Acta Geodaetica et Cartographica Sinica

Continuous indoor visual localization using a perceptual Hash algorithm and spatial constraint

ZHANG Xing, LIN Jing, LI Qingquan, LIU Tao, FANG Zhixiang

2021, 50(12): 1639-1649. doi:10.11947/j.AGCS.2021.20200286

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Visual localization achieves indoor localization by matching visual data (collected by camera) with visual features of an environment. However, visual feature matching requires a long computation time, which makes it difficult to provide a continuous localization result. Besides, for environment with sparse visual data (e.g. images), it is also difficult to achieve continuous indoor localization using only visual feature matching. To solve this problem, this study proposes a continuous indoor localization approach using perceptual Hash algorithm (pHash) and spatial-constrained image searching strategies. It realizes accurate indoor localization by matching the collected video frames (from smartphone) with the images from a generated indoor image dataset. To improve the efficiency of visual feature matching, a two-level image searching and matching strategy is designed, including a pHash-based global searching strategy and a local strategy considering motion continuity. Based on the two-level strategy, a continuous indoor visual localization algorithm is proposed, which can increase the spatial continuity of localization result by integrating both visual localization and dead reckoning. Besides, this algorithm employs a structure from motion method to improve its heading estimation accuracy. Experimental results show that the localization errors of the image querying, continuous offline localization and online localization of this method are approximately 0.70, 0.86 and 0.93 m, respectively, which achieves sub-meter level localization accuracy. In the online localization condition, its average computation time is about 0.42 s, which can provide continuous visual localization.

Construction of navigable road network based on taxi trajectories

ZHANG Caili, XIANG Longgang, LI Yali, WANG Wenlong

2021, 50(12): 1650-1662. doi:10.11947/j.AGCS.2021.20200470

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Taxi trajectories not only reflect the static geometrical and topological structures of urban road network, but also carry dynamic navigation information such as turning relationship. Considering that the location, topological connection and turning rules of road intersection are the basis and key to the construction of navigable road network, this paper made full use of the spatial distribution and dynamic connection information of taxi trajectories and proposed a navigable road network generation method guided by intersection information, which adopts the research idea of “intersection location—road segment geometry—navigation attribute”. Firstly, an integrated intersection recognition technology considering multi-mode features was designed, and then the random forest algorithm was introduced to explore a zero-label supervised anti-counterfeiting strategy for eliminating false intersections; on this basis, a multi-stage road segment generation method based on Delaunay triangulation network was proposed; finally, the turning relationship and one/two-way information of road segment, which are closely related to intersections, were further captured so as to derive a navigable road network. Compared with other methods, the proposed method achieves higher completeness and accuracy for the extraction of navigable road network information such as road intersection, road segment and turning relationship.

A evaluation model of relative influence among landmarks considering distance constraint

TAN Yongbin, LI Xiaolong, CHENG Penggen, CAI Lianbin, TANG Yao

2021, 50(12): 1663-1670. doi:10.11947/j.AGCS.2021.20200363

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Whether the location is accurately described or not directly affects the quality of service of LBS. Relative location description is a common way of location information exchange within the text big data, and its core element lies in the selection of reference landmarks. Accurate and effective evaluation of landmark influence is an important foundation for correct selection of a reference landmark. The models involved in the current research focus on the impact on the inherent characteristics of landmarks. However, non-isolated landmarks are always subject to the dual constraints on their internal characteristics and external landmarks at the same time. Therefore, the influence of distance between landmarks can not be ignored in actual scenes. This paper evaluates the individual influence of landmarks from the aspects of cognition, vision and scene, and calculates the relative influence of each landmark on the point to be located by using the distance between the point and the landmark. A landmark influence evaluation model considering distance constraints is proposed to solve above problems. Finally, take multiple adjacent landmarks in the surrounding area of Bayi square in Nanchang as an example, an on-site questionnaire surveys was performed to verify the effectiveness of the evaluation model. The results show that the model proposed in the paper has a good performance and most recommended landmarks could meet people's psychological expectations than the existing models.

