NB/T 11664-2024Technical Specification for 3D Real Scene Modeling of Oblique Photography for Hydropower Projects (English PDF)

Foreword

This document was issued on 24 September 2024 by the National Energy Administration of the PRC and takes effect on 24 March 2025.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.140, Chinese classification P59.

This specification was prepared in accordance with the requirements of the Notice of the General Department of the National Energy Administration on Issuing the 2021 Plan for the Formulation and Revision of Energy Sector Industry Standards and the Plan for Translation of Foreign-Language Versions (Guo Neng Zong Tong Ke Ji [2021] No. 92). The drafting group conducted extensive investigation and research, carefully summarized practical experience, referred to advanced standards at home and abroad and, on the basis of wide solicitation of opinions, formulated this specification.

The main technical contents of this specification are: general provisions; terms and abbreviated terms; basic requirements; schematic design of aerial photography; photo control point layout and survey; data acquisition; aerotriangulation; construction of 3D real scene models; post-processing of 3D real scene models; and quality inspection, assessment and acceptance of results.

The National Energy Administration is in charge of the administration of this specification. China Renewable Energy Engineering Institute proposed it and is responsible for its routine management, and the Engineering Survey Subcommittee of the Energy Industry Standardization Technical Committee for Hydropower Survey and Design (NEA/TC15/SC1) is responsible for the interpretation of its specific technical contents.

Any comments or suggestions arising during the implementation of this specification should be sent to China Renewable Energy Engineering Institute (address: No. A57 and No. B57 Andingmenwai Street, Dongcheng District, Beijing; postcode: 100011).

Chief development organization: PowerChina Kunming Engineering Corporation Limited.

Participating development organization: PowerChina Beijing Engineering Corporation Limited.

Participating development organization: PowerChina Guiyang Engineering Corporation Limited.

Participating development organization: Sichuan Zhongshui Chengkanyuan Cehui Gongcheng Youxian Zeren Gongsi.

Chief drafting staff: Wu Xiaodong, Wang Ying, Luan Youkun, Zhang Wei, Xiong Li, Liu Guangqing, Wang Chong, Ning Yao, Zhu Qinghui, Lu Qinxiu, Wen Daoping, Lu Xiaocang, Liu Xiaobo, He Jie.

Chief drafting staff (continued): Wu Jie, Wu Xianjun, Yang Yanmei, Zhang Gaoming, Feng Gang, Qian Junjin, Xie Fei, Zhao Jidan, Hu Rui, Wen Ping, Zheng Jiang, Yang Linbo, Yang Yongxi, Zhang Jiaze.

Chief reviewers: Zhang Dongsheng, Guo Jiming, Liu Dongqing, Guo Bingxuan, Shi Kun, Xie Zhiqiang, Xie Jinping, Zhou Chuansong, Huang Yanfeng.

Chief reviewers (continued): Hou Zhiqun, Liu Guangchao, Zhang Chengzeng, Zha Hailin, Deng Yong, Wang Hanbin, Guo Decun, Liu Yaowu, Li Shisheng.

Publication information

Standard number: NB/T 11664-2024. Energy Industry Standard of the People's Republic of China, category P.

English title printed on the cover: Technical Specification for 3D Real Scene Modeling of Oblique Photography for Hydropower Projects.

Classification codes printed on the cover: ICS 27.140; CCS P 59.

Issued on 2024-09-24 and implemented from 2025-03-24, as printed on the cover.

Issued by the National Energy Administration.

Chief editing department: China Renewable Energy Engineering Institute.

Approval department: National Energy Administration.

Implementation date stated on the title page: 24 March 2025.

Published by China Water & Power Press, Beijing, 2025.

National Energy Administration Announcement 2024 No. 3

In accordance with the Standardization Law of the People's Republic of China and the Administrative Measures for Energy Standardization, the National Energy Administration has approved 384 energy industry standards, including the Design Code for Rock-Filled Concrete Dams (Attachment 1), 32 foreign-language versions of energy industry standards, including the Operation code for liquefied natural gas receiving terminal (Attachment 2), and 2 amendment notices of energy industry standards, including that of the Code for Monitoring of Rock and Soil Mass of Hydropower Projects (Attachment 3), and hereby publishes them.

Attachment 1: Catalogue of industry standards.

