Data storage and service method and device based on COG raster data

Abstract

The invention provides a data storage and service method and device based on COG raster data, and belongs to the technical field of geographic information.The data storage and service method based on the COG raster data comprises the steps that remote sensing satellite image data are obtained, whether the remote sensing satellite image data are in a COG format or not is verified, and if yes, the remote sensing satellite image data are stored; when the remote sensing satellite image data is not in the COG format, converting the remote sensing satellite image data into the COG format; uploading the remote sensing satellite image data to object storage, acquiring metadata of the uploaded remote sensing satellite image data, and storing the metadata in an object-relational database; and based on a received data access request of the user, determining target remote sensing satellite image data according to the metadata, and sending the target remote sensing satellite image data to the user. According to the data storage and service method based on the COG raster data, the storage space and the storage cost can be greatly saved by converting the data into the COG format and combining object storage, and on-demand access can be provided to achieve efficient data distribution and map service based on COG rendering.

Description

Technical Field

[0001] The present invention relates to the field of geographic information technology, and in particular to a data storage and service method and device based on COG raster data. Background Art

[0002] Raster data is a data model used to represent geographic spatial information. It is widely used in GIS (Geographic Information System) to store and process various types of geospatial data, such as satellite images, aerial photographs, terrain elevation data, etc. Common raster data formats include TIFF (Tagged Image File Format), Geo TIFF, Net CDF (Network Common Data Form), HDF (Hierarchical Data Format), etc. The data volume of a remote sensing satellite image orthophoto fusion product is GB, and as the accuracy of satellite sensors increases, the size of a single image file is getting larger, which brings greater challenges to the efficient application of data. Summary of the invention

[0003] In view of this, the present invention provides a data storage and service method and device based on COG raster data, which is used to solve the current problems of large amount of remote sensing satellite image data resulting in excessive storage space, high storage cost, and inefficient application.

[0004] In order to solve the above technical problems, in a first aspect, the present invention provides a data storage and service method based on COG grid data, the method comprising: Acquire remote sensing satellite image data, verify whether the remote sensing satellite image data is in COG format, and if the remote sensing satellite image data is not in COG format, convert the remote sensing satellite image data into COG format; Upload remote sensing satellite image data in COG format to object storage, obtain metadata of the uploaded remote sensing satellite image data and store it in an object-relational database; Based on the received data access request of the user and according to the metadata, the target remote sensing satellite image data is determined, and the target remote sensing satellite image data is sent to the user.

[0005] Optionally, after the steps of uploading the remote sensing satellite image data in COG format to the object storage, obtaining metadata of the uploaded remote sensing satellite image data and storing it in the object-relational database, the method further comprises: At least part of the remote sensing satellite image data in the object storage is published as a WMTS service that complies with the OGC standard.

[0006] Optionally, the method further includes: Based on the received WMTS service request of the user, searching whether a corresponding target tile exists in a distributed file storage system storing tiles corresponding to the WMTS service; If the target tile exists, sending the target tile to the user; If the target tile does not exist, the corresponding remote sensing satellite image data is obtained from the object storage and sliced ​​into tiles, and the sliced ​​tiles are sent to the user and stored in the distributed file storage system.

[0007] Optionally, the step of determining the target remote sensing satellite image data based on the metadata based on the received user's data access request includes: Based on the access location corresponding to the data access request, find the target access range in the entire remote sensing satellite image and obtain corresponding row and column information; The number of bytes required to offset the target access range is determined according to the metadata and the row and column information, and then the target remote sensing satellite image data is determined.

[0008] Optionally, the step of acquiring remote sensing satellite image data, verifying whether the remote sensing satellite image data is in COG format, and converting the remote sensing satellite image data into COG format when the remote sensing satellite image data is not in COG format includes: The GDAL raster spatial data conversion library encapsulation verification method is used to verify whether the remote sensing satellite image data is in COG format; In the case where the remote sensing satellite image data is not in the COG format, the remote sensing satellite image data is converted into the COG format using a conversion method provided by the GDAL raster spatial data conversion library.

