Model application of digital terrain model

Digital terrain model is widely used in engineering construction (such as route selection, alignment and engineering quantity calculation of highway, railway, high-voltage line and other projects), which provides conditions for realizing the automation of design work. In map drawing, orthographic images can be printed according to digital terrain model, and topographic maps can be drawn automatically. In addition, in close-range photogrammetry, when the observed objects are some objects with complex surface structures, this principle can also be cited for surveying and mapping.

The original data points needed to establish digital terrain model can come from the stereo model of photogrammetry, the results of ground survey or the existing topographic map. Measuring data points with stereo plotter is a widely used method to record and store data points, which can be carried out regularly or selected at will (such as selecting geomorphic feature points). One of the most important methods to obtain data points is to obtain the data needed by digital terrain model while scanning the cross section of orthophoto (see orthophoto technology). In order to improve the quality, on the basis of topographic points measured in cross-section, additional measured geomorphic characteristic lines or some independent elevation points representing geomorphic features can be added. This can improve the accuracy of interpolation. Another method is to record the contour lines represented by numbers on the stereo plotter, and then get the digital terrain model with regular distribution of data points through calculation.

The measured data points, even if they reach a considerable density, are generally not enough to represent the complex ground morphology. Therefore, after having a certain number of data points, it is often necessary to supplement the points needed by the digital terrain model by interpolation. The so-called interpolation is to find the elevation of the fixed point according to the data of the surrounding points and a certain functional relationship. Interpolation methods can be distinguished according to whether the interpolation function used is a global function or a local function. Because there are many data points used in digital terrain model, local function interpolation (block interpolation) is generally used, that is, the reference space is divided into several blocks, and each block uses different functions. Typical local interpolation includes linear interpolation, local polynomial interpolation, bilinear interpolation or spline function, fitting estimation (collocation method), multi-layer quadric surface method and finite element method. There is also a point-by-point interpolation method, that is, a new interpolation function is defined for each fixed point. The use of point-by-point interpolation method is very flexible, with high accuracy and simple calculation. It does not need a computer with large memory capacity, but it takes a long time to calculate. Typical point-by-point interpolation methods include weighted average method and moving fitting method. The choice of interpolation method should consider the structure of data points, the required accuracy, the calculation speed and the requirements for computer memory.

Digital terrain model has practical significance only when it is combined with computer processing. At present, the function of computer can make the original data, intermediate results or final results of digital terrain model be used not only for the original purpose, but also for many other purposes. The data of digital terrain model is stored in the peripheral equipment of the data processing system together with the graphics representing the ground features and other data, and a "geographic database" is established. According to the need, the computer can output topographic maps, orthophoto maps or other different forms of data of various scales.