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    25 September 2024, Volume 53 Issue 8
    The Geographical Cognition of Spatio-temporal Big Data
    Six geographic application paradigms of big data
    Lun WU, Yuanqiao HOU, Yu LIU
    2024, 53(8):  1465-1479.  doi:10.11947/j.AGCS.2024.20230199
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    With the advent of the big data era, multi-source big data is on the rise, leading to the integration of data-driven research paradigms with geography. Geospatial big data based on individual behavior offers observations of massive individual behavior patterns, thereby achieving “from people to places” social perception and supporting various applications such as urban management, transportation, and public health. This article delineates six application paradigms focusing on geospatial big data from an application perspective, ranging from describing spatio-temporal distributions at a low level to optimizing spatial decision-making at a high level. The first direction involves a simple characterization of the spatio-temporal features of geographic phenomena and elements, while the second to fourth directions focus on exploring the rules and mechanisms behind spatio-temporal distribution characteristics. The last two directions provide support at the decision-making level. Furthermore, this article highlights issues in data acquisition, analysis methods, and application goals in big data applications.

    Refined accounting and spatio-temporal characteristics of land use carbon budget
    Jia LI, Limin JIAO
    2024, 53(8):  1480-1492.  doi:10.11947/j.AGCS.2024.20230324
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    This research delves into refining the method of carbon budget accounting for land use, which is crucial for advancing low-carbon land utilization and aiding in achieving China's dual carbon goals. Utilizing geospatial big data, the study accounts for the carbon budget of China's land use from 2008 to 2020 across various land use types and examines their spatio-temporal trends. Key findings reveal a consistent annual increase in carbon emissions from land use, with minor changes in carbon sequestration. Construction land represents the predominant carbon source, while agricultural land has reached its carbon peak. Ecological land offsets only about 7% of carbon emissions, and soil carbon stocks are mostly experiencing net losses. The study also highlights significant spatial clustering and lock-in effects, with high carbon emissions from construction land often found in resource-rich or economically developed cities, and major grain-producing areas showing high agricultural carbon emissions. High carbon sequestration areas in ecological lands are located on both sides of the "Hu Line" and in the southeast, with soil carbon stock losses generally higher in the east. The study underscores the immense pressure of achieving carbon peak in land use across various city types, with active and passive peaks, plateau phases, and cities yet to reach their carbon peak constituting 10%, 5%, 31%, and 54%, respectively. The findings advocate for the adoption of geospatial big data-driven refined management of land resources, formulating low-carbon land use transition strategies tailored to different regions and land use types, thereby facilitating the optimization of low-carbon national land space and carbon neutrality.

    Spatio-temporal anomaly detection: connotation transformation and implementation path from data-driven to knowledge-driven modeling
    Yan SHI, Da WANG, Min DENG, Xuexi YANG
    2024, 53(8):  1493-1504.  doi:10.11947/j.AGCS.2024.20230341
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    As one of the critical technologies of geo-spatial data mining, spatio-temporal anomaly detection has the capacity of providing key breakthroughs for deeply revealing the evolution mechanism of geographic processes. Promoted by the big data and artificial intelligence technology, the transformation from data-driven to knowledge-driven modeling is the development tendency for the intelligent detection of spatio-temporal anomalies from geographic big data. This paper systematically sorts out the development process and the mainstream study ideas of current spatio-temporal anomaly detection. Through analyzing the dialectical relationships among data, information and knowledge, a unified description framework of spatio-temporal knowledge is constructed by integrating geographic variables, space basis, spatio-temporal relationships and knowledge types. Then, the connotation of bidirectional driving between spatio-temporal knowledge and spatio-temporal anomalies is elaborated with the help of practical cases. The implementation path for intelligent detection of spatio-temporal anomalies is further proposed, which includes spatio-temporal knowledge correlation modeling, spatio-temporal anomaly intelligent detection and spatio-temporal anomaly-based knowledge dynamic updating, so as to support both the reliable spatio-temporal anomaly detection and the credible spatio-temporal knowledge services.

