An interoperable interface for data provision

One of the goals of the MEA system design is to be applicable to multiple sensors at different resolutions. From data pre-processing perspective, the availability of a classified maps catalogue gives the opportunity to use it as a connection point between the multi temporal system and different data archives. This approach allows also to define an interoperable interface that is proven suitable for data access automation by its actual integration into the system. The interface presented hereafter is used to automatically search and retrieve classification maps by the automated data ingestion component that builds the MEA archive.

The ASQuLD SOAP interface, defined in [34] is based on four synchronous opera- tions and a customised type to define data collections consisting of files accessible over

4.2. A LAND COVER MAPS PROVIDER SERVICE 63 the network. Two of the four operations, “Search” and “Present” are based on the EOLI standard and provide functions to browse the catalogue. As defined in [17] these functions allow to query the catalogue by date and geographic area. An extension point is also provided in the standard that can be used to add the thematic content to the query parameters. Even if the extension is defined as Satellite domain conditions it is basically an array of key-value pairs that is also well suited for thematic content. Practically the implemented catalogue recognized specific keys added to the Search op- eration that correspond to aggregation sets of its output classes paired with a pair of values to bound its occurrence. This solution allows queries that identify all classified maps over a given area of interest (a square in the geographic coordinate system), that is produced from data acquired between two dates with a given thematic content. The thematic content is expressed by triples in the form of (class, min, max) where: class is one of the aggregate set while min and max are minimum and maximum percentage of the given class to be present in the image, with respect to its entire content (e.g Bare soil between 10 and 40 % of the granule). The identified granules keep reference to the source dataset that originate them, hence the catalogue provides also a thematic view over the underlying data archive and can be used as a query system to search the original data archive based on its thematic content.

The EOLI standard uses the concept of CollectionsId to identify groups of products and it is assumed that each product is uniquely identified within the same collection by a resTitle . Under these assumptions, each product (be it raw satellite data or any derived product) can be uniquely identified by the CollectionId.resTitle pair. From the perspective of processing automation, considerable overhead is introduced by the EOLI standard Search operation that provides as minimal output an entire tree of meta-data while a minimal product identification would suffice to proceed to data retrieval. The “Present” operation is already defined to provide additional information about any given result: thematic content is also added to the output of that operation via its key-value pair array extensibility.

Based on the SSE interface [43], two operations are provided to obtain information

about and use a service. The “RFQ” (Request For Quotation) is used to obtain

information about available product typologies for a given collection. Input to the operation is a list of collection identifiers and the output provides, for each collection identifier, the list of available product typologies, as listed in Table 4.3. The “Order”

64 CHAPTER 4. IMPLEMENTATION: BUILDING THE DