Circular No. 39/2014/TT-BTNMT on technical procedures for establishing digital elevation models using LiDAR technology

CIRCULAR

Technical regulations for establishing digital elevation model

1. Voluntary drug rehabilitation participants using LiDAR scanning technology

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Pursuant to Decree No. 12/2002/NĐ-CP dated January 22, 2002 of the Government on surveying and mapping activities;

Pursuant to Decree No. 21/2013/NĐ-CP dated March 4, 2013, promulgated by the Government, stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment;

The Minister of Natural Resources and Environment issues this Circular providing technical regulations for updating the geographic base database at a scale of 1:10,000 using satellite images.

The Minister of Natural Resources and Environment issues this Circular to stipulate technical regulations for establishing digital elevation models using LiDAR scanning technology.

PART I

GENERAL PROVISIONS

Article 1. Scope of Regulation

This Circular sets forth technical requirements for establishing digital elevation models using LiDAR scanning technology.

In cases where there is a requirement to combine digital photography during LiDAR scanning, the establishment of digital orthophoto maps shall be carried out in accordance with the technical regulations set forth in this Circular.

Article 2. Applicability

This Circular applies to state management agencies responsible for surveying and mapping; organizations and individuals operating in the field of surveying and mapping within the territory of the Socialist Republic of Vietnam.

Article 3. Explanation of Terms

In this Circular, the following terms are understood as follows:

1. Laser (Light Amplification by Stimulated Emission of Radiation): is a technology/device that amplifies light through stimulated emission.

2. LiDAR (Light Detection And Ranging): is a technology for measuring distance using laser beams.

3. DEM (Digital Elevation Model): is a digital elevation model representing the height of the terrain surface.

4. DSM (Digital Surface Model): is a digital surface model representing the uppermost layer of the Earth's surface visible from above.

Digital orthophoto map: is a general term for digital photographs that have been corrected for topographic relief effects, positioned in the coordinate system of the map to be established, resampled to match the scale of the map to be established, cut and joined according to map sheets, and named according to the corresponding map sheet designations.

5. EGM2008 (Earth Gravitational Model 2008): is the Earth gravity model published by the United States in 2008, commonly referred to as the global geoid model 2008.

6. IMU (Inertial Measurement Unit): is an inertial measurement unit consisting of a cluster of devices for measuring acceleration and rotation angles in space.

7. GNSS (Global Navigation Satellite System): is a common name for global positioning systems using satellites such as GPS (United States), Galileo positioning system (European Union), GLONASS (Russian Federation), BeiDou (China), etc.

8. PDOP (Position Dilution Of Precision): is a precision degradation index indicating a measure of three-dimensional positioning accuracy due to the relative positions of GPS/GNSS satellites relative to a GPS/GNSS receiver.

9. WGS-84 (World Geodetic System 1984): is the World Geodetic System 1984, including data on Earth coordinate reference frames, reference ellipsoid, and geoid surface published by the United States Department of Defense in 1984.

10. GRID: is a special format used to store digital elevation models and digital surface models in grid form, which can be in binary file format or ASCII (American Standard Code for Information Interchange) file format.

11. GeoTIFF: is a special format used to store digital image data along with geographic location information of the image.

12. GNSS Base Station: is a GNSS base station, a GNSS receiver placed at a known accurate coordinate point, used to distribute correction information to mobile GNSS receivers within a certain range in post-processing differential GNSS measurements or real-time differential GNSS measurements.

13. Point cloud: is a point cloud, a collection of points with coordinates and heights determined through laser scanning data processing.

14. Ground points: is the ground point layer, comprising points located on the terrain surface after removing surface cover objects such as houses, architectural structures, vegetation, etc.

15. Non-ground points: is the non-ground point layer, comprising points located on the surfaces of objects covering the ground when viewed from above.

16. Metadata: is metadata, including information describing the characteristics of data such as content, format, quality, conditions, and other features to guide access methods, managing authorities, access addresses, storage locations, and data preservation.

17. LAS format: is a common format for storing and converting laser point cloud data.

18. Intensity: is the intensity of the reflection, defined as the ratio of the strength of reflected light to emitted light, primarily influenced by the reflective properties of reflecting objects.

19. Intensity image: is an intensity image, representing a digital image file storing the intensity of laser reflections received and encoded on a grayscale scale.

20. First return: is the first return signal.

21. Last return: is the last return signal.

Chapter II

TECHNICAL REGULATIONS

Article 4. Mathematical basis for the digital elevation model

1. The digital elevation model is established according to the National Reference System and National Coordinate System VN-2000, projection zone, and central meridian implemented in accordance with Circular No. 973/2001/TT-TCĐC dated June 20, 2001, issued by the Land Administration General Department guiding the application of the national reference system and national coordinate system VN-2000.