[0009] Optionally, the step of obtaining metadata of the uploaded remote sensing satellite image data and storing it in an object-relational database includes: Use the Tifffile library to obtain the metadata of the uploaded remote sensing satellite image data, which includes IFD tag data; The metadata is stored in a PostgreSQL object-relational database.

[0010] In a second aspect, the present invention further provides a data storage and service device based on COG grid data, the device comprising: An acquisition module is used to acquire remote sensing satellite image data, verify whether the remote sensing satellite image data is in COG format, and convert the remote sensing satellite image data into COG format if the remote sensing satellite image data is not in COG format; A storage module is used to upload remote sensing satellite image data in COG format to object storage, obtain metadata of the uploaded remote sensing satellite image data and store it in an object-relational database; The data sending module is used to determine the target remote sensing satellite image data based on the received data access request of the user and the metadata, and send the target remote sensing satellite image data to the user.

[0011] Optionally, the device further comprises: The publishing module is used to publish at least part of the remote sensing satellite image data in the object storage as a WMTS service that complies with the OGC standard.

[0012] Optionally, the device further comprises: A search module, configured to search, based on a received WMTS service request from a user, whether a corresponding target tile exists in a distributed file storage system storing tiles corresponding to the WMTS service; A tile sending module, configured to send the target tile to the user if the target tile exists; The tile processing module is used to obtain the corresponding remote sensing satellite image data from the object storage and slice it into tiles if the target tile does not exist, and send the sliced ​​tiles to the user and store them in the distributed file storage system.

[0013] Optionally, the data sending module includes: A search unit, configured to find a target access range in the entire remote sensing satellite image and obtain corresponding row and column information based on an access position corresponding to the data access request; The determination unit is used to determine the number of bytes that the target access range needs to be offset according to the metadata and the row and column information, and then determine the target remote sensing satellite image data.

[0014] Optionally, the acquisition module includes: A verification unit, used for verifying whether the remote sensing satellite image data is in COG format by using a GDAL raster spatial data conversion library encapsulation verification method; The conversion unit is used to convert the remote sensing satellite image data into the COG format by using the conversion method provided by the GDAL raster spatial data conversion library when the remote sensing satellite image data is not in the COG format.

[0015] Optionally, the storage module includes: An acquisition unit, used to use a Tifffile library to acquire metadata of uploaded remote sensing satellite image data, wherein the metadata includes IFD tag data; The storage unit is used to store the metadata in a PostgreSQL object-relational database.

[0016] The beneficial effects of the above technical solution of the present invention are as follows: In an embodiment of the present invention, by uniformly converting remote sensing satellite image data into the COG format, storage space can be greatly saved. Combining object storage can reduce storage costs, provide on-demand access to achieve efficient data distribution and COG-rendered map services, and provide standard OGC services at the same time, so that each map engine can be directly loaded without modification. When accessing the service, the tiles are stored in the distributed file storage system MongoDB. Tile caching is achieved through this mechanism to achieve efficient map services. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a flow chart of a data storage and service method based on COG grid data in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the structure of a data storage and service device based on COG grid data in Embodiment 2 of the present invention. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field belong to the scope of protection of the present invention.

[0019] Raster data is a data model used to represent geographic spatial information. It is widely used in GIS to store and process various types of geospatial data, such as satellite images, aerial photographs, terrain elevation data, etc. Common raster data formats include Geo TIFF, TIFF, Net CDF, HDF, etc. The data volume of a remote sensing satellite image orthophoto fusion product is GB, and as the accuracy of satellite sensors increases, the size of a single image file is getting larger, which brings greater challenges to the efficient application of data.

[0020] Therefore, the industry has proposed a COG (Cloud Optimized Geo TIFF) format specification. COG is an optimized Geo TIFF format designed specifically for cloud computing environments. It aims to improve the efficiency and service performance of cloud-based geospatial data and make data easier to access and process through a series of optimization measures.

[0021] The inventors of this application found in further research that the traditional large-capacity shared file storage is expensive, and the use of object storage can reduce storage costs and improve access efficiency, but it puts forward higher technical requirements on how to combine new data formats with storage management, service management, etc. In other words, how to combine object storage to store and distribute raster data and provide efficient WMTS services (Web Map Tile Service) that comply with the OGC (Open Geospatial Consortium) standard is a technical problem that needs to be solved urgently.

