What are the main methods of 3D terrain modeling based on fractal theory?

Based on the basic surveying and mapping data such as digital line drawing with elevation value, the ground elevation model (DEM) is established to simulate the terrain fluctuation completely and truly. The ground elevation model here can be made into different scales as needed. There are many sources of DOM data, which can be remote sensing images with different resolutions, high-resolution aerial images, and even landform data obtained through low-altitude photography and close-range photography. The accuracy requirement is to select image data with different resolutions according to different needs. According to the unified spatial reference system, the orthophoto is superimposed on the ground elevation model to form a three-dimensional model representing the terrain.

(2) Three-dimensional model of existing buildings (structures)

Existing buildings (structures) are the most important components in the three-dimensional simulation model of the city, including houses, roads, footbridges, bridges, tunnels, dams, parks, green spaces, trees and other important features, as well as urban ancillary facilities such as street lamps, fire hydrants, manhole covers and bus stops. The production of this kind of three-dimensional model often consumes the most manpower and material resources and the longest time period, so when starting this part of the work, we should choose different modeling methods according to the actual needs and different accuracy requirements. For buildings (structures) with low or unimportant requirements, batch modeling can be carried out by using the results of basic surveying and mapping data, especially for buildings (structures) with regular shapes, and even the texture at the top of buildings (structures) can be mapped by using high-resolution images; For buildings (structures) with irregular shapes or needing to be emphasized, interactive manual fine modeling is also adopted. This part of the texture is shot on the spot and processed by professional image processing software, which reflects the aesthetics and realism of the building model.

(3) Three-dimensional model of urban design

The production of this three-dimensional model is mainly to meet the needs of urban planning and design and to assist urban planning and management. Planning designers design in 3D software such as AutoCAD or 3Dmax, and integrate the design results with the current 3D model. At the same time, all or part of the planning model can be modified, adjusted and replaced according to the surrounding environment of the current three-dimensional model. Or regenerate a new model on the three-dimensional simulation system, including designing and defining position, orientation, shape, height, external tone and texture. These modifications and adjustments need to be carried out in proportion in a unified spatial reference system.