Attachment 2: Catalogue of foreign-language versions of industry standards.

Attachment 3: Amendment notices of industry standards.

The announcement is signed by the National Energy Administration and dated 24 September 2024.

Attachment 1, entry No. 66: standard number NB/T 11664-2024; standard title: Technical Specification for 3D Real Scene Modeling of Oblique Photography for Hydropower Projects; replaced standard: none; adopted standard: none; approval date: 2024-09-24; implementation date: 2025-03-24.

Structure of the specification

Chapter 6, Data Acquisition, continues beyond clause 6.2.4 with the remainder of 6.2 Oblique Photography and with 6.3 Nap-of-the-Object Photography, 6.4 Terrestrial Photography, 6.5 Quality Inspection and 6.6 Data Compilation.

Chapter 7, Aerotriangulation, comprises 7.1 General Requirements, 7.2 Data Preparation and Preprocessing, 7.3 Connection Point Matching, 7.4 Freenet Adjustment, 7.5 Absolute Orientation and Block Adjustment, 7.6 Quality Inspection and 7.7 Data Compilation.

Chapter 8, Construction of 3D Real Scene Models, comprises 8.1 General Requirements, 8.2 Model Reconstruction, 8.3 Model Modification, 8.4 Quality Inspection and 8.5 Data Compilation.

Chapter 9, Post-processing of 3D Real Scene Models, comprises 9.1 General Requirements, 9.2 Model Refinement, 9.3 Construction of Single Entity Models, 9.4 Quality Inspection and 9.5 Data Compilation.

Chapter 10, Quality Inspection, Assessment, and Acceptance of Result, comprises 10.1 General Requirements, 10.2 Quality Inspection, 10.3 Quality Assessment, 10.4 Acceptance of Results and 10.5 Summary and Submission of Results.

Appendix A: Calculation Formulas of Flight Line Coverage.

Appendix B: Ground Surface Marker Arrangement and Description and Results Templates of Photo Control Points.

Appendix C: Flight Record Form of Aerial Photography.

Appendix D: IMU/GNSS-Assisted Aerial Photography Flight Data Inspection Results Analysis Form.

Appendix E: IMU/GNSS Data Processing Checklist.

Appendix F: 3D Real Scene Model Construction Procedure.

Appendix G: 3D Real Scene Single Entity Model Construction Procedure.

Appendix H: Quality Evaluation of Surveying and Mapping Results for 3D Real Scene Modeling.

The specification closes with the Explanation of Wording in This Specification, the List of Quoted Standards and, as an addition, the Explanation of Provisions.

1 Scope

NB/T 11664-2024 is the Chinese technical specification for building three-dimensional real scene models of hydropower projects by oblique photography. A real scene model is a textured 3D reconstruction of the site produced from overlapping aerial images taken at several angles at once, from drones or crewed aircraft: it gives designers a measurable model of a steep valley that conventional topographic mapping describes poorly, and it gives owners a record of the dam, the spoil areas and the reservoir rim that can be compared survey after survey. The specification sets the general provisions, the defined terms and abbreviations, and the basic requirements - the coordinate and height systems, the model accuracy classes by design stage and purpose, the ground resolution required for each class, and the tabulated limits on planimetric and height error. It then covers the flight: the choice of platform and camera, route planning over mountainous terrain, image overlap and oblique angle, weather and illumination conditions, and the placement and survey of ground control and check points. Data processing follows, with aerial triangulation, dense matching, mesh generation, texture mapping, the treatment of water surfaces and vegetation, and the repair of model defects. Quality inspection of the finished model against the check points, data compilation, and the delivery of results with their metadata complete the specification.

1.0.1 This specification is formulated with a view to standardizing the technical requirements and work procedures for 3D real scene modeling by oblique photography for hydropower projects.

1.0.2 This specification is applicable to 3D real scene modeling by oblique photography for hydropower projects.

1.0.3 In addition to this specification, 3D real scene modeling by oblique photography for hydropower projects shall comply with the current relevant standards of the nation.

2 Terms and Abbreviated Terms

2.1.1 oblique photography: A type of photography in which the principal optical axis of the camera deviates from the plumb line or from the horizontal direction and images are taken at a certain tilt angle.