    Geodesy and Navigation
    Construction of series ultra-high-degree Earth's gravity field models DQM2022 and their precision evaluation
    Yunpeng WANG, Xiaogang LIU, Qi LI, Duan LI, Liu FANG
    2024, 53(8):  1505-1516.  doi:10.11947/j.AGCS.2024.20230530
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    Based on ellipsoidal harmonic analysis, the regional integral correction iteration method and the global integral correction iteration method for construction of ultra-high-degree Earth's gravity field model (EGM) are proposed in this paper. The limitations of the traditional regional integral correction method are solved, and the precision of the improved EGMs applied in China is effectively improved. Taking EGM2008 and EIGEN-6C4 EGMs as the initial models, and series ultra-high-degree EGMs DQM2022 are constructed based on the latest 5′×5′ measured grid mean gravity anomaly data in China and its surrounding areas, and their complete degree and order is 2190. The precision of improved EGMs are evaluated by measured gravity anomaly on the ground, GNSS/leveling and astrogeodetic vertical deflection. The results show that the precision of the improved EGMs are significantly improved when indicating the gravity field in China, compared with the initial models, i.e., the precision of gravity anomaly, elevation anomaly, and vertical deflection are improved by 2.4~2.8 mGal, 1.0~2.4 cm, and 0.07″~0.15″, respectively. When indicating the gravity field in China, the improved EGMs based on the EIGEN-6C4 initial model have the highest precision, the precision of elevation anomaly and vertical deflection are about 10.6 cm, and 2.1″, respectively.

    Application of remove-restore method in iterative algorithm for submarine terrain analysis based on gravity anomaly data
    Bang AN, Yaoyao YU, Huan XU, Jinhai YU, Yuwei TIAN
    2024, 53(8):  1517-1530.  doi:10.11947/j.AGCS.2024.20230558
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    High-precision seafloor topography maps are of great significance in earth science research. Due to the difficulties of measuring high-resolution global coverage of bathymetry directly, a large-scale prediction of seafloor topography relies on gravity data mostly. So, various methods are employed for predicting seafloor topography from gravity data, such as the frequency-domain polarization method, geological-gravity method and iterative analytical method. However, there are some drawbacks in these methods. In this paper, remove-restore method is applied to deal with the issues related to the far-field influences in the iterative analytical method. By combining existing bathymetric models, the simulated experiments are designed to discuss separation methods for the far-field influences and their accuracies are also assessed in inverting seafloor topography. Ultimately, the effectiveness of the remove-restore method is validated in improving seafloor topography by applying to the region of 13.85°N—14.85°N latitude and 117.25°E—118.25°E longitude in the South China Sea. Compared with actual bathymetric data from NGDC, it is concluded that the root mean square error is 90.3 meters and relative error is up to 2.11%. This provides a potential means for improving sparse bathymetric data in ship-measuring areas with high-precision gravity data and the analytical iterative algorithm.

    Ultra-high-order Earth gravity field modeling method based on ico_HEALPix grid
    Zhanpeng ZHANG, Xinxing LI, Changjian LIU, Haopeng FAN, Xianyong PEI
    2024, 53(8):  1531-1539.  doi:10.11947/j.AGCS.2024.20230151
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    Aiming at the problem of data redundancy in the high latitude region in the construction of gravity field model based on traditional geographic grid data subdivision, this paper introduces the hierarchical equal area isolatitude pixelation (HEALPix) grid structure into the calculation of the Earth's gravity field for the first time, and proposes the construction theory of ultra-high-degree Earth's gravity field model using icosahedral HEALPix (ico_HEALPix) grid. The efficient construction of the global 3600-degree spherical harmonic coefficient is realized, which solves the problems of uneven distribution of traditional geographic grid points and redundancy of high-latitude data. At the same time, the normal matrix of the ico_HEALPix grid is not a strict block diagonalization structure in the process of spherical harmonic analysis. An iterative algorithm is designed to effectively improve the accuracy of model construction. Experiments show that the accuracy of the global Earth gravity field model constructed by the iterative method can be better than that of the geographic grid under the premise that the data volume of the ico_HEALPix grid data is less than about 5 million of the geographic grid, and the error order RMS of spherical harmonic coefficient is increased by 1~2 orders of magnitude. It also solves the problems of north and south pole distortion and data redundancy of the geographic grid, and improves the data utilization of the grid.