A method of polygon aggregation for complex buildings based on shortest adjacent lines

WANG Yong, LUO An, WANG Huihui, CAO Yuanhui, LIU Jiping

2021, 50(12): 1671-1682. doi:10.11947/j.AGCS.2021.20200493

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The aggregation of buildings is generally based on the disjoint in spatial topological relationships. However, with the diversification of data sources, the spatial topological relationships between polygons inside large buildings also become diversified and even inconsistent, making it difficult to aggregate building polygons, while the multi-scale visualization of buildings in the smart city requires rapid and accurate extraction of the contour shapes of complex buildings. For this reason, this paper proposes an aggregation method of buildings based on the shortest adjacent line between vertexes interpolated. First, recursively search for adjacent polygons based on the aggregation distance to determine the clustering regions, which is convenient for processing the map data in every region. Then, for each clustered region, encrypt vertices for each polygon using a linear interpolation method according to the order of the points on it; after that, search and construct the shortest adjacent line between each vertice and the adjacent polygon through the grid index, and delete the three types of shortest adjacent lines which are abnormal; then, merge the effective shortest adjacent lines in a certain order to form the shortest adjacent line group, and further filter out three kinds of special cases, so as to determine the aggregation area of adjacent polygons; finally, the recursive search is performed clockwise according to the minimum rotation angle, and the contour of polygons including the inner ring and the outer ring is extracted. Experiments show that this method can quickly detect the aggregation area between the adjacent polygons. Experiments in this paper show that this method can quickly detect the adjacent aggregation areas between building polygons and accurately extract the outer contour lines of complex building entities in real-time. The overall contours extracted are close to the actual situation, which provides conditions for building visualization.

Shape similarity measurement based on DNA alignment for buildings with multiple orthogonal features

WEI Zhiwei, GUO Qingsheng, CHENG Lu, LIU Yang, TONG Ying

2021, 50(12): 1683-1693. doi:10.11947/j.AGCS.2021.20200227

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Shape similarity between buildings is useful for building matching, classification, retrieval, etc. As buildings tend to be designed with square angles, we proposed a method to measure shape similarity for buildings with multiple orthogonal features based on Needleman-Wunsch (NW) and Smith-Waterman (SW) algorithms. While encoding building shape into a sequence, adjacent two angles are taken as codes. Type, angle and length characteristics of the codes are described, and their similarity are then defined. NW and SW algorithms are applied to compute similarities between two encoding sequences of buildings. Shape-based query experiments show that proposed method in this paper can effectively measure the shape similarity between buildings. And the experimental results are also in line with human visual perception.

Model and implementation of geographic data transaction certificate and copyright protection based on blockchain and digital watermarking

ZHU Changqing, XU Dingjie, REN Na, CUI Hanchuan, ZHAO Yazhou

2021, 50(12): 1694-1704. doi:10.11947/j.AGCS.2021.20200559

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In order to effectively solve the trust problem of both parties in the process of geographic data transaction, this paper proposes a geographic data transaction certificate and copyright protection model combining zero-watermark, IPFS and smart contract technology. The model uses the IPFS system to build a new mechanism for zero-watermark registration, which can eliminate the dependence on third-party IPR institutions. Through the smart contract design, the zero-watermark information is registered, and the transaction information is stored permanently in the blockchain to realize the time-stamp authentication of zero-watermark registration. The tamper-resistant of blockchain and the automatic execution mechanism of smart contract in the model enable both parties to reach a consensus and achieve mutual trust without the supervision of a third party. Based on this model, this paper uses “Ant OpenChain” as a smart contract development platform to develop a transaction and copyright protection system for geographic data. With this system, both sides of the transaction can call the smart contract to obtain the transaction certificate from the blockchain. Then, the watermark information is extracted by the zero-watermark downloaded from the IPFS network. Finally, the data copyright can be confirmed and the source can be traced effectively for protecting rights or solving disputes by combining the transaction certificate and watermark information to confirm. Experiments show that the proposed method can protect the rights and interests of both parties and protect the security of transaction data simultaneously.

A two-tuple model based spatial direction similarity measurement method

GONG Xi, XIE Zhong, ZHOU Lin, HE Zhanjun

2021, 50(12): 1705-1716. doi:10.11947/j.AGCS.2021.20200361

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Aiming at problems that in direction relation matrix model, recognition ability for distinguishing direction changes in the same cardinal direction is insufficient, the direction distance references for different cardinal directions are defined incompletely, and the distance calculation between arbitrarily direction relation matrices is not accurate enough, this paper proposes a direction relation two-tuple model, which combines grid-based direction relation matrix and centroid-based direction relation matrix to concern both distribution ratio variations and centroid position variations for objects, thus distinguishing the direction difference in the same cardinal direction. Meanwhile, the traditional neighborhood graph is optimized based on human spatial cognition, and a centroid direction distance reference suitable for arbitrarily direction relationships is established. Finally the Earth mover's distance (EMD) is utilized to further improve the accuracy of distance calculation between direction relation two-tuples. Experiments indicate the method is simple and feasible, the measurement results are more consistent with human cognition, and can be better applied to tasks like cartographic generalization results evaluation.