[0022] Therefore, please refer to Figure 1 , Figure 1 A flowchart of a data storage and service method based on COG grid data provided in the first embodiment of the present invention, the method comprising the following steps: Step 11: Acquire remote sensing satellite image data, verify whether the remote sensing satellite image data is in the COG format, and convert the remote sensing satellite image data into the COG format if the remote sensing satellite image data is not in the COG format.

[0023] In this embodiment, the remote sensing satellite image data obtained is raster data. By verifying whether the remote sensing satellite image data is in COG format, when the remote sensing satellite image data is in COG format, there is no need to convert the data. When the remote sensing satellite image data is not in COG format, it is converted to COG format, which can effectively reduce the data size and thus reduce the storage space occupied. In this application, for different formats of the same example, the COG format reduces the space occupied by more than half compared to the Geo TIFF or IMG (ERDAS IMAGINE Image File) format.

[0024] Step 12: Upload the remote sensing satellite image data in COG format to the object storage, obtain the metadata of the uploaded remote sensing satellite image data and store it in the object-relational database.

[0025] In this embodiment, remote sensing satellite image data in COG format is stored in an object storage manner. Files and objects have metadata related to the data they contain, but objects are characterized by extended metadata. Each object is assigned a unique identifier, allowing a server or end user to retrieve the object without knowing the physical address of the data. This storage method is very helpful for automating and simplifying data storage in a cloud computing environment, so that the present application can realize data distribution capabilities based on the sharing function provided by object storage, and support on-demand access (range download) and concurrent download of large objects.

[0026] At the same time, metadata is obtained from the uploaded remote sensing satellite image data and stored in an object-relational database for subsequent downloading, retrieval, etc.

[0027] Step 13: Based on the received data access request from the user and according to the metadata, the target remote sensing satellite image data is determined, and the target remote sensing satellite image data is sent to the user.

[0028] In this step, data distribution, on-demand access (range download), and concurrent download of large objects are realized based on the sharing function provided by the object storage. When receiving the user's data access request, based on the received data access request and the metadata obtained in the previous step, the target remote sensing satellite image data can be determined from the remote sensing satellite image data stored in the object storage, and then sent to the user for rendering and presentation. In this way, the remote sensing satellite image data distribution capability and on-demand access are realized, which greatly improves the efficient application of remote sensing satellite image data. Among them, the data access request contains information such as the number of bytes to be accessed.

[0029] The data storage and service method based on COG raster data provided by the embodiment of the present invention can significantly save storage space by uniformly converting remote sensing satellite image data into COG format. In combination with object storage, storage costs can be reduced. On-demand access is provided to achieve efficient data distribution and map services based on COG rendering. At the same time, standard OGC services are provided so that each map engine can be directly loaded without modification. When accessing the service, tiles are stored in the distributed file storage system MongoDB. Tile caching is implemented through this mechanism to achieve efficient map services.

[0030] The following example illustrates the data storage and service method based on COG raster data.

[0031] In one optional specific implementation manner, after the steps of uploading the remote sensing satellite image data in COG format to the object storage, obtaining metadata of the uploaded remote sensing satellite image data and storing it in the object-relational database, the method further includes: At least part of the remote sensing satellite image data in the object storage is published as a WMTS service that complies with the OGC standard.

[0032] The above-mentioned embodiment can already provide map services for map engines that support COG rendering, such as Open Layers (a JavaScript library package specially provided for Web-GIS client development, used to implement access to map data published in a standard format), that is, it supports directly obtaining remote sensing satellite image data from object storage and performing COG rendering through Web-GIS engines, etc. In this embodiment, in order to better support other map engines such as Cesium (a three-dimensional geospatial platform), the server is further constructed to publish remote sensing satellite image data as a WMTS service that complies with the OGC standard, that is, the remote sensing satellite image data is further processed into tile data. As a result, users can be further allowed to access tile maps, thereby improving the service response speed.

[0033] In this embodiment, optionally, remote sensing satellite image data can be downloaded on demand from the object storage, sliced ​​and processed into tile data, and stored in the MongoDB database. MongoDB is a database based on distributed file storage, which has the characteristics of high performance, easy deployment, easy use, and very convenient data storage. Thus, the data is published from the MongoDB database as a WMTS service that complies with the OGC standard.

