Study On Theory And Method Of Cloud Base Height Survey Based On Stereo Images

Cloud observation is an important content of daily meteorological tasks, mainly including cloud form, cloud base height, and cloudiness. Traditional methods for measuring the CBH come down to two major ones, i.e., visual measurement and instrument aided measurement. However, the former is a simple quantitative method of great subjectivity due to men's distinct experiences and incapable of roundly and continuously reflecting weather conditions as a result of limited observation frequencies, while the latter lacks experimental conditions for objective evaluations though it is able to measure the CBH. Hence, according to the requirements of daily meteorological observations, this paper adopts the technologies of photogrammetry to measure the CBH,develops a special CBH stereo measuring instrument based on CCD non-metric cameras, and designs a particular control field for the calibration of the measuring instrument. Meanwhile, the images of clouds obtained by each photographing station are mosaiced to form whole sky nephograms. which further enlarges the application ranges of the instrument. The research work and innovative points are as follows.1) It develops a stereo measuring instrument of CBH based on non-metric CCD cameras.The core of this stereo measuring instrument is two integrated high speed global camera separated by a certain distance. Besides, it includes a leveling system, a communication system, video capture cards and other additional devices. This instrument is characterized by its realization of projecting the sky control points to the ground though the control of stepped motor, which cuts down the cost and helps arrive at a probably large-scale popularization.2) It builds a special indoor control field of five directions, i.e., south, west, north, east and zenith, which serves to calibrate the instrument.In order to calibrate the inner orientation elements, the systematic error of rotation angles, and the distortion elements pertaining to the instrument, 773 mark points are set up with 15 centimeters apart, 20 centimeters apart or 40 centimeters apart in the five directions. Meanwhile, the shape, size and structure of the mark points are designed so as to realize the automatic extraction and make it convenient for the instrument calibration.3) It studies the fast calibration method for the stereo measuring instrument of the CBH.In order to fasten the speed of the mark point extraction during the calibration of the speed, an automation extraction technology is researched and realized. It adopts Canny algorithm, eight neighborhood tracking and least square ellipse fitting method. According to the positions of the automatically extracted mark points, direct linear transform (DLT) and space resection of single image are utilized to roughly calibrate the measuring instrument. The image coordinates are readjusted according to the mark point errors until the error is less than 0.2 pixel.4) It accomplishes accurately measuring the CBH on the basis of self-adapting match of different pyramid layer.In order to improve the accuracy of image matching in the process of calculating the CBH, an algorithm named Retinex is detailedly researched and realized, which improves the effect of the image matching. Then, an adaptive stratified image matching algorithm is utilized to ascertain the same points. The concrete process includes: 1) diving the cloud image into a three-layer structure; 2) applying SIFT matching to the top layer; 3) applying the Fostner feature matching algorithm to the second layer; 4) utilizing the least square matching method to obtain sub-pixel accuracy to the bottom layer. At the same time, a direct solution formula of relative orientation for single model is derived to insure the stability of the measuring instrument.5) It studies the mosaic method for the whole sky Nephogram.As for the 36 images of whole sky obtained by the instrument, the cloud images are firstly transformed into a spherical space with a radius of focal length according to their azimuth angles and pitching angles. And the forward and inverse formulae of the transformation among the image plane, the spherical plane and the horizontal plane are derived. Secondly, a clip method is researched to moderately clip the horizontally projected cloud image in the radius and tangent direction, which reduces the overlaps between images, decrease the operation load, and increases the speed of image mosaic. Finally, a weighted smoothing method is utilized to mosaic the clipped images to acquire the whole sky nephogram.This paper has 102 figures,25 tables and 154 references.