Collection method of 4D products

Data acquisition of digital elevation model usually includes the following methods:

ground survey

On-site measurement adopts automatic recording distance measuring theodolite (commonly used electronic rapid measuring theodolite or all-elegant theodolite). This theodolite is generally equipped with a microprocessor, which can automatically record and display relevant data, and can also calculate various stations. The recorded data can be input into the computer through serial communication for processing.

Digitization of existing maps

Digitizer is used to digitize information (such as contour lines) on existing maps. At present, commonly used text digitizers include manual tracking digitizer and scanning digitizer.

Airborne sensor

Use GPS, radar and laser altimeter to collect data. As early as 2000, after the space shuttle Endeavour completed its nine-day flight around the earth, it used high-tech technologies such as spaceborne imaging radar and synthetic aperture radar to collect the ground elevation information of the area where human beings can move normally (about 80% of the total area of the table). After processing, it can be made into digital elevation models and three-dimensional topographic maps. The surveying and mapping results obtained in this plan have wide coverage, high precision and unified benchmarks. They are not only widely used in the civil field, but also have important value in military activities such as missile launch, battlefield management and logistics planning, so they are widely concerned by military circles and media in various countries.

Digital photogrammetry method

This is one of the most commonly used and effective methods to obtain DEM data. Manual, semi-automatic or full-automatic measurement is used to obtain data by using the attached automatic recording device (interface) stereo mapping instrument or stereo coordinate instrument, analytical mapping instrument and digital photography system.

Lidar +CCD camera

Lidar, also known as airborne lidar, is an airborne laser detection and ranging system installed on aircraft. It is a comprehensive application system of GPS (Global Positioning System), INS (Inertial Navigation System) and laser ranging. For example, ALTM3 100 system produced by Canada's OPTECH Company and LiteMapper5600 system produced by Germany's IGI Company, ALTM3 100 and LiteMapper5600 airborne laser scanning remote sensing systems also integrate CCD cameras to record the image information of detection points synchronously with laser detection and ranging systems. Therefore, it can directly obtain high-precision digital elevation model (DEM), digital terrain model (DTM) and digital orthophoto map (DOM) of an area. Because this method can directly obtain high-precision orthophoto data, it saves the link of image processing, and its results can be widely used in urban surveying and mapping, planning, forestry, transportation, electric power, disasters and other departments.

Digital raster map (DRG)

Digital raster map is a digital raster map formed by scanning paper or other simulated topographic maps with a scanner, generating one-dimensional array images, quantifying the gray level (or color separation) of each system at the same time, and then binarizing, graphic orientation, geometric correction and other treatments. It needs to go through the following steps:

(1) Graphic scanning: scan with monochrome or color scanner, and the scan resolution shall be no less than 500dpi.

(2) Map orientation: the raster map is converted from the scanner to the rectangular coordinates of the Gaussian projection plane.

(3) Geometric correction: eliminate the geometric distortion caused by the bottom of the picture and scanning. Relevant software can be used to correct the distortion of raster images. The correction should be carried out according to the kilometer grid, and the map sheet can be corrected by affine transformation and bilinear transformation.

(4) Color correction: Editing raster images with PHOTOSHOP and other software caused a sensation. Monochrome maps are artificially colored according to elements, and color maps are color corrected. In order to unify colors, several tones should be selected according to the specified RGB ratio. (DOM)

Digital orthophoto map is a digital orthophoto data set generated by using digital elevation model (DEM) to correct and mosaic the projection difference of scanned digital aerial photos or satellite image data collected at high altitude pixel by pixel, and cutting it according to the scale of national basic scale topographic map.

For aerial photos, the full digital photography system is used to restore the photography posture during aerial photography, establish a three-dimensional model, detect, edit and generate DEM in the system, and finally generate a DOM with high accuracy.

For satellite image data, DOM data can be generated by single-chip digital differential correction using existing DEM data. (DLG)

Digital line drawing is the core digital product of basic geographic information system, and its collection methods are many, mainly including the following aspects:

(1) Flat panel measurement: non-digital products are measured and collected by the splint instrument, and the final result is a paper or film diagram. It also needs to be digitized and edited in the office to generate DLG. At present, flat panel survey is no longer the main means of GIS field data collection, and is gradually being replaced by all-field digital survey.

(2) All-field digital measurement: the electronic notebook, portable computer or palmtop computer are connected with the total station, and the measurement results are directly stored in digital form without indoor digital processing.

(3)GPS measurement: A real-time dynamic GPS measurement system is adopted, and two or more GPS receivers work together. As a base station, one receiver is placed at a known point, and other receivers measure space targets, and the collected data are stored in a portable computer or a PDA.

(4) Map Digitization: Map Digitization has two working modes: handwriting digitization and scanning vectorization. Handheld tracking digitization refers to digitizing the map with a digitizer, and scanning vectorization refers to screen tracking vectorization of the scanned digital raster map with special software.

(5) Photogrammetry: Photogrammetry has gone through the stages of analog photogrammetry and analytical photogrammetry. With the development of computer technology and its application, digital image processing, pattern recognition, computer vision and other disciplines, it has entered the stage of digital photogrammetry. For a long time, photogrammetry has occupied a very important position in the production of basic scale maps, especially in the current digital photogrammetry stage. Photogrammetry plays an unparalleled role with its high efficiency and complete data products.