[0034] In some embodiments of the present application, the method further comprises: Based on the received WMTS service request of the user, searching whether a corresponding target tile exists in a distributed file storage system storing tiles corresponding to the WMTS service; If the target tile exists, sending the target tile to the user; If the target tile does not exist, the corresponding remote sensing satellite image data is obtained from the object storage and sliced ​​into tiles, and the sliced ​​tiles are sent to the user and stored in the distributed file storage system.

[0035] That is to say, after publishing part of the remote sensing satellite image data as a WMTS service that complies with the OGC standard, when receiving the user's WMTS service request, it is preferred to search whether there is a corresponding target tile in the distributed file storage system (such as the above-mentioned MongoDB database) that stores the tiles corresponding to the WMTS service. If it exists, the target tile data is directly returned to the user. If the requested tile does not exist in the distributed file storage system, the server will download the corresponding remote sensing satellite image data from the object storage according to the user's request range, and then slice the downloaded remote sensing satellite image data and store it in the distributed file storage system, and send it to the user at the same time for rendering and display on the user side. Among them, when the remote sensing satellite image data exists at the next level, the remote sensing satellite image data of the next level is downloaded at the same time to improve the response efficiency when the tile may be accessed later.

[0036] In some embodiments, optionally, when a user's WMTS service request is received, WMTS standard parameter information will be received. Taking the Get Tile request (the Get Tile request is a request for obtaining a specified tile in the WMTS service) as an example, the level and row and column number (i.e., xyz information) of the tile to be accessed is obtained from the parameters, and the xyz information is used to query whether there is a corresponding tile in the distributed file storage system (i.e., MongoDB).

[0037] In some other embodiments of the present application, the step of determining the target remote sensing satellite image data based on the metadata based on the received user's data access request includes: Based on the access location corresponding to the data access request, find the target access range in the entire remote sensing satellite image and obtain corresponding row and column information; The number of bytes required to offset the target access range is determined according to the metadata and the row and column information, and then the target remote sensing satellite image data is determined.

[0038] In this embodiment, when accessing data, the number of bytes to be accessed is set by passing in a Range request header through an HTTP (Hypertext Transfer Protocol) request, wherein the Range request header is used to indicate that the server only needs to return a portion of the resource, which is usually used for breakpoint resuming or downloading large resources in segments. The number of bytes to be accessed is determined based on the metadata obtained in the above steps and the range of the location to be accessed, and the row and column information of the access location is obtained by calculating the range of the access location in the entire remote sensing satellite image, and the number of bytes required to offset the access range is calculated in combination with the metadata, and then the target remote sensing satellite image data is determined.

[0039] In some embodiments of the present application, the steps of acquiring remote sensing satellite image data, verifying whether the remote sensing satellite image data is in COG format, and converting the remote sensing satellite image data into COG format when the remote sensing satellite image data is not in COG format include: The GDAL raster spatial data conversion library encapsulation verification method is used to verify whether the remote sensing satellite image data is in COG format; In the case where the remote sensing satellite image data is not in the COG format, the remote sensing satellite image data is converted into the COG format using a conversion method provided by the GDAL raster spatial data conversion library.

[0040] That is to say, in this embodiment, when verifying whether the remote sensing satellite image data is in COG format, you can use Python's GDAL (Geospatial Data Abstraction Library, open source raster spatial data conversion library) library encapsulation validate method to verify whether the data is in COG format specification. If it is already in COG format, there is no need to convert the data format. If it is not in COG format, you can use the gdal_translate method provided by GDAL for conversion. The format parameter is specified as COG, and the compression format is specified as DEFLATE (a lossless data compression algorithm that uses both LZ77 algorithm and Huffman Coding). Other parameters are adjusted according to the actual data situation.

[0041] In some other embodiments of the present application, the step of obtaining metadata of the uploaded remote sensing satellite image data and storing it in an object-relational database includes: Use the Tifffile library to obtain the metadata of the uploaded remote sensing satellite image data, which includes IFD tag data; The metadata is stored in a PostgreSQL object-relational database.