A method of road network selection based on case and ontology reasoning

GUO Xuan, QIAN Haizhong, WANG Xiao, LIU Junnan, ZHONG Ji

2021, 50(12): 1717-1727. doi:10.11947/j.AGCS.2021.20200360

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The selection of road network is a fuzzy decision-making process. Aiming at problems such as noise, conflict and human decision-making in the cartographic generalization algorithm based on general case reasoning, this paper proposes a road network selection method which uses ontology to organize cases and conduct knowledge reasoning. In this paper, we extract expert cases from series scale map and describe them formally according to ternary model. To realize knowledge reasoning, these cases are used to build a domain ontology repository, reorganizing their representation format. Then, ontology rule extension helps to identify and eliminate noise and conflict cases. Additionally, with methods of semantic mapping and graph query, the semantic and geometric features of the road network to be selected are associated with the ontology repository to realize road network selection automatically.

Pseudo-stable constellation bias error of BDS-3 and its high-precision prediction

YANG Yufei, YANG Yuanxi, CHEN Jinping, TANG Chengpan, LI Chong, GUO Hairong, YANG Jianhua, LIU Jinhuo, YANG Bin

2021, 50(12): 1728-1737. doi:10.11947/j.AGCS.2021.20210084

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The BDS-3 satellite is equipped with Ka-band inter-satellite link (ISL) payload to realize ranging and communication between satellites. By taking all the spaceborne atomic clocks of the constellation as a group, the clock error within the group can be measured in nearly real-time by ISL. With some of the more stable clocks as the reference, the constellation pseudo-stable atomic timescale can be established. The deviation between the constellation pseudo-stable clock and the BeiDou time (BDT) can be obtained using only one or a few ground stations. The performance of the spaceborne atomic clock and the accuracy of the clock prediction can be obtained using the pseudo-stable constellation clock errors, which avoids the influence of the satellite orbit error on the clock error estimation and can get more realistic and reliable analysis results. The results show that the spaceborne atomic clocks of the BDS-3, especially the passive hydrogen maser (PHM), have good stabilities. The average Hadamard deviation with an integration time of 1 day is 3.5E-15 for MEO satellite PHM, 2.8E-15 for IGSO satellite PHM and 8.2E-15 for MEO rubidium atomic frequency standards (RAFSs). The RMS of 1 hour prediction error of the constellation pseudo-stabilized clock bias is about 0.1 ns, which can improve the BDS-3 signal in space accuracy to even better than 15 cm. Obviously, the huge potential of ISL needs to be further explored.

Evaluation of multi-GNSS orbit and clock extrapolating error and their influence on real-time PPP during outages of SSR correction

SHU Bao, WANG Li, ZHANG Qin, HUANG Guanwen

2021, 50(12): 1738-1750. doi:10.11947/j.AGCS.2021.20200580

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For real-time precise point positioning (PPP) users, the State Space Representation (SSR) messages from the server are usually transmitted by wireless data link. Due to unstable communication links in practical applications, it is common that the real-time orbit and clock streams are delivered to the users at certain time delays. For multi-GNSS combined positioning, the satellite orbit and clock types for GPS, GLONASS, BDS, and Galileo are different, the ability to maintain orbit and clock accuracy will be of different characteristics when SSR delay occurs. In this condition, it is difficult to maintain the real-time precision positioning accuracy and availability with conventional models. In this study, the orbit and clock extrapolating errors are evaluated based on GPS, GLONASS, BDS-2, BDS-3, and Galileo SSR products from CNES; and the performance of real-time positioning during outages of SSR correction are also analyzed for these four systems. The experiment results show that the accuracy of the orbital clock extrapolation decreases with the increase of the SSR delay time. However, significant differences in the orbital clock extrapolation errors are observed for different GNSS satellites under the same SSR delay. For 10 minutes SSR delay, the GLONASS signal-in-space range errors (SISREs) caused by orbit and clock offset extrapolation can reach 20.9 and 7.3 cm, respectively, while the BDS and Galileo SISREs caused by orbit and clock offset extrapolation can be kept within 2.2 cm, the clock offset has been converted to distance by the light speed. The performance of BDS and Galileo real-time positioning under SSR delays are significantly better than that of GLONASS and GPS. Compared with the traditional model, the four-system combined real-time positioning performance under SSR delay situation is significantly improved when ultra-rapid precise orbits are used and the clock extrapolation error characteristics for different satellites are considered. Using this new model, the positioning accuracy in the horizontal and vertical directions can maintain at 1.7 and 2.1 cm under 10 minutes SSR delay. This research has important reference value for improving the robustness of real-time positioning and optimizing the SSR update rate.

