be observed using DJ6 theodolite for one observation set in each direction，and shall not have a circumferential angle misclosure larger than 40.Each leg length shall be measured uni-directionally using distance measurement instrument for an observation set.

2　Where polar coordinate technique for electro-optical distance measurement is used，the survey shall be performed at the control points with accuracy ratings or the mapping control points established with one connecting traverse，and shall be connected to two directions with known azimuths.See Table 5.2.3-2 for the main technical requirements.All the mapping control points surveyed by such technique shall not be used for further densification of mapping control points.

3　The maximum permissible measurement length for total station shall be as specified in Table 5.2.3-3.

4　The complementary mapping control points used for analytic computations may be obtained by intersecting check legs or angles at an angle of 30°-150°，with the intersected legs not longer than 0.5m on map.In grouped computations，coordinate discrepancies shall not exceed 0.2mm on map.

5　Where GPS technique is used for mapping control survey，the coordinates and elevations of mapping control points should be directly measured by GPS-RTK technique.For GPS-RTK technique，the survey radius should not exceed 5km，each mapping control point shall be surveyed for twice with respect to the same reference station or different reference stations，and the requirements of 5.2.1 shall apply for discrepancy of horizontal results.

Ⅱ　Vertical Mapping Control

5.2.4　For the purpose of vertical mapping control，use of mapping leveling or electro-optical distance measurement trigonometric leveling may be allowed.The main technical requirements for vertical mapping control are as follows:

1　Mapping leveling routes may be arranged as connecting route，loop route or junction network based on mapping control points，starting and ending with vertical control points of fourth or higher order accuracy.See Table 5.2.4-1 for the main technical requirements.

For open leveling routes，bidirectional observation is required，and the total route length shall not exceed2.5km.For leveling routes sharing a single junction point，the length of each route shall not exceed 3.7km.

2　Electro-optical distance measurement trigonometric leveling shall meet technical requirements specified in Table 5.2.4-2.The length of leveling routes shall not exceed 5km for connecting routes and not exceed 2.5km for open routes.Instrument and target heights shall be measured to the nearest 1mm.The leveling routes shall start and end with vertical control points of fourth or higher order.

5.2.5　Where the density of mapping control points is insufficient for mapping，additional stations can be established by arranging open traverses，or using polar coordinate method or free station method.Such stations shall have a position RMSE with respect to the adjacent control point not greater than 0.1mm on map，and have an elevation RMSE not greater than 1/6 of the basic contour interval.See 5.2.3 for the technical requirements for open traverses and polar coordinate survey method.

Where free station method is used，at least two known points shall be observed，and the horizontal angle and distance shall be each observed for one observation set.The angular discrepancy in half an observation set shall not be greater than 30"，and the reading discrepancy of the measured distance shall not greater than 10mm.The difference between the computed and observed horizontal angles in the same direction shall not exceed 40"for the mapping scale of 1:500，and shall not exceed 20"for 1:1000 and 1:2000.

5.2.6　For the purpose of office computations，angles shall be taken to the nearest 1"；leg length or coordinates shall be taken to the nearest 0.001m；and the solutions of coordinates and elevations shall be taken to the nearest 0.01m.