[0042] Specifically, in this embodiment, Python's GDAL and Tifffile libraries are used to obtain the metadata (or meta-information) of the uploaded remote sensing satellite image data, where the metadata includes all IFD (Information Field Definition) tag data corresponding to the remote sensing satellite image data, and the IFD contains various information and metadata of the image, such as resolution, color depth, author information, etc. The IFD standard defines the structure and content of these information fields to ensure that different software and systems can correctly read and interpret the information in the image file. The information field in the IFD exists in the form of a tag, and each tag corresponds to a type of information.

[0043] After obtaining the metadata of remote sensing satellite image data, it can be stored in the PostgreSQL object-relational database to facilitate subsequent data download and retrieval. PostgreSQL supports most SQL standards and provides many other modern features, such as complex queries, foreign keys, triggers, views, transaction integrity, multi-version concurrency control, etc.

[0044] In summary, in the embodiments of the present invention, by uniformly converting remote sensing satellite image data into the COG format, storage space can be greatly saved. Combining object storage can reduce storage costs, provide on-demand access to achieve efficient data distribution and COG-rendered map services, and provide standard OGC services at the same time, so that each map engine can be directly loaded without modification. When accessing the service, the tiles are stored in the distributed file storage system MongoDB. Tile caching is achieved through this mechanism to achieve efficient map services.

[0045] See also Figure 2 , Figure 2 2 is a schematic diagram of a data storage and service device based on COG grid data provided by the second embodiment of the present invention. The device 20 includes: The acquisition module 21 is used to acquire remote sensing satellite image data, verify whether the remote sensing satellite image data is in COG format, and convert the remote sensing satellite image data into COG format if the remote sensing satellite image data is not in COG format; The storage module 22 is used to upload the remote sensing satellite image data in COG format to the object storage, obtain the metadata of the uploaded remote sensing satellite image data and store it in the object-relational database; The data sending module 23 is used to determine the target remote sensing satellite image data based on the received data access request of the user and according to the metadata, and send the target remote sensing satellite image data to the user.

[0046] Optionally, the device further comprises: The publishing module is used to publish at least part of the remote sensing satellite image data in the object storage as a WMTS service that complies with the OGC standard.

[0047] Optionally, the device further comprises: A search module, configured to search, based on a received WMTS service request from a user, whether a corresponding target tile exists in a distributed file storage system storing tiles corresponding to the WMTS service; A tile sending module, configured to send the target tile to the user if the target tile exists; The tile processing module is used to obtain the corresponding remote sensing satellite image data from the object storage and slice it into tiles if the target tile does not exist, and send the sliced ​​tiles to the user and store them in the distributed file storage system.

[0048] Optionally, the data sending module includes: A search unit, configured to find a target access range in the entire remote sensing satellite image and obtain corresponding row and column information based on an access position corresponding to the data access request; The determination unit is used to determine the number of bytes that the target access range needs to be offset according to the metadata and the row and column information, and then determine the target remote sensing satellite image data.

[0049] Optionally, the acquisition module includes: A verification unit, used for verifying whether the remote sensing satellite image data is in COG format by using a GDAL raster spatial data conversion library encapsulation verification method; The conversion unit is used to convert the remote sensing satellite image data into the COG format by using the conversion method provided by the GDAL raster spatial data conversion library when the remote sensing satellite image data is not in the COG format.

[0050] Optionally, the storage module includes: An acquisition unit, used to use a Tifffile library to acquire metadata of uploaded remote sensing satellite image data, wherein the metadata includes IFD tag data; The storage unit is used to store the metadata in a PostgreSQL object-relational database.

[0051] In an embodiment of the present invention, by uniformly converting remote sensing satellite image data into the COG format, storage space can be greatly saved. Combining object storage can reduce storage costs, provide on-demand access to achieve efficient data distribution and COG-rendered map services, and provide standard OGC services at the same time, so that each map engine can be directly loaded without modification. When accessing the service, the tiles are stored in the distributed file storage system MongoDB. Tile caching is achieved through this mechanism to achieve efficient map services.