    Terrain corrections for airborne gravity gradiometry
    Jiaxi HUANG, Shaofeng BIAN, Bing JI
    2024, 53(8):  1540-1551.  doi:10.11947/j.AGCS.2024.20230342
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    Terrain correction is a critical part for airborne gravity gradient data processing, the quality of which is not only depends on elevation resolution and accuracy, but also related to the correction model. Based on the prism integration method, this paper studies the effects of terrain accuracy and resolution, and survey height error on the terrain correction results, then derived an evaluation model. To accelerating calculation of terrain correction without any approximations that may lead to a loss of accuracy, the prism method was parallelized on Nvidia's GPU card based on CUDA interface. Our model and paralleled algorithm were validated in both moderate and rugged terrain. The result shows that a 10 m resolution terrain dataset with accuracy better than 0.5 m can guarantee the terrain correction accuracy better than 1 E when survey altitude is higher than 40 m. Meanwhile, the parallel algorithm achieves speedup of a factor of 15 on consumer GPU and a factor of 150 on professional GPU, which helps to quickly accomplish terrain corrections even in large survey areas. We confirm that our model, a simple analytic formula, presents a clear guideline for both position and terrain requirements in gravity gradient survey, our parallel algorithm proves to be practical and dramatically reduce the calculation cost while retaining the accuracy.

    BTTB-MRNSD method for downward continuation of gravity field based on nonnegative constraints
    Tianyou LIU, Xiaoniu ZENG, Xihai LI
    2024, 53(8):  1552-1563.  doi:10.11947/j.AGCS.2024.20230161
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    As an important technology for processing and interpreting gravity data, downward continuation has attracted the attention of researchers because of its ill-posedness. The space domain continuation method generally has high continuation accuracy, but the computational complexity is usually large. In this paper, firstly, according to the characteristics of gravity anomaly data and the idea of image restoration, an equivalent mathematical model of gravity downward continuation is proposed. Then, based on the block-Toeplitz Toeplitz-block (BTTB) structure of the coefficient matrix, we propose a space-wavenumber mixed domain downward continuation iterative method with nonnegative constraints. The method overcomes the disadvantage of large computational complexity in space domain continuation and has high computational efficiency. The comparison experiments of theoretical gravity model and real anomaly data show that the gravity field downward continuation method proposed in this paper has high downward continuation accuracy and stability, and has good convergence.

    A RSSI ranging algorithm based on GWO-BP neural network
    Yiruo LIN, Kegen YU, Feiyang ZHU, Jinwei BU
    2024, 53(8):  1564-1573.  doi:10.11947/j.AGCS.2024.20220693
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    Recently, the research on received signal strength indication (RSSI) based ranging has received a significant attention, especially in the field of Internet of things and indoor positioning. Precise distance measurement is the basis for high-precision positioning based on ranging algorithms, but the RSSI signal is highly fluctuating due to measurement noise and multi-path effects, which leads to a non-uniform mapping relationship between RSSI and the real physical distance in space. In order to enhance the mapping relationship between RSSI and real physical distance and improve the precision of RSSI ranging, this paper proposes a RSSI ranging algorithm based on GWO-BP neural network, which makes use of back propagation (BP) neural network and gray wolf optimization (GWO) algorithm. GWO algorithm has faster convergence and greater stability than particle swarm optimization (PSO), genetic algorithm (GA), differential evolution (DE), evolutionary programming (EP) and evolution strategy (ES). Furthermore, in this paper, the results of the experiments conducted in two different environments by collecting real data through the developed smartphone software show that: the root mean square error (RMSE) of the path loss model (PLM) based ranging were 2.218, 2.059 m, the RMSE of the traditional BP neural network ranging algorithm were 1.541, 1.551 m, and the RMSE of the GA algorithm-based optimized BP neural network ranging algorithm were 1.269, 1.201 m, respectively, and the RMSE of the GWO-BP neural network ranging algorithm proposed in this paper were 1.054, 0.833 m, respectively. The results indicate that the RSSI ranging algorithm proposed in this paper has higher ranging precision and better robustness.