2. The height system used in establishing the digital elevation model is the current national height system.

3. The Geoid model used is the local Geoid model with the highest accuracy currently available. In cases where a local Geoid model has not been established in the survey area and the survey area is narrow (approximately 50km x 50km), it is permitted to use the global Geoid model EGM2008. In cases where a local Geoid model has not been established but the survey area is wide or located in mountainous regions, a local Geoid model must be established for that area. The plan for establishing the local Geoid model must be clearly stated in the Technical Design - Budget Estimate.

4. The digital elevation model (DEM) is established using LiDAR technology and is represented in the form of a square grid (GRID) or other formats as required.

Article 5. Procedure for establishing the digital elevation model and digital orthophoto map using LiDAR technology

The procedure for establishing the digital elevation model and digital orthophoto map using LiDAR technology includes the following main steps:

Step 1. Preparation work.

Step 2. LiDAR scanning and digital photography.

Step 3. Data processing.

Step 4. Establishing the digital elevation model and digital orthophoto map.

Step 5. Product inspection and acceptance.

Step 6. Submission of data and products.

Article 6. Preparation Work

1. Develop the LiDAR flight design: carried out in accordance with Article 7 of this Circular.

2. Prepare all resources that will be mobilized for the implementation of the task and project.

3. Calibration, inspection, installation of equipment systems:

a) The equipment system must be flown for calibration according to the manufacturer's regulations: cycle time, survey area scale for calibration flights, number of control points, control point specifications, horizontal accuracy, and height accuracy of control points serving calibration flights;

b) Regular aerial inspection work must be conducted at least once a year;

c) Inspect the ground-based equipment system before installation on the aircraft;

d) Install and inspect the equipment system before the aircraft takes off.

4. Arrange and measure connections for GNSS Base stations and weather observation teams: carried out in accordance with Article 8 of this Circular.

5. Construct calibration fields: carried out in accordance with Article 9 of this Circular.

Article 7. Development of LiDAR Flight Design

1. The construction area of the project to establish the digital elevation model (referred to as the survey area) is divided into zones to ensure convenience for the arrangement of GNSS Base stations and for LiDAR scanning and digital photography operations.

2. The design of the LiDAR flight path and digital photography should cover the entire survey area, ensuring accuracy and cost-effectiveness.

3. Basic information to be determined when designing the flight path includes: defining the boundaries of the zones, flight altitude, flight direction, coverage for LiDAR scanning paths, camera focal length, vertical and horizontal coverage for photography, and settings for LiDAR and digital photography equipment.

4. Specialized software may be used to design the flight path.

5. The design of the flight paths includes:

a) Main flight paths: average overlap between adjacent LiDAR scan paths (horizontal overlap) is 30%. Digital photography ensures an average vertical overlap of 60% and an average horizontal overlap of 30%. The flight range must extend beyond the survey area boundary by at least 1/5 of the flight path width. The flight direction is selected based on the shape of the survey area, terrain conditions, and air traffic conditions to minimize flight time;

b) Cross-flight paths are designed to intersect the main flight paths (preferably perpendicular to the main flight path direction) to detect and minimize systematic errors and gross errors; the maximum distance between cross-flight paths is 50km.

6. Other parameters such as flight altitude, speed, scan angle, scan frequency, scan swath width, and scan point density depend on the type of LiDAR equipment, aircraft type, and the accuracy required for the DEM must be presented in the Technical Design - Budget Estimate.

Article 8. Arrangement and connection of GNSS Base stations and weather monitoring teams

1. Each sector must be arranged with two GNSS Base stations. The distance from the GNSS Base station to the flight scanning boundaries shall not exceed 30 km, prioritizing the placement of the GNSS Base station in the middle of the measurement area. Points selected for GNSS Base stations must have planimetric and height accuracy higher than the required DEM accuracy by 1.4 times.

2. During the LiDAR flight scanning process, GNSS receivers with two frequencies must be placed to continuously receive signals at the GNSS Base stations with a signal reception frequency of one signal per second throughout the flight scanning period. The GNSS receiver must be turned on and receiving signals before the aircraft starts its engine and can only be turned off after the aircraft has stopped, according to the instructions of the person in charge of the flight monitoring team.

3. Weather monitoring teams must be arranged throughout the construction period in the measurement area or in each sector to promptly report reliable information about the weather conditions in that area to the person in charge of the flight monitoring team, ensuring the effectiveness of each flight.