[0052] The embodiment of the present invention is a product embodiment corresponding to the above-mentioned method embodiment 1, so it will not be described in detail here. Please refer to the above-mentioned embodiment 1 for details.

[0053] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A data storage and service method based on COG grid data, characterized in that: include: Acquire remote sensing satellite image data, verify whether the remote sensing satellite image data is in COG format, and if the remote sensing satellite image data is not in COG format, convert the remote sensing satellite image data into COG format; Upload remote sensing satellite image data in COG format to object storage, obtain metadata of the uploaded remote sensing satellite image data and store it in an object-relational database; Based on the received data access request of the user and according to the metadata, the target remote sensing satellite image data is determined, and the target remote sensing satellite image data is sent to the user.

2. The method according to claim 1, characterized in that After the steps of uploading the remote sensing satellite image data in COG format to the object storage, obtaining metadata of the uploaded remote sensing satellite image data and storing it in the object-relational database, the method further comprises: At least part of the remote sensing satellite image data in the object storage is published as a WMTS service that complies with the OGC standard.

3. The method according to claim 2, characterized in that The method further comprises: Based on the received WMTS service request of the user, searching whether a corresponding target tile exists in a distributed file storage system storing tiles corresponding to the WMTS service; If the target tile exists, sending the target tile to the user; If the target tile does not exist, the corresponding remote sensing satellite image data is obtained from the object storage and sliced ​​into tiles, and the sliced ​​tiles are sent to the user and stored in the distributed file storage system.

4. The method according to claim 1, characterized in that The step of determining the target remote sensing satellite image data based on the received user's data access request and the metadata comprises: Based on the access location corresponding to the data access request, find the target access range in the entire remote sensing satellite image and obtain corresponding row and column information; The number of bytes required to offset the target access range is determined according to the metadata and the row and column information, and then the target remote sensing satellite image data is determined.

5. The method according to claim 1, characterized in that The steps of acquiring remote sensing satellite image data, verifying whether the remote sensing satellite image data is in COG format, and converting the remote sensing satellite image data into COG format when the remote sensing satellite image data is not in COG format include: The GDAL raster spatial data conversion library encapsulation verification method is used to verify whether the remote sensing satellite image data is in COG format; In the case where the remote sensing satellite image data is not in the COG format, the remote sensing satellite image data is converted into the COG format using a conversion method provided by the GDAL raster spatial data conversion library.

6. The method according to claim 1, characterized in that The step of obtaining metadata of the uploaded remote sensing satellite image data and storing it in an object-relational database includes: Use the Tifffile library to obtain the metadata of the uploaded remote sensing satellite image data, which includes IFD tag data; The metadata is stored in a PostgreSQL object-relational database.

7. A data storage and service device based on COG grid data, characterized in that: include: An acquisition module is used to acquire remote sensing satellite image data, verify whether the remote sensing satellite image data is in COG format, and convert the remote sensing satellite image data into COG format if the remote sensing satellite image data is not in COG format; A storage module is used to upload remote sensing satellite image data in COG format to object storage, obtain metadata of the uploaded remote sensing satellite image data and store it in an object-relational database; The data sending module is used to determine the target remote sensing satellite image data based on the received data access request of the user and the metadata, and send the target remote sensing satellite image data to the user.

8. The device according to claim 7, characterized in that The device also includes: The publishing module is used to publish at least part of the remote sensing satellite image data in the object storage as a WMTS service that complies with the OGC standard.

9. The device according to claim 8, characterized in that The device also includes: A search module, configured to search, based on a received WMTS service request from a user, whether a corresponding target tile exists in a distributed file storage system storing tiles corresponding to the WMTS service; A tile sending module, configured to send the target tile to the user if the target tile exists; The tile processing module is used to obtain the corresponding remote sensing satellite image data from the object storage and slice it into tiles if the target tile does not exist, and send the sliced ​​tiles to the user and store them in the distributed file storage system.

10. The device according to claim 8, characterized in that The data sending module comprises: A search unit, configured to find a target access range in the entire remote sensing satellite image and obtain corresponding row and column information based on an access position corresponding to the data access request; The determination unit is used to determine the number of bytes that the target access range needs to be offset according to the metadata and the row and column information, and then determine the target remote sensing satellite image data.

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