2.1.2 nap-of-the-object photography: A type of photography that takes a specific surface as the photographic object and acquires a series of ultra-high-resolution images by bringing the camera close to the object being photographed.

2.1.3 ground imitation flight: A flight operation mode in which, during photographic flight operations, the aircraft automatically adjusts its flight altitude according to the ground elevation and terrain, so that a constant height difference is kept between the aircraft and the ground.

2.1.4 3D real scene model: A measurable continuous triangulated mesh model with real scene texture information, produced from data sources such as real scene images and point clouds.

2.1.5 3D real scene single entity model: A measurable 3D model of an individual ground feature with real scene texture information, produced from data sources such as real scene images and point clouds.

2.1.6 model tile: The smallest unit in which a continuous triangulated mesh model is clipped and stored according to the grid size.

2.1.7 texture: A mapped image reflecting the surface quality of the modeled object and the distinctive forms and pattern features it presents in terms of colour, light and shade, structure and the like.

DEM: digital elevation model.

DOM: digital orthophoto map.

DSM: digital surface model.

GNSS: global navigation satellite system.

IMU: inertial measurement unit.

POS: position and orientation system.

PPK: post processed kinematic, a GNSS positioning technique using post-processed differential computation of carrier phase.

RTK: real-time kinematic, a real-time kinematic carrier phase differential technique.

3 Basic Requirements

3.0.1 The geodetic coordinate system shall be the China Geodetic Coordinate System 2000. Where another coordinate system is used, a relationship with the China Geodetic Coordinate System 2000 shall be established.

3.0.2 The height datum shall be the 1985 National Height Datum. Where another height datum is used, a relationship with the 1985 National Height Datum shall be established.

3.0.3 In this specification the mean square error shall be used as the criterion for measuring accuracy, and twice the mean square error shall be taken as the limit error.

3.0.4 The data types of the 3D real scene model shall include geometric data, texture data, attribute data and metadata. The data types and data formats of the 3D real scene model should meet the requirements of Table 3.0.4.

Table 3.0.4, geometric data: .OSGB, .OBJ, .FBX, .3DS, .DAE, 3D Tiles, 3MX, etc.

Table 3.0.4, texture data without alpha channel: .JPG, .TIFF, .PNG, etc.

Table 3.0.4, texture data with alpha channel: .DDS, .TGA, .TIFF, .PNG, etc.

Table 3.0.4, animated texture: .AVI, .MPG, etc.

Table 3.0.4, attribute data: .XML, .XLS, .DBF, .TXT, .KML, .SHP, etc.

Table 3.0.4, metadata: .XML, .XLS, .DBF, .TXT, .KML, .SHP, etc.

3.0.5 The 3D real scene model is classified into four grades, namely special grade, grade 1, grade 2 and grade 3, according to the ground resolution of vertical images and the mapping scale. The ground resolution of vertical images and the mapping scale corresponding to each grade of model should comply with Table 3.0.5.

Table 3.0.5 (ground resolution of vertical images and mapping scale for each grade of model), ground resolution: not more than 0.02 m for the special grade, not more than 0.03 m for grade 1, not more than 0.05 m for grade 2 and not more than 0.10 m for grade 3.

Table 3.0.5, mapping scale: 1:200 for the special grade, 1:500 for grade 1, 1:1000 for grade 2 and 1:2000 for grade 3.

3.0.6 The planimetric position accuracy and elevation accuracy of the 3D real scene model shall not exceed the values specified in Table 3.0.6.

Table 3.0.6 (planimetric position accuracy and elevation accuracy of the 3D real scene model, in m), special grade: planimetric position accuracy 0.21 for flat and hilly land and 0.28 for mountainous and high mountainous land; elevation accuracy 0.10 for flat land, 0.14 for hilly land, 0.42 for mountainous land and 0.56 for high mountainous land.

Table 3.0.6, grade 1: planimetric position accuracy 0.30 for flat and hilly land and 0.40 for mountainous and high mountainous land; elevation accuracy 0.14 for flat land, 0.20 for hilly land, 0.60 for mountainous land and 0.80 for high mountainous land.

Table 3.0.6, grade 2: planimetric position accuracy 0.60 for flat and hilly land and 0.80 for mountainous and high mountainous land; elevation accuracy 0.30 for flat land, 0.40 for hilly land, 0.60 for mountainous land and 0.80 for high mountainous land.

