# Big Data Integration in Remote Sensing across a Distributed Metadata-Based Spatial Infrastructure
## 3 Distributed Integration of Multi-Source Remote Sensing Data
### The ISO 19115-Based Metadata Transformation  
Remote sensing metadata represent descriptive information about remote sensing images, as well as data identification, imaging time, imaging location, product level, quality, the spatial reference system, and other characteristic information. At present, the metadata forms of different remote sensing data vary greatly. For example, Landsat 8 collects images of the Earth with a 16-day repeat cycle, referenced to the Worldwide Reference System-2 [24]. The spatial resolution of the Operational Land Imager (OLI) sensor onboard the Landsat 8 satellite is about 30 m; its collected images are stored in GeoTIFF format, with Hierarchical Data Format Earth Observation System (HDF-EOS) metadata [25; 26]. The Moderate-Resolution Imaging Spectroradiometer (MODIS) instruments capture data in 36 spectral bands ranging in wavelength from 0.4 \(\upmu\)m to 14.4 \(\upmu\)m and at varying spatial resolutions (2 bands at 250 m, 5 bands at 500 m, and 29 bands at 1 km). Most of the MODIS data are  
Figure 1: An object-oriented data technology (OODT) framework. available in the HDF-EOS format, and it is updated every 1 to 2 days [27]. The charge-coupled device (CCD) sensor, which is carried by the Huan Jing (HJ)-1 mini satellite constellation, has an image swath of about 360 km, with blue, green, red, and near infrared (NIR) bands, 30-m ground pixel resolution, and a 4-day revisit period. Its collected images are stored in GeoTIFF format, and their customized metadata are in eXtensible Markup Language (XML) format [28]. These different metadata formats have resulted in great difficulties for data integration and management, which could be solved by transforming them into a uniform metadata format for uniform retrieval and management [29; 30]. ISO 19115-2:2009 is the geographic information metadata standard which was published by the International Standardization Organization (ISO). It mainly defines the metadata schema of geographic information and services, including the identification, quality, space range, time horizon, content, spatial reference system, distribution, and other characteristic information [31]. Currently, ISO 19115-2:2009 has been integrated into the Common Metadata Repository (CMR) as one of the most popular standards for data exchange [32], data integration, and data retrieval across international geographic information organizations and geographic data centers. On the basis of the ISO 19115-2:2009 geographic information standard, we proposed a uniform remote sensing metadata format. All of the remote sensing metadata in the distributed sub-centers should be transformed into this uniform format before data integration. In this paper, the transformational rules we established are mainly aimed at NASA EOS HDF-EOS format metadata (Aster and Landsat series satellites included) and the customized XML-based metadata of the CCRSDA (HJ-1A/B, GF and ZY series satellites included) (see Table 1). It should be noted that in Table 1, the struck-through (-) shows the field does not exist, and it will be assigned a null value after metadata transformation. In the ISO metadata column, the term spatialResolution describes the ability of the remote sensor to distinguish small details of an object, generally in meters, thereby making it a major determinant of image resolution. Hence, the spatialResolution is mapped to NadirDataResolution in the HDF-EOS metadata column and pixelSpacing in the CCRSDA metadata column. The terms scenePath and sceneRow are orbit parameters of the satellite in the Worldwide Reference System (WRS), just mapping to WRS_PATH and WRS_ROW in the HDF-EOS metadata column. The term imageQualityCode is a characteristic of a remote sensing image that measures the perceived image degradation, and has the same meaning as the overallQuality in the CCRSDA metadata column. The term processingLevel denotes the type of the remote sensing data, and is mapped to the DATA_TYPE in the HDF-EOS metadata column and productLevel in the CCRSDA metadata column.