Article 9. Construction of calibration reference sites for planimetric and height adjustment

1. Number of calibration reference sites: depends on the LiDAR flight scanning range, the shape of the sector, and the distinctive topographical features within the sector for constructing calibration reference sites. It is required to build a minimum of two (02) calibration reference sites to serve planimetric and height adjustments and checks.

2. Arrangement of calibration reference sites: calibration reference sites need to be arranged in accordance with the characteristics and topography of the measurement area, not placing them close together but rather in relatively even positions within the measurement area.

3. Selection of survey areas for calibration reference sites: calibration reference sites must be chosen in areas with flat terrain, easily recognizable, and optimal for reflecting only one laser pulse. When surveying to select calibration reference sites, priority should be given to choosing areas such as vacant land, sports fields, squares, and major roads.

4. Surveying calibration reference sites:

a) A control network must be established at the calibration reference site if it is not possible to directly use planimetric and height control points for detailed measurements;

b) At each calibration reference site, accurate surveying equipment such as electronic total stations, leveling instruments, and GNSS receivers must be used to determine the coordinates and heights of at least fifty ground detail points and at least five distinct land objects with significant contrast differences from surrounding objects like houses, yards, and grasslands.

5. The coordinate and height determination errors of detail points are specified in Table 1.

Table 1

6. In cases where distinct land objects are not surveyed, markers must be constructed according to the following regulations:

a) Markers must be made in areas with relatively flat terrain and without surrounding vegetation cover. The shape of the marker may be circular or cross-shaped, with dimensions depending on the density of laser pulse reflection points, under the condition that the arms of the cross (or diameter of the circle) must not be less than twice the distance between reflection points. The surface material of the marker must ensure good reflection of laser pulses and the color must have high contrast with the surrounding terrain;

b) If the marker is circular, the coordinates and height of the center of the marker must be accurately determined on-site. For a cross-shaped marker, the positions of the eight corner points of the four arms must also be measured. The coordinate and height determination errors of points are specified in Table 1.

Article 10. LiDAR Scanning and Digital Photography

1. The GNSS receiver installed on the aircraft must be a dual-frequency GNSS receiver with parameters similar to those of the GNSS Base station. The flight time selection depends not only on weather conditions and flight permission but also on the GNSS satellite geometry at the time of flight. Scanning flights should only be conducted during periods when the GNSS satellite geometry has a PDOP value less than 4.0 and both the GNSS receiver on the aircraft and the GNSS Base station simultaneously receive signals from at least five strong satellites.

2. When installing equipment on the aircraft, ensure that the scanning angle of the laser transmitter and the shooting angle of the digital camera are not obstructed by the body or floor of the aircraft. After installation, accurately measure the offset distance (dX, dY, dZ) between the centers of the devices: GNSS antenna, digital camera, laser transmitter, and IMU device, with precision up to centimeters.

3. During LiDAR scanning and digital photography flights, continuously monitor the operation of all equipment, the progress of the scanning and photography process, particularly paying attention to the operation of the LiDAR scanner, coverage between flight paths, areas without scan data or with weak signal strength, flooded areas... to decide whether to conduct additional flights directly.

4. Upon completion of primary and check flight routes, perform result checks, data dumping, and backups for processing and calculations. Data formats depend on specific scanning equipment types.

5. At the end of each measurement session, immediately conduct quality checks of LiDAR scanning and digital photography in the scanned area, including flight quality checks, calculation of open area errors, and planning for compensatory flights.

Article 11. Data Processing

1. The LiDAR scanning data processing combined with digital photography includes the following main steps:

Step 1. Calculate flight path trajectory.

Step 2. Create point cloud along flight strips and merge into blocks.

Step 3. Transform point cloud data to the VN-2000 coordinate system and elevation through transformation parameters and Geoid model.

Step 4. Check and adjust the point cloud according to corrected ground survey measurements.

Step 5. Classify and filter points.

2. The flight path trajectory is determined through processing GNSS measurements between the receiver at the GNSS Base station and the receiver on the aircraft using accompanying software to determine the coordinates and elevations of the GNSS antenna, laser transmitter, and camera centers at each GNSS signal reception moment. The root mean square error of point positions in the horizontal plane and average height after processing must be less than 0.1 meters across the entire measurement area.

3. Coordinates and elevations of points within the data cloud in the WGS-84 system are determined through: coordinates and elevations of the laser transmitter center, laser measurement data, and IMU measurement data applying the Kalman filter.