Table 3.0.6, grade 3: planimetric position accuracy 1.20 for flat and hilly land and 1.60 for mountainous and high mountainous land; elevation accuracy 0.30 for flat land, 0.40 for hilly land, 1.20 for mountainous land and 1.60 for high mountainous land.

Note to Table 3.0.6: in areas that are difficult to photograph, such as forest areas and areas concealed or covered by shadow, 1.5 times the values specified in the table may be applied.

3.0.7 The planimetric relative accuracy and height relative accuracy of the 3D real scene model should not exceed the values specified in Table 3.0.7.

Table 3.0.7 (planimetric relative accuracy and height relative accuracy of the 3D real scene model, in m), planimetric relative accuracy: 0.18 for the special grade, 0.25 for grade 1, 0.50 for grade 2 and 1.00 for grade 3.

Table 3.0.7, height relative accuracy: 0.18 for the special grade, 0.25 for grade 1, 0.50 for grade 2 and 1.00 for grade 3.

3.0.8 The detail representation of the 3D real scene model should meet the requirements of Table 3.0.8.

Table 3.0.8 (detail representation of the 3D real scene model), reservoir area: fine modeling (star) for the special grade; main-body modeling (circle) for grades 1, 2 and 3.

Table 3.0.8, dam-site river reach and reservoirs of pumped storage power stations: fine modeling (star) for the special grade; main-body modeling (circle) for grades 1, 2 and 3.

Table 3.0.8, dam site, sluice site, canal head and spillway: fine modeling (star) for the special grade and grades 1 and 2; main-body modeling (circle) for grade 3.

Table 3.0.8, inlets and outlets of culverts and culvert pipes, surge shafts and powerhouses: fine modeling (star) for the special grade and grades 1 and 2; main-body modeling (circle) for grade 3.

Table 3.0.8, protection works areas and landslide areas: fine modeling (star) for the special grade and grade 1; main-body modeling (circle) for grades 2 and 3.

Table 3.0.8, natural borrow areas, spoil areas and construction sites: fine modeling (star) for the special grade and grade 1; main-body modeling (circle) for grades 2 and 3.

Table 3.0.8, strip-shaped terrain areas such as embankments, water conveyance lines, power transmission lines and towers, roads, channels and tunnels: fine modeling (star) for the special grade and grade 1; main-body modeling (circle) for grades 2 and 3.

Table 3.0.8, construction land acquisition and resettlement works: fine modeling (star) for the special grade and grade 1; main-body modeling (circle) for grades 2 and 3.

Note 1 to Table 3.0.8: the star indicates fine modeling representation, in which the main structure and detailed structure of the key geographic elements in the model are represented by fine geometric modeling, and the facades use images that accurately reflect the hue, saturation, brightness and detail features of the objects.

Note 2 to Table 3.0.8: the circle indicates main-body modeling representation, in which only the basic outline and external structure of the model are represented by geometric modeling, and the facades use images that reflect the hue, saturation, brightness and similar features of the objects.

3.0.9 The textures of the 3D real scene model shall use current-state photographs, and the degree of texture modification of the 3D real scene model shall meet the requirements of Table 3.0.9.

Table 3.0.9 (degree of texture modification of the 3D real scene model), description of texture modification: modified real texture for the special grade and grade 1; unmodified real texture for grades 2 and 3.

Table 3.0.9, texture modification content, occluding objects: occlusion to be treated for the special grade, grade 1 and grade 2; appropriate treatment for grade 3.

Table 3.0.9, texture modification content, perspective deformation: treatment required for the special grade and grade 1; appropriate treatment for grades 2 and 3.

Table 3.0.9, texture modification content, texture seams: treatment required for the special grade and grade 1; appropriate treatment for grades 2 and 3.

Table 3.0.9, texture modification content, texture glare: treatment required for the special grade and grade 1; appropriate treatment for grades 2 and 3.

Note 1 to Table 3.0.9: real texture refers to texture formed by photographing on site. Modified real texture refers to texture generated after interfering information such as pedestrians and plants on the real texture has been retouched; unmodified real texture refers to texture in which interfering information on the real texture is not retouched.

