5　Topographic Survey

5.1　General Provisions

5.1.1　The basic mapping scales of topographic map shall be as specified in Table 5.1.1.

Table 5.1.1　Basic mapping scales

Note:1　For the purpose of pre-feasibility and feasibility studies，the existing topographic maps of survey area collected may be applied directly，or the survey area can be remapped by the techniques such as digital photogrammetry and satellite remote sensing survey.
2　Topographic mapping may also be performed at the scales as required in different design stages.

5.1.2　Basic contour interval of topographic map shall be determined in accordance with Table 5.1.2.

Table 5.1.2　Basic contour interval of topographic map（m）

Note:1　Identical basic contour interval should be used for the maps of a survey area at the same scale.
2　Elevation notes may be used in lieu of contours where necessary in plain areas and building areas.
3　For topographic maps of water areas，the basic isobath interval can be determined based on the inclination of underwater ground and the mapping scale.

5.1.3　Topographic survey areas can be classified into general area，building area，and water area.

5.1.4　The basic mapping accuracy requirements for topographic surveys are as follows:

1　The position RMSE of the ground feature points with respect to the adjacent mapping control points on maps shall not exceed the limits specified in Table 5.1.4-1.

Table 5.1.4-1　Maximum permissible position RMSE of ground feature points with respect to the adjacent mapping control points on maps

Note:The maximum permissible position RMSE specified above may be increased by 50% in forest and concealed areas and by 100% in areas with exceptional difficulties in surveying.

2　The elevation RMSE of interpolation point between contours or isobaths with respect to the adjacent mapping control points shall not exceed the limits specified in Table 5.1.4-2.

Table 5.1.4-2　Maximum permissible elevation RMSE of interpolation point between contours or isobaths

Note:1　Hd is the basic contour interval of topographic map，m.
2　The maximum permissible elevation RMSE above may be increased by 50% in concealed areas or general areas where it is difficult to perform surveys.
3　For underwater topographic maps，the elevation RMSE specified above shall be increased by 150%；where surveys are difficult to perform，and water depth exceeds 20m，or the required mapping accuracy is not high，an additional increase of 100% is allowed.

3　The accuracy of digital elevation models shall meet the following requirements:

1）The digital elevation models generated from field digital survey data should be irregular grid models，and the elevation RMSE of the points comprising the irregular grid with respect to the adjacent mapping control points shall not be lower than the required accuracy for the maps with corresponding scale.

2）Regular grid digital elevation model can be obtained by interpolating points in the irregular grid digital elevation model.The elevation RMSE of regular grid points shall not be greater than those of interpolation points between contours required for the specific map scale.

4　The position and elevation RMSE of the detail survey points in building area shall be as specified in Table 5.1.4-3.

Table 5.1.4-3　Maximum permissible position and elevation RMSE of detail survey points in building area

Note:The maximum permissible position and elevation RMSE specified above may be increased by 50% in forest and concealed areas and by 100% in areas with exceptional difficulties in surveying.

5　The elevation notes should be made on the obvious ground features or representative terrain points with a density of 5 to 20 notes per 100cm2.The spacing of terrain points shall be as specified in Table 5.1.4-4，and may be reduced on terrain lines and fracture lines appropriately taking into account the terrain changes.

Table 5.1.4-4　Requirements on spacing of terrain points

Note:In flat areas and simple terrain areas，the required spacing may be increased appropriately.

6　The elevation notes of survey points on topographic maps shall be rounded to 0.1m.Where the basic contour interval is 0.5m，the elevation notes shall be rounded to 0.01m.

5.1.5　Topographic maps should be divided into 50cm×50cm or 40cm×50cm sheets，or divided otherwise as required.Division and numbering of topographic maps should comply with the provisions in Appendix L.Alternatively，the points at the southwest corner of each map sheet may be numbered according to the kilometers，with Y coordinate preceded by X coordinate.For survey areas that have once been mapped，the map sheet division and numbering of the existing topography maps may be used.

5.1.6　The cartographic symbols and topographic element classification codes of topography maps shall comply with the current national standards GB/T 20257.1，Cartographic Symbols for National Fundamental Scale Maps-Part 1:Specifications for Cartographic Symbols-1:500，1:1000，and 1:2000 Topographic Maps and GB/T 20257.2，Cartographic Symbols for National Fundamental Scale Maps-Part 2:Specifications for Cartographic Symbols-1:5000 and 1:10000 Topographic Maps.For nuclear power stations，supplementary cartographic symbols and topographic element classification codes not covered in such standards may be developed on an as-needed basis.The same supplementary cartographic symbols and topographic element classification codes shall be used for the same survey area and shall be noted on maps.

5.1.7　Total station or GPS-RTK digital surveying technique should be used for mapping at a scale of 1:500 through 1:2000.Digital aerophotogrammetry technique should be used for mapping at scales of 1:5000 and 1:10000.

5.1.8　The mapping results shall be sorted and checked in accordance with the following requirements:

1　The field data shall be sorted timely after survey and shall be documented in graphical form.

2　Any problem found during field check shall be corrected timely，but in no case shall the original observation data be modified.

3　The data after being checked and corrected shall be saved in a timely manner and backed up.

4　The points selected for checking shall be distributed evenly on topography map，and the obvious ground features shall be selected randomly.The number of horizontal and vertical check points shall be determined according to the actual complexity of ground features，generally 20 to 50 points for each sheet.

5　The coordinates and elevations of each check point shall be surveyed using field discrete point method at the accuracy of the survey station.At least 20 legs between adjacent ground features should be measured in each map sheet with steel tape or distance measurement instruments.Where gross errors are found，the ground features or elevation points in question may be resurveyed as appropriate.Whenever the RMSE in a map sheet exceeds twice the RMSE specified in 5.1.4，the RMSE distribution shall be analyzed，then the adjacent sheets shall be randomly sampled for checking，and those with unacceptable RMSE shall be resurveyed.

5 Topographic Survey

5.1 General Provisions

5　Topographic Survey

5.1　General Provisions