    Efficiency analysis of polarizing filter enhanced signal to noise ratio for daytime star measurement
    Wanxiang GOU, Chonghui LI, Yinhu ZHAN, Yuan YANG, Yong ZHENG
    2024, 53(8):  1574-1585.  doi:10.11947/j.AGCS.2024.20230416
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    Now, astronomical geodesy can only observe the celestial bodies in the visible band at night, and has not yet achieved all-time measurement. Therefore, studying daytime star measurement technology is very meaningful. In order to enhance the signal-to-noise ratio(SNR) of star measurement in daytime, this paper constructs a polarization filtering SNR enhancement model and analyzes the effectiveness of using polarization filtering method for star measurement SNR enhancement. Firstly, the characterization model of atmospheric polarization state is analyzed, and then a polarization filtering model is put forward based on the Rayleigh scattering model. Then, an enhancement model for improving the SNR is derived, and the SNR enhancement efficiency and influencing factors are analyzed. Finally, two outdoor experimental platforms were built to verify the effectiveness of the model. The experimental results show that in the band of 900~1700 nm, the filtering model constructed based on the Rayleigh scattering model has high accuracy, with 77% of the point polarization angle errors being smaller than 5° and 100% smaller than 12°, which can meet the accuracy requirements for the practical polarization filter model. Through observations of 6 stars, including Arcturus and Kochab, it was shown that the SNR of stars with different observation orientations can be improved by 22%~109%, which is consistent with the SNR enhancement model. The use of polarization filtering method for daytime star measurement in the entire sky has important application value. When the maximum polarization degree of the entire sky is 0.65, the average polarization degree can reach 0.34, and the corresponding star measurement SNR can be improved by about 51.5%.

    Photogrammetry and Remote Sensing
    Weakly supervised building change detection integrating multi-scale feature fusion and spatial refinement for high resolution remote sensing images
    Xin YAN, Li SHEN, Junjie PAN, Yanshuai DAI, Jicheng WANG, Xiaoli ZHENG, Zhi-lin LI
    2024, 53(8):  1586-1597.  doi:10.11947/j.AGCS.2024.20230118
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    To alleviate the heavy dependence of deep learning methods on large-scale high-cost pixel-level annotations, in this paper, we propose a novel weakly supervised method, named MDF-LSR-Net, for high-resolution remote sensing building change detection. Specifically, the proposed method first designs a multi-scale difference feature aggregation module to make better use of multi-scale difference features to generate change heatmaps. Then, by utilizing the local spatial consistency of the low-level fused difference features, MDF-LSR-Net presents a local spatial refinement module to enhance the integrity and accuracy of change regions in heatmaps. Finally, the change detection model is trained based on the high-quality change heatmaps. Experimental results on publicly available datasets, including WHU and LEVIR, demonstrate that our proposed method can obtain more integral and accurate change heatmaps, leading to significantly improved detection performance of the final change detection model. The final model has achieved over 65% points in IOU and over 79% points in F1 on the WHU dataset.

    River SAR image segmentation using L1 norm based hybrid active contours
    Yibo XING, Bin HAN, Bingkun BAO
    2024, 53(8):  1598-1609.  doi:10.11947/j.AGCS.2024.20220690
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    To solve the problem that the existing active contour models are difficult to segment river SAR images accurately, this paper presents a hybrid active contour model based on the L1 norm. First, the median values of the pixel intensities in the inner and outer regions of the contour curve are calculated as the region fitting centers to suppress the influence of the interference regions in SAR images on their accuracies. Second, the L1 norm is used to construct a new energy constraint term and the edge indicator function is introduced into the model's energy functional to further enhance the segmentation performance. Finally, the median and mean energy constraint terms based on the L1 norm are combined and additional region-fitting center constraint terms are added to improve the overall stability of the model. The segmentation experiments on real river SAR images show that the proposed model can segment river SAR images more accurately and stably than the existing models.