Note 2 to Table 3.0.9: occluding objects are unreasonable elements that block the texture of the target object itself; perspective deformation refers to perspective distortion of the texture caused by the shooting angle; texture seams are seams produced when a texture is tiled repeatedly; texture glare refers to an unsuitable brightness distribution or extreme brightness contrast in an image that causes visual discomfort and reduces the visibility of objects.

Note 3 to Table 3.0.9: treating occlusion means treating the unreasonable elements that block the texture of the target object itself, so as to ensure the completeness and good appearance of the object. Appropriate treatment means treatment that does not affect the appearance or texture content of the treated object and ensures the completeness of the object.

3.0.10 The instruments and related equipment used in the production of 3D real scene models shall be verified at the prescribed intervals and their routine maintenance shall be strengthened; the software used shall have passed testing or validation.

3.0.11 Field work for the production of 3D real scene models shall fully consider working conditions such as the meteorology, hydrology, topography and landforms and distribution of structures in the survey area, shall ensure that all components of the acquisition system are tightly connected and operate normally and effectively, shall ensure the safety of personnel and equipment, and shall meet environmental protection requirements.

3.0.12 The technical design for the production of 3D real scene models shall comply with the current industry standard Specifications for Technical Design of Surveying and Mapping CH/T 1004, and shall also include the following: 1 the extent of the survey area; 2 the main technical indicators and specifications of the results; 3 the technical indicators and requirements for work processes such as schematic design of aerial photography, photo control point layout and survey, data acquisition, aerotriangulation, construction of 3D real scene models and post-processing of 3D real scene models.

3.0.13 Airspace applications shall be made in accordance with the relevant national regulations on aircraft flight management.

4 Schematic Design of Aerial Photography

4.1.1 Before the schematic design of aerial photography, the following materials should be collected and analysed: 1 meteorological, communication, transport, human geography and natural geography materials and the like; 2 existing control data, map data, image data and the like; 3 existing results such as digital elevation models, 3D real scene models and single entity models.

4.1.2 A site reconnaissance should be carried out before the schematic design of aerial photography, and conditions relevant to aerial photography in the survey area, such as take-off and landing sites, tall buildings and structures and hazard sources, shall be ascertained.

4.1.3 The schematic design of aerial photography shall select flight platforms, aerial cameras and other instruments and equipment that meet the corresponding accuracy requirements according to the task requirements and the environment of the survey area.

4.1.4 On completion of the schematic design of aerial photography a design report shall be prepared, which should include the following: 1 zoning of aerial photography; 2 flight lines and flight altitude; 3 aerial photography design parameters, mainly camera focal length, sensor size, pixel size, ground resolution, overlap, photographing interval and flight line spacing.

4.2.1 The flight plan of aerial photography shall be prepared according to the needs of the oblique photography 3D real scene modeling task and shall include the following main contents: 1 the extent of the photographic area and the characteristics of its ground features and landforms; 2 the grade of the 3D real scene model and the ground resolution of vertical images at the datum plane; 3 the type and technical parameters of the flight platform; 4 the type and parameters of the oblique aerial camera and the parameters of other auxiliary equipment; 5 the acquisition method of aerial photography data, the method of laying flight lines, and the forward and side overlap of images; 6 the climate and time for carrying out the aerial photography task; 7 the names, types and quantities of aerial photography results.

4.2.2 Image overlap shall comply with the following requirements: 1 the forward overlap of vertical images shall not be less than 60 % and the side overlap shall not be less than 30 %; 2 for photographic areas such as steep mountain areas and areas dense with high-rise buildings, the forward overlap should be 70 % to 80 % and the side overlap should be 40 % to 80 %.

4.2.3 Aerial photography shall be carried out in periods when the meteorological conditions over the photographic area are favourable.

4.2.4 When aerial photography is carried out over deserts, gobi, rivers, lakes, seas, salt flats, saline-alkali land, tidal flats and similar areas, photographs should not be taken within 1 h to 2 h around noon; in steep mountain areas and areas dense with tall buildings, aerial photography should be carried out within 2 h before and after noon.

