The ability to accurately extract urban digital surface models (DSMs) at a reasonable cost is quite valuable. In this study, the ZiYuan-3 (ZY-3) satellite across-track and along-track modes were used to build stereo pairs and elevation models featuring different views. Image matching processing was used to generate optical point clouds for multiple stereo pairs, which were used to analyze the effects of point cloud fusion under different scenarios and optimize the ZY-3 multiview point cloud integration. Then, with the assistance of a high-precision airborne light detection and ranging point cloud, 12 indices, including the maximum, minimum, mean, and B10 to B90, were selected from the building roof optical point cloud in order to construct an urban building elevation model. The results were as follows: (1) The across-track optical point cloud formed by stereo pairs of ZY-3 nadir images is more suitable for urban DSM extraction due to the high spatial resolution and small base/height ratio of these images. (2) The B70 index performs best in the estimated urban building elevation model [R2 = 0.91, root mean square error (RMSE) = 5.59 m]; this model accurately identified 95.39% and 95.25% of the whole-building elevations in a narrow-scale test of 3170 buildings and a broad-scale test of 1,510,606 buildings, respectively. Moreover, the model R2 and number of accurate building elevation recognitions gradually increased with increasing numbers of fused point clouds, while the RMSE decreased significantly. (3) A combined ZY-3 multiview point cloud fusion and multispectral data analysis technique delivered a more refined urban DSM and better characterization of building elevation under complex underlying surface scenarios than were obtained using traditional stereo pair technology. This research presents the methods and theory relating to the application of ∼2- to 6-m resolution satellite images for complex urban underlying surface scenarios.
Image segmentation is the key step for image processing to image analysis. In order to deal with the
difficulties in automatic selection and the vast computation of multi- threshold image segmentation，a
multi-threshold segmentation algorithm based on Ant Colony is proposed. Compared with traditional multi-threshold segmentation algorithm, the proposed algorithm has better segmentation result and
decreases the computation greatly. The experimental results demonstrate that the algorithm given in this paper can cope with multil-threshold image segmentation efficiently and rapidly.
The urban heat island (UHI) refers to the temperature rise of any man-made area, which can make it distinctively
protrude as a 'warm island' among the 'cool sea' that the lower temperature of their natural landscape represents. This
paper focuses on the monitoring of UHI effect with seasonal change using Advance Spaceborne Thermal Emission and
Reflection Radiometer (ASTER) data which is onboard the satellite Terra. Our study area is in central urban area of
Beijing within fifth ring road. The ASTER data on January, 27, April 9, June 4, August 31, 2004 in daytime were
collected and regarded as winter, spring, summer, and autumn respectively. In order to calculate the urban heat island
intensity, the land surface temperatures were retrieved using an iterative way to compare the temperature difference
between the urban and surrounding rural area. Some pre-processing procedures such as geometric rectification,
radiometric correction, delineating and masking of features, and land cover classification should be performed using
satellite images before calculating the UHI. The UHI was divided into normal, weak, strong, very strong, severe and
extreme based on the calculated UHI intensity. Our result indicates that the UHI effect is weak, strong and extreme in
sprint, summer and autumn respectively, and in winter, the central urban area of Beijing is in an urban heat sink. It means
that the UHI effect is different with the seasonal change.
The paper presents a new method of measuring 3D small angles. In the method, a collimated parallel ray is projected on a double-face-reflector, a CCD detects the position changes of the returned light points. Compared with the traditional autocollimation system, the presented system can distinguish the small change of angle around axis z, therefore, the system realizes the measurement of 3D small angles. By matrix transform, the paper sets up the mathematics model between the position changes of the imaging points on CCD and the changes of spatial small angles. It describes the characteristics of this method and gives its resolving method. By simulations, the paper testifies the correctness of the system model and its resolving method. Furthermore, the paper analyses the effect of the system parameters on the system resolutions. The results can be used as the reference and basis while designing system. When the system parameters apply the values supposed in the paper, the resolutions of alpha and beta can reach 0.025'. The resolution of gamma, less than that of alpha and beta, is about 2'.