4. The vertical deviation of each point formed by reflected laser pulses in the processed point cloud must be less than or equal to two-thirds of the allowable error of the required DEM model.

5. Use measurements from check flight paths to adjust between primary flight paths and check flight paths.

6. Horizontal and vertical adjustment between point cloud data and ground survey points at calibration sites is performed using specialized software based on the following determined values:

a) Horizontal deviations are determined through comparison of measured points at calibration sites and corresponding points in the point cloud or on images.

b) Vertical deviations are determined through comparison of measured points at calibration sites and corresponding points in the point cloud.

7. Automatically classify point cloud data into ground points and non-ground points using specialized software based on the combination of intensity return images and digital photographs to serve the establishment of DEM and DSM models.

Article 12. Establishment of Digital Elevation Model and Digital Image Map

1. The process of establishing the Digital Elevation Model and Digital Image Map includes the following main steps:

Step 1. Creating a Digital Surface Model (DSM), Digital Elevation Model (DEM), and intensity image according to map sheets.

Step 2. Calculating external orientation elements of the images.

Step 3. Converting formats and enhancing image quality.

Step 4. Rectifying images and establishing digital image maps.

Step 5. Normalizing the Digital Elevation Model (DEM).

2. The Digital Elevation Model (DEM) is constructed from the last return data. The Digital Surface Model (DSM) is constructed from the first return data.

3. The intensity image is created in the VN-2000 coordinate system based on the intensity data of laser beams and is used for point data classification. The grid cell value of the image is interpolated from the intensity values at discrete LiDAR points. The resolution of the intensity image is determined based on the average distance between laser pulse return points.

4. Determining external orientation parameters for the images is carried out using software accompanying the equipment based on external orientation parameters which are GNSS measurement data, IMU data, and the time of image capture.

5. Converting image formats (to GeoTIFF format) and enhancing image quality from original aerial photographs using specialized software.

6. The digital image map is established based on digital image data converted from original photographs taken during the LiDAR scanning flight and is used as a basis for normalizing the DEM model and serving the creation of topographic maps. The resolution of the digital image map is determined based on the average distance between laser pulse return points.

7. Normalizing the DEM model requires survey results and other reference materials, particularly paying attention to areas with weak LiDAR intensity, and flooded areas at the time of photography. In necessary cases, additional field measurements must be conducted.

8. The accuracy requirements for the DEM model to be established are as follows:

a) The edge error between different DEM sections in different flight scan zones shall not exceed 1.5 times the allowable error of the DEM according to the Technical Design - Estimate.

b) The absolute error of the DEM is evaluated through ground check points. The root mean square height difference between the measured heights and those interpolated from the DEM shall not exceed the accuracy of the DEM according to the Technical Design - Estimate. The limit error shall not exceed twice the root mean square error. Deviations of the check measurements shall not exceed the limit error, and the number of measurements within the range of (70% - 100%) of the limit error shall not exceed 10%.

Article 13. Inspection and Acceptance of Products

Inspection and acceptance work shall be carried out in accordance with Circular No. 02/2007/TT-BTNMT dated February 12, 2007, issued by the Minister of Natural Resources and Environment guiding inspection, review, and acceptance of surveying and mapping works and products.

Article 14. Submission of Data and Products

Submitted data and products include:

1. All original data from the laser scanner, IMU, GNSS, digital photographs, and intermediate calculation results.

2. The Digital Elevation Model (DEM) stored in two binary GRID and ASCII formats along with metadata (Metadata).

3. Point cloud data file in LAS format (LAS format).

4. Digital orthophoto files in GeoTIFF format.

5. The Digital Surface Model (DSM) stored in two binary GRID and ASCII formats along with metadata (Metadata) or in other formats if required.

6. Field measurement results: construction of calibration benchmarks, supplementary measurements (if any), GNSS base station measurements, local Geoid model construction (if any).

7. Technical summary report: detailing information about equipment used, processing software, LiDAR flight scan parameters, adjustment results based on data from flight lines and calibration benchmarks, point classification filtering results, evaluation of processing step errors, technical parameters of the DEM.

Chapter III

IMPLEMENTING PROVISIONS

Article 15. Effective Date

This Circular takes effect from August 18, 2014.

Article 16. Responsibility for Implementation

1. Ministries, ministerial-level agencies, agencies under the Government, People's Committees of provinces and centrally governed cities, and relevant organizations and individuals are responsible for implementing this Circular.

2. During implementation, if there are any difficulties, agencies, organizations, and individuals should promptly report to the Ministry of Natural Resources and Environment for consideration and decision./.