4.3.1 The basic geographic data used for zoning of aerial photography shall comply with the following requirements: 1 the most recently produced topographic maps, image maps, DEM or DSM of the photographic area shall be selected; the scale of topographic maps and image maps should not be smaller than 1:10000, and the scale of DEM and DSM should not be smaller than 1:50000. 2 For photographic areas where the basic geographic data contain unmarked hazardous obstacles such as high-voltage lines, tall buildings and structures and tall vegetation, zoning of aerial photography should use a pre-produced DSM or existing model results as basic data. 3 Zoning for nap-of-the-object photography should use a pre-produced DSM or existing model results as basic data.

4.3.2 The datum plane elevation of a zone shall be determined according to the terrain relief of the zone, flight safety conditions and the like; the average of the elevations of most of the low points and high points within the zone should be taken as the datum plane elevation.

4.3.3 The division of aerial photography zones shall comply with the following requirements: 1 the division shall take into account factors such as model grade, flight safety, flight efficiency and flight direction; 2 when aerial photography is carried out with UAVs without ground imitation flight function, the terrain height difference within a single zone shall not be greater than 1/4 of the relative flight altitude for flat land, hilly land and mountainous land, and shall not be greater than 1/3 of the relative flight altitude for high mountainous land; 3 for UAV flight platforms with ground imitation flight function, the number of zones shall be optimized on the premise that the performance and safety of the flight platform are satisfied; 4 where flying an existing zone is difficult to carry out, the area may be re-zoned on the principle that the ground resolution at the lowest point is not lower than 1.5 times the datum plane resolution, or several small zones within the photographic area whose area exceeding the resolution limit accounts for not more than 10 % may be merged into adjacent larger zones.

4.4.1 The flight line coverage of a photographic area shall comply with the following requirements: 1 the forward coverage and side coverage of the photographic area shall extend beyond the boundary line of the photographic area, and the formulas for calculating flight line coverage shall comply with Appendix A of this specification; 2 for photographic areas of buildings and structures such as powerhouses, camps and dams, the distance by which the forward coverage and side coverage extend beyond the boundary line of the photographic area shall not be less than 1.5 times the relative flight altitude. The requirements for flight line coverage of zones are the same as those for photographic areas.

4.4.2 The laying of flight lines shall comply with the relevant provisions of the current national standard Technical Specification for Oblique Digital Aerial Photography GB/T 39610 and shall also meet the following requirements: 1 in reservoir area photographic areas, the flight line direction shall be parallel to the flow direction of the main stream or tributary, and flight lines should avoid the principal points of vertical images falling on water; 2 in the project hub area, flight lines shall be laid according to the front facades of buildings and the terrain, and the flight line direction should be perpendicular to the front facades of most buildings; 3 in other photographic areas, flight lines may be laid according to the terrain and should be flown in parallel in the east-west or north-south direction; under specific conditions they may be flown along the direction of routes, rivers and the like.

5 Photo Control Point Layout and Survey

5.1.1 The basic control survey of the survey area shall comply with the relevant provisions of the current industry standard Code for Engineering Survey of Hydropower Projects NB/T 35029.

5.1.2 Before the field survey of photo control points, a site reconnaissance shall be carried out to select working routes, characteristic ground features, marker positions and the like.

5.1.3 For 3D models of all grades, ground markers for photo control should be laid before the aerial photography is carried out; for grade 3 3D models, characteristic points of ground features may also be selected after the aerial photography. The layout method of ground markers for photo control points, the point descriptions and the format of the results table should comply with Appendix B of this specification.

5.1.4 Photo control points should be laid by the block network method. The distribution of photo control points shall control the whole survey area and meet the accuracy requirements of the results, and photo control points between adjacent stereo pairs, adjacent flight lines and adjacent zones should be shared.

5.2.1 The selection of photo control points shall comply with the following requirements: 1 photo control points should be selected at positions such as intersections of small linear ground features, obvious corner points of ground features, corner points of ground marker lines, and centres of point-like ground features not larger than 6 pixels by 6 pixels in the original images; the point target shall be clearly imaged and easy to identify, and shall be at a place without obvious terrain relief, permanently fixed and easy to locate and measure; 2 elevation control points shall be selected at positions where the terrain elevation changes little; 3 in areas of the survey area where suitable photo control point targets are difficult to find, ground markers shall be laid in advance, and the layout method of ground markers for photo control points should comply with Section B.1 of this specification; 4 the photo control points laid should be capable of being shared, located within the six-photo or five-photo overlap range in the forward and side directions of the vertical images and close to the centre line of the side overlap of the photos, not less than 150 pixels from the photo edge and not less than 50 pixels from the various marks on the photo; 5 the area controlled by photo control points shall be larger than the modeling extent.

