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Analysis of Topographic Map Mapping Technology Based on Airborne Lidar Data

1  Introduction to Airborne Lidar Technology

1. 1  Airborne lidar system composition

The basic principle of lidar is to use a laser to emit laser pulses to the detected target. After being reflected or scattered by the detected target, the laser pulse returns to the laser, and the detected target is detected by analyzing the returned laser pulse. According to different specific principles, Lidar can be divided into Range Finder Lidar, Doppler Lidar and Differential Absorption Lidar. Ranging lidar can be divided into ground lidar and airborne lidar according to different operating platforms.

The airborne lidar system usually consists of the following four parts: flight platform, laser scanner, positioning and inertial measurement unit, and control unit. Among them, the flight platform can be either a fixed-wing aircraft or a helicopter, and the positioning and inertial measurement unit are determined by inertial measurement.

Inertial Measurement Unit (IMU) and differential GPS (DGPS)

1. 2  The working principle of the airborne lidar system

The distance d between the center of the scanner and the laser spot on the ground, and the center coordinates of the scanner (X0, Y0, Z0) are determined by DGPS, and the instantaneous spatial attitude parameters (φ, ω, κ) of the laser scanner can be determined by using IMU. . According to these geometric parameters and spatial geometric relationships, the three-dimensional coordinates of the ground laser point can be determined

Among them, the working principle of the laser scanner is the same as that of ordinary microwave radar, except that its wavelength is much smaller than that of ordinary microwave radar. There are two ranging methods for laser scanners: one is pulse ranging by measuring the time it takes for the wave emitted by the laser to return from the moment of emission to the reflection of the target, and the other is by measuring the wave emitted by the laser. The phase difference with the received wave reflected by the target determines the distance between the laser and the target

The typical scanning methods of laser scanners are wired scanning, conical scanning and fiber scanning. The scan trace of the line scan mode is parallel or "Z" shape. The scanning trajectory of the conical scanning method is elliptical.

1. 3  Airborne lidar data

  Airborne lidar data usually includes laser point cloud data and echo intensity image data. In addition, in order to facilitate the processing and application of laser point cloud data, most of the current airborne lidar systems are integrated with high-resolution aerial digital cameras. Therefore, aerial digital images can also be regarded as airborne lidar data.

In fact, the airborne lidar system directly obtains data system laser scanning data (recording the time information of each laser pulse from launch to return), differential GPS (DGPS) data, IMU data, and aviation data during the flight data collection process. Digital image data. By using laser scanning data, differential GPS data and IMU data to perform post-processing and positioning solutions, only laser point cloud data and echo intensity image data are obtained.

In practical applications, not all airborne lidar data are used directly. Under normal circumstances, the laser point cloud data can be classified (divided into surface laser point cloud data and non-surface laser point cloud data) to generate DEM (Digital Elevation Model) data. And by using DEM data and the internal and external orientation elements of aerial digital images

2  The topographic map based on airborne lidar data is converted into 32-bit elements (in the airborne lidar aerial flight data collection, the external orientation elements of the aerial digital image can be obtained through DG PS and IMU), and the aerial digital image can be carried out. Ortho-correction to generate DOM (Digital Orthophoto Map) data. Of course, we can also directly use laser point cloud data to generate DSM (Digital Surface Model) or extract information such as houses, roads, and vegetation.

2. 1  Technical Route

As the airborne lidar system integrates laser scanners, differential GPS, inertial measurement unit (IMU), and aerial digital cameras at the same time, this allows the system to obtain laser scanning data (or laser point cloud data) while also Obtain high-resolution aerial digital image data and corresponding positioning and spatial attitude parameters. Therefore, the system can be used as a POS auxiliary aerial photogrammetry system in addition to obtaining airborne lidar data products DEM and DOM. It is based on this that two technical routes for topographic mapping based on airborne lidar technology have been derived.

2. 1. 1  Based on DEM + DOM method

Use high-precision DEM data products to automatically generate contour vector data, and use high-resolution DOM data products for feature collection. Of course, in the process of feature collection, DEM data synchronization support is required, that is, to obtain features in DOM The plane coordinate data of a point is used to obtain the elevation data of the corresponding feature point through DEM data, and the required digital topographic map product is generated by superimposing the contour vector data with the vector data obtained by the feature collection.

2. 1. 2  POS auxiliary aerial photogrammetry method

Use a professional digital photogrammetry system to establish a stereo pair, and collect features in the stereo mode. There are two ways to acquire the photo control points needed to establish a stereo pair:

(1) Measure the control points of the survey area in the field according to the method of regional network point layout, and then carry out the indoor air three encryption.

(2) Based on laser point cloud data and DOM data, directly read the control points of the photo.www.isurestar.net