Optimal allocation of risk probability based on ARAIM algorithm

HAN Qingqing, WANG Li, LUO Silong, SHU Bao, YUE Cong

2021, 50(12): 1751-1761. doi:10.11947/j.AGCS.2021.20200260

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The advanced receiver autonomous integrity monitoring (ARAIM) algorithm distributes the probability of continuity risk and integrity risk equally to all visible satellites, resulting in the relatively conservative vertical protection level (VPL). Therefore, a maximum minimization method was adopted, and fminimax function of Matlab was used to reasonably allocate the intact risk and the continuity risk, improving the availability of single constellation ARAIM. The results of the calculation examples of the measured data show that: whether it is static or dynamic data, the maximum minimization method can reasonably allocate the risk. The average improvement rate of VPL for GPS and BDS single systems is 12.14% and 10.63%, respectively, and the average improvement rate of STD (standard deviation) is 78.69% and 72.33%, respectively. Compared with other two optimization algorithms, the optimization algorithm has a better effect of improving the availability of ARAIM under LPV-200. Global simulation results show that after using the maximum minimization method, GPS ARAIM availability is greater than 90% and 99.5% coverage has increased from 26.60% and 0% to 38.69% and 17.35%, respectively. BDS ARAIM availability is greater than 90%. The coverage of 99.5% has increased from 30.73% and 19.91% to 34.28% and 22.33%, respectively.

Analysis and comparison of three unsupervised learning clustering methods for GNSS multipath signals

ZHU Bin, YANG Cheng, LIU Yan

2021, 50(12): 1762-1771. doi:10.11947/j.AGCS.2021.20210233

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In urban environments, the GNSS signals could be easily blocked and reflected by buildings, which leads to low positioning accuracy and discontinuity. This paper employs k-means++, Gaussian mixed clustering (GMM) and fuzzy c-means (FCM) clustering methods to separate the line-of-sight signals (LOS), multi-path and non-line-of-sight signals (NLOS). The pseudorange residuals, signal-to-noise ratio, elevation angle, as well as pseudorange rate consistency are considered in the three methods. The performance comparisons of the three methods are carried out by static testing of GPS/BDS integrated system in known position point. The results show that FCM has the greatest silhouette coefficient and the best clustering performance. The positioning performance is therefore been greatly improve by eliminating the NLOS. The root mean squares error (RMSE) of point positioning indicates that positioning results can be improved by 50% after NLOS been eliminated by k-means++ and FCM. Compared with the supervised clustering methods, the unsupervised clustering is easy to implement without prior information, has lower computation burden and has certain advantages in improving the positioning accuracy.

Precise GNSS phase velocity determination for GRACE Follow-On satellites

LIU Han, WEI Hui, ZOU Xiancai

2021, 50(12): 1772-1779. doi:10.11947/j.AGCS.2021.20200140

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In this study, we composed the velocity and acceleration of GRACE Follow-On satellites directly estimated from the onboard carrier phase observations based on the FIR (finite impulse response) differentiator instead of the orbit differentiation method. Using the level 1B data of GRACE Follow-On from DOY 305 to 314 in 2018, CODE(Center for Orbit Determination in Europe)precise ephemeris and 5 s clock corrections, the results showed that based on the ninth-point differentiator, the 3D root mean square (RMS) of satellite C and D velocity can achieve the accuracy at 0.227 6 mm/s and 0.238 4 mm/s (differentiator interval=60 s); Meanwhile, the 3D RMS values of acceleration can achieve the accuracy at 4.1 μm/s² and 4.5 μm/s² for satellite C and D respectively (differentiator interval=90 s). The GNSS phase direct differentiation method does not require a fixed ambiguity, which weakens the influence of the correlation between orbit epochs, respect to the kinematic orbit difference method and provides high-precision velocity and acceleration information for gravity field model determination.

Research on high-precision seamless positioning model and method based on multi-sensor fusion

LIU Fei

2021, 50(12): 1780-1780. doi:10.11947/j.AGCS.2021.20200396

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