5.2.2 The division of blocks shall meet the following requirements: 1 blocks shall be divided according to the aerial photography zones, flight lines, terrain conditions and the like, and should be rectangular; 2 the size of the blocks and the spacing of photo control points shall be determined according to factors such as the model accuracy, the aerial photography data and the method of handling systematic errors.

5.2.3 The layout of photo control points in blocks shall meet the following requirements: 1 where there are modeling requirements of different grades within the same block, the layout shall meet the modeling accuracy of the highest grade; 2 where photos taken at different photographic resolutions, or at the same photographic resolution with different aerial cameras, are included in the same block, one planimetric-and-height control point and one elevation control point shall be added at the forward junction; 3 one planimetric-and-height point used for checking shall be laid in the central part of the block; 4 in large special difficult areas such as deserts, gobi, swamps, forests, lakes, rivers, tidal flats and islands and reefs, the number of photo control points shall be increased in accessible areas.

5.2.4 The span between photo control points in a block shall be determined in combination with the ground resolution of vertical images. When points are laid in blocks without GNSS-assisted or IMU/GNSS-assisted aerial photography, the span of photo control points should be less than 10000 times the ground resolution of vertical images; when points are laid in blocks with GNSS-assisted or IMU/GNSS-assisted aerial photography, the span should be less than 20000 times the ground resolution of vertical images.

5.2.5 Where the principal points or standard point positions of the photographic area fall on water, or the area lies in bay and island regions, aerial photography gaps or similar areas, photo control points shall be laid in accordance with the relevant provisions of the current industry standard Specifications for Control Survey of Digital Aerophotogrammetry CH/T 3006.

5.2.6 Check points should be laid as planimetric-and-height points and should be evenly distributed.

5.3.1 Photo control points and check points shall be surveyed with the accuracy of mapping control points. For special grade models, the allowable mean square error of point position and the allowable mean square error of elevation of mapping control points relative to adjacent basic control points shall both be not greater than 0.035 m; below the special grade, the provisions of the current industry standard Code for Engineering Survey of Hydropower Projects NB/T 35029 shall apply. In large special difficult areas such as deserts, gobi, swamps, forests, lakes, rivers, tidal flats and islands and reefs, the planimetric position mean square error and elevation mean square error of photo control points may be 1.5 times the corresponding allowable mean square errors.

5.3.2 The identification and pricking of photo control points shall comply with the relevant provisions of the current national standard Specifications for Field Work of Aerophotogrammetry of 1:500 1:1000 1:2000 Topographic Maps GB/T 7931 and shall also meet the following requirements: 1 site photographs of photo control points should be taken, and they shall clearly show the accurate field position of the photo control point and its relative orientation to the surrounding ground features; 2 the position of the photo control point should be marked on the digital image with a corresponding description attached; 3 the point position determined on site shall meet the requirements for pricking and observation of photo control points.

5.3.3 The pricking and annotation of field photo control points shall comply with the relevant provisions of the current national standard Specifications for Field Work of Aerophotogrammetry of 1:500 1:1000 1:2000 Topographic Maps GB/T 7931 and shall also meet the following requirements: 1 the pricking and annotation on control photos shall be uniform in format, clear and neat, the text description shall be concise and accurate, and the point sketch, the description and the pricked hole shall be consistent with one another; 2 when points are pricked on digital images, point description files of photo control points shall be prepared, and their format should comply with Section B.2 of this specification. The point descriptions shall be filled in completely, the text shall be concise and accurate, and the point sketch and the point description shall correspond to each other.

5.3.4 The observation data of photo control points shall be checked, adjusted and coordinate-transformed. The check of the coordinate transformation shall use points that did not take part in the calculation of the coordinate transformation parameters.

5.3.5 The final results data of photo control points shall be compiled and the results table of photo control points shall be prepared; the format of the results table should comply with Section B.3 of this specification.