    Object-oriented high-resolution image classification using inductive graph neural networks
    Zhiwei XIE, Shuaizhi ZHAI, Fengyuan ZHANG, Min CHEN, Lishuang SUN
    2024, 53(8):  1610-1623.  doi:10.11947/j.AGCS.2024.20230224
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    Traditional object-oriented classification methods mostly use spectral features of image objects and ignore the spatial features among image objects. In this paper, an object-oriented classification method for high-resolution remote sensing images using improved inductive graph neural network is proposed. The method is able to adaptively adjust the fusion coefficient of spectral-spatial composite node similarity and automatically determine the optimal sampling number of neighboring nodes. First, we improved the K-nearest neighbor (KNN) graph construction method. The standard deviation informativeness evaluation method was used to determine the fusion coefficients for constructing the composite node similarity of spectral and spatial features. Then, the optimal sampling number of neighboring nodes was determined using the feedback curve method, and feature representation was accomplished using GraphSAGE node embedding. Finally, the classifications of the nodes were predicted by Softmax function. We used GID-15 and BDCI2017 datasets as experimental data. The proposed graph construction method has improved the classification accuracy. The average Kappa coefficient of the proposed method was better than CART, GCN, GAT, LANet, CCTNet, and SLCNet by 0.31, 0.14, 0.13, 0.12, 0.08, and 0.02. The average overall accuracy, on the other hand, was better than 42.31%, 7.4%, 6.73%, 8.69%, 6.03%, and 1.52%. Meanwhile, our method had good robustness in vegetation and built-up land extraction. The method proposed in this paper provides an effective tool for land cover classification of high-resolution remote sensing images.

    Integrated graph convolution and multi-scale features for the overhead catenary system point cloud semantic segmentation
    Tao XU, Yuanwei YANG, Xianjun GAO, Zhiwei WANG, Yue PAN, Shaohua LI, Lei XU, Yanjun WANG, Bo LIU, Jing YU, Fengmin WU, Haoyu SUN
    2024, 53(8):  1624-1633.  doi:10.11947/j.AGCS.2024.20230198
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    Accurately segmenting the catenary is essential for extracting its components and detecting geometric parameters. In fact, the catenary scene is complex, with significant differences in size between internal components. There are many components with similar and connected semantic information, which makes it difficult for existing deep learning methods to accurately complete catenary point cloud semantic segmentation tasks. To address this issue, this paper proposes a neural network named GDM-Net that leverages graph convolution and multi-level features. GDM-Net includes a graph-based local feature extractor that enhances local feature extraction of the catenary point cloud, a double efficient channel attention module that considers the extraction of global and salient features of the catenary point cloud, and a refinement module of multi-scale feature fusion that improves segmentation accuracy by extracting and fusing multi-scale information of the catenary. The proposed network significantly improves the point cloud segmentation ability of catenary components, particularly at the intersection. Based on qualitative and quantitative analysis of the overhead catenary system dataset, the method is verified to achieve the highest accuracy among five other point cloud deep learning methods. The OA, mIoU, and F1 accuracy indices reach 96.73%, 91.06%, and 95.28%, respectively. Qualitative comparisons demonstrate that the proposed network effectively reduces the misclassification problem of component links and improves the integrity of catenary component segmentation.

    Cartography and Geoinformation
    Autoencoder neural network method for curve data compression
    Pengcheng LIU, Hongran MA, Yang ZHOU, Ziqin SHAO
    2024, 53(8):  1634-1643.  doi:10.11947/j.AGCS.2024.20230580
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    Vector spatial data compression stands as the most direct and effective means for reducing storage space and conserving network transmission bandwidth. This study introduces a compression and decompression model tailored for vector curve data, leveraging autoencoder neural networks. Confronting curves of varying data volume and complexities, the model initiates with segmenting, resampling processes and normalizing input vectors. Through the optimization of autoencoder structure and adjustment of the loss function, a high-precision compression-decoding module is achieved. The stability of the data is ensured through closed coordinate differences of arc segment data. Compression and restoration experiments were carried out on the 1∶1 000 000 county-level arc segments in Shanxi, Hunan, and Jiangxi provinces. The analysis reveals that the accuracy of data restoration decreases with decrease in the compression rate. When the compression rate falls below 35%, the accuracy of data restoration exhibits a fluctuating trend. Therefore, a compression rate of 25% is recommended to ensure the required level of accuracy. Comparison with Fourier series and Bézier curve fitting methods indicates advantages in compression accuracy and processing speed within specific compression rate ranges for the proposed model. Additionally, the model highlights the potential of deep learning in extracting geometric features of spatial elements. In summary, this research presents a model for vector spatial data compression based on autoencoder neural networks, demonstrating promising performance and emphasizing the potential of deep learning in spatial feature extraction.

    Spatio-temporal cross K-function for geographical flows
    Mengjie ZHOU, Mengjie YANG, Huiying CHEN, Yumeng TIAN, Yiliang WAN, Jizhe XIA
    2024, 53(8):  1644-1655.  doi:10.11947/j.AGCS.2024.20230258
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    Geographical flows represent meaningful interactions of geographical objects between pairs of locations. Mining the spatio-temporal association patterns of geographical flows is of great significance for uncovering the spatio-temporal dependency and heterogeneity among flows, as well as understanding the underlying flow mechanisms and spatio-temporal interactions. Currently, there is an increasing number of methods for spatial association analysis of geographical flows. However, there is limited research considering the spatio-temporal coupling characteristics of geographical flows and focusing on the impact of time effects on association pattern detection. Accurately capturing the spatio-temporal dynamics of dependencies between geographical flows remains a challenging issue in the flow association analysis field. To address this gap, this paper extends the spatio-temporal cross K-function of point process to the flow spatio-temporal cross K-function. The method takes the geographical flow as the research object, which can be used to detect spatio-temporal association patterns between any two types of geographical flows. Specifically, the global flow spatio-temporal cross K-function can detect the overall association patterns of geographical flows in the study area, while the local flow spatio-temporal cross K-function can identify the spatio-temporal associations at the local scale that does not follow the global pattern. This work utilizes the flow spatio-temporal cross K-function to analyze the spatio-temporal association patterns between taxi flows and ride-hailing flows on Xiamen Island. The global results show an isolated pattern between taxi flows and ride-hailing flows, suggesting that there is no intense positive competition between the two types of vehicles on Xiamen island. Whereas, the local results indicate competition between the two types of vehicles during the morning, afternoon, and evening peak hours. This competition is mainly concentrated in several specific areas, such as residential areas to industrial parks, residential areas to the airport, train stations to business districts, and business districts to tourist attractions.

    Summary of PhD Thesis
    Complexity-based optimization of cartographic design for multi-scale image-map generation
    Qian PENG
    2024, 53(8):  1656-1656.  doi:10.11947/j.AGCS.2024.20230164
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    Spatio-temporal prediction of urban road congestion based on graph process neural network
    Jianlong WANG
    2024, 53(8):  1657-1657.  doi:10.11947/j.AGCS.2024.20230167
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    Development of a UV-based remote sensing technology for sulphur dioxide monitoring from ship emissions
    Keru LU
    2024, 53(8):  1658-1658.  doi:10.11947/j.AGCS.2024.20230172
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    Research on GNSS tropospheric delay modeling and spatial-temporal characteristics analysis of bias
    Junsheng DING
    2024, 53(8):  1659-1659.  doi:10.11947/j.AGCS.2024.20230177
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    Urban mobility structure detection via spatio-temporal representation learning
    Xiaoqi DUAN
    2024, 53(8):  1660-1660.  doi:10.11947/j.AGCS.2024.20230209
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    Evaluation of landslide hazard risk rating based on bipolar fuzzy set evaluation method——take Ludian county as an example
    Wenfei XI
    2024, 53(8):  1661-1661.  doi:10.11947/j.AGCS.2024.20230215
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    Research on ambiguity resolution of INS-aided high-precision GNSS in urban environment
    Chao CHEN
    2024, 53(8):  1662-1662.  doi:10.11947/j.AGCS.2024.20230223
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