5.3　Surveying and Mapping Techniques

Ⅰ　Total Station

5.3.1　Instruments and computer applications used for total station mapping shall comply with the following requirements:

1　The total station should have an angular measurement accuracy of not lower than DJ6，and a nominal distance measurement accuracy of not lower than class IV.

2　The mapping applications shall meet the basic requirements for office data processing and graphical editing.

3　After the survey data is transmitted，they should be converted into common formats.

5.3.2　The mapping techniques，including coding method，sketch method or the real-time mapping method which integrates office and field work，may be used as appropriate.

5.3.3　For the purpose of setting up instruments and checking survey stations，the following requirements shall apply:

1　The centering error of instrument shall not be greater than 5mm.Instrument and reflector heights shall be measured to the nearest 1mm.

2　Where azimuth is determined based on a remote station，checking computations shall be made for another station.The computed position error of the check station shall not be greater than 0.2mm on map，and the elevation discrepancy not greater than 1/5 of the basic contour interval.

3　The determined azimuth shall be rechecked whenever data acquisition is finished at each station.Where the recheck results exceed the limits specified in the previous sub-clause，the survey results of detail points shall be recomputed，and at least two detail points shall be checked.

5.3.4　The maximum permissible measurement length shall be as specified in Table 5.3.4.

5.3.5　Digital topography mapping shall comply with the following requirements:

1　The observation record of detail points shall include station No.，instrument height，observation point No.，code，target height，sloop distance，vertical angle，horizontal angle，and connection type，etc.

2　Where sketch method is used，the sketches shall be mapped by survey stations and the survey points shall be numbered.The station numbers shall be consistent with the recording numbers of instrument.The sketches should be prepared to briefly indicate the positions，properties and correlations of topographic elements.

3　The independent features that can be mapped in scale shall be surveyed according to their actual shapes，and，in case of independent features that cannot be mapped in scale，their orientation points or routing points shall be precisely surveyed.For directional independent features，their positioning points shall be acquired first，and then orientation points.

4　Linear features，common edges of areal features，and the common boundary between linear feature and areal feature shall be acquired for only once，but the relationship between various attributes shall be handled appropriately.

5　Linear features shall be surveyed to reflect each change in terrain and route.The feature points may be added where necessary.

6　Where detail points are acquired synchronously with control survey，the coordinates of detail points shall be computed using the adjusted coordinates of control points.If the control survey data exceeds the limits，the control points shall be resurveyed，and the coordinates of detail points shall be recomputed.

7　For code mapping method，the general coding formats should be employed，or user coding system shall be established with the customized and expansion functions of software.Field data should be saved as text files in any designated format，and shall be submitted along with format notes.

5.3.6　Where the mapping instruments without automatic recording function are used for mapping，horizontal and vertical angles shall be read and recorded to the least graduation of circles；observation target height shall be measured to the nearest centimeter，distance shall be read to the nearest millimeter，and rounding misclosure as well as index errors of vertical angles shall not exceed 1.

5.3.7　The data acquired shall be inspected and processed.After that，the invalid data shall be deleted or marked，and the unacceptable，incorrect and missing data shall be resurveyed.Any modification shall be saved in original data files and backed up.

Ⅱ　GPS-RTK

5.3.8　Prior to GPS-RTK surveys，the following data shall be collected:

1　Control points survey data and GPS survey data of survey area；and

2　Conversion parameters between WGS-84 coordinate system and the local coordinate system of survey area，and those between the geodetic height datum of WGS-84 system and local height datum.

5.3.9　GPS-RTK conversion shall be in accordance with the following requirements:

1　For datum conversion，conversion parameters can be determined using the common points.

2　Coordinate and elevation conversion parameters should be determined separately based on identical position references.At least four common points shall be selected from both the periphery and the center of survey area.Elevation fitting in accordance with Section 4.4 is permitted for elevation conversion.

3　Alternatively，the conversion parameters may be computed by 2D constrained adjustment of the GPS network of survey area.

4　Where survey area is so large that it has to be subdivided for the purpose of computing conversion parameters，adjoining subdivisions shall have at least two common points.

5　Conversion parameters should be computed separately for multiple point combinations，and then the results shall be optimized.

5.3.10　GPS-RTK conversion parameters shall be used in accordance with the following requirements:

1　GPS-RTK conversion parameters shall be used within the application range of their original computation.The input spatial rectangular coordinates of reference stations shall be identical to the spatial rectangular coordinates used for computing the horizontal and elevation conversion parameters.

2　Prior to use，the conversion parameters shall be analyzed for accuracy and reliability，and shall be checked against some stations in the center and periphery of survey area.The check results shall have a horizontal discrepancy not more than 50mm，and an elevation discrepancy not more than （D is the distance from the reference station to check station，km）.Otherwise，it shall be necessary to analyze the causes and reestablish the conversion relationship.

3　Where considerable variation occurs locally，more restrict checking is required；if the result of such check is unacceptable，the elevation fitting parameters shall be computed more precisely.

5.3.11　The positions of GPS-RTK reference stations shall be determined to meet the following requirements:

1　Reference stations shall be arranged evenly based on the acreage of survey area，surface feature and landform，and communication coverage of data links.

2　Reference stations shall be selected at high places where in the vertical angles above 15° there is neither any obstacle nor any object that may intensely interferes with the receipt of satellite signals or reflects satellite signals.

3　Reference stations shall have an effective operating radius not more than 5km.

5.3.12　Reference stations shall be arranged in accordance with the following:

1　The antennae of receivers shall be accurately leveled and centered with a deviation not more than 5mm.The antenna height shall be measured to the nearest 1mm.

2　The antennae cables，power cables and communication cables shall be connected correctly.A distance of at least 3m should be provided between receiver antenna and radio station antenna.

3　The required data on reference stations shall be input correctly.

4　The frequency of radio station shall not collide with other radio communication frequencies in the same work area.

5.3.13　Surveying at GPS-RTK mobile stations shall comply with the following requirements:

1　Each mobile station should operate with at least five effective satellites，shall have a PDOP（position dilution of precision）less than six，and shall use fixed solutions.

2　Survey mode，reference parameters，conversion parameters，and the communication frequency of data link shall be determined and set as appropriate，and shall be consistent with the reference station.

3　Mobile stations shall be initialized in open areas.

4　Prior to survey，check survey should be done against at least two known points with accuracy not lower than the mapping control accuracy.Both horizontal and vertical discrepancies between the check results and the known results shall not exceed 10cm.In mountainous areas，however，such discrepancies may be doubled.

5　Digital topography mapping shall comply with Section 5.3.5.

6　During surveys，if satellite signals suffer loss of lock，signals shall be reinitialized，and checks shall be made against the common points before proceeding with the surveys.

7　Checks shall be made against the known points whenever survey activities end.

8　Upon completion of observations each day，the survey data shall be saved and backed up on a timely basis.

5.3.14　Where GPS-RTK operates with different reference stations，checks shall be made at the mobile stations against a number of common feature points，and the position discrepancy shall meet the requirements specified in 5.1.4.

5.3.15　The acquired data shall be reviewed.Delete or note the invalid data.Resurvey the unacceptable data，false data and missing data.