5.4.1 The quality inspection of photo control point layout and survey shall comply with the following requirements: 1 the layout of points shall be reasonable and shall evenly cover the whole survey area; 2 the selection of points shall be reasonable and the markers shall be clear and unobstructed; 3 the accuracy of the survey results shall comply with the relevant provisions of the current industry standard Code for Engineering Survey of Hydropower Projects NB/T 35029; 4 the annotation of the survey results shall comply with the relevant provisions of the current national standard Specifications for Field Work of Aerophotogrammetry of 1:500 1:1000 1:2000 Topographic Maps GB/T 7931; 5 materials such as results tables and point descriptions shall be complete and their contents shall be complete.

5.4.2 All photo control point data shall be checked in the office, and any problems found shall be remedied promptly by supplementary survey or resurvey.

5.5 The compilation of photo control point survey data shall include the following: 1 results and descriptions of photo control points and check points; 2 raw observation data, observation field books and calculation materials of photo control points and check points; 3 point description files of photo control points; 4 layout diagram of photo control points; 5 instrument calibration certificates.

6 Data Acquisition

6.1.1 Safety checks shall be carried out before acquisition to ensure that the acquisition system operates normally and effectively.

6.1.2 For aerial photography without GNSS or IMU/GNSS assistance, data acquisition shall comply with the relevant provisions of the current national standard Specifications for Digital Aerial Photography — Part 1: Frame Digital Aerial Photography GB/T 27920.1. When GNSS-assisted or IMU/GNSS-assisted photography is used, the airborne IMU/GNSS system of the flight platform shall comply with the relevant provisions of the current national standard Technical Specifications for IMU/GPS Supported Aerial Photography GB/T 27919; for large-area aerial photography, the data recording frequency of the IMU should not be less than 200 Hz.

6.1.3 The setting of base stations during oblique photography operations shall comply with the following requirements: 1 the base station should be located at a relatively high position in the survey area, without surrounding obstacles with an elevation angle exceeding 15 degrees and without sources of satellite signal interference; 2 the instrument height of the base station should not be less than 1.3 m; 3 the minimum sampling interval shall not be greater than 1 s; 4 the observation time of the base station shall be extended by at least 10 min before the start and after the end of the oblique photography operation.

6.1.4 Image data for oblique photography 3D real scene modeling may be supplemented by acquisition methods such as nap-of-the-object photography and terrestrial photography.

6.2.1 The UAV flight platform for oblique photography shall comply with the following requirements: 1 it shall be capable of safe flight under general meteorological conditions, with a wind resistance of not lower than force 5; 2 it shall have a fixed-point exposure or equidistant exposure control function; 3 it shall have a function for recording the position and attitude information at the moment of camera exposure.

6.2.2 The basic performance of the oblique digital aerial camera shall comply with the following requirements: 1 the interior orientation elements and distortion parameters of the camera can be accurately determined; 2 the pixel count of a single camera should not be less than 20 million; 3 it shall have an exposure signal feedback device capable of stably outputting and recording exposure pulse signals; 4 for aerial camera equipment composed of several cameras, the relative positions and attitudes between the cameras shall be kept rigidly fixed; 5 the exposure time difference between the cameras shall not be greater than 120 ms; 6 the ground resolution at the centre point of oblique images should not be lower than the ground resolution at the centre point of vertical images.

6.2.3 The calibration items and calibration methods of the oblique digital aerial camera shall comply with the relevant provisions of the current standard Calibration Code for Digital Aerial Cameras CH/T 8021, and calibration shall be carried out in any of the following cases: 1 the camera has not been calibrated or its calibration validity period has expired; 2 after overhaul, or after main components have been disassembled or replaced; 3 after severe vibration during use or transport; 4 other circumstances that may affect the stability of the oblique digital aerial camera.

6.2.4 Oblique photography data acquisition shall comply with the following requirements: 1 a detailed flight plan and an emergency plan shall be prepared before the aerial photography is carried out; 2 take-off and landing sites and backup sites shall be selected according to the performance of the aircraft; 3 the equipment and accessories used for the sortie shall be carefully checked before flight; 4 aerial photography should be carried out in periods when the solar elevation angle is greater than 40 degrees or the shadow factor is less than 1.2; 5 the wind force during aerial photography shall be lower than the wind resistance rating of the flight platform; 6 flight parameters shall be monitored in real time during operation; 7 at the end of each sortie a flight record form of aerial photography shall be filled in, and the form should comply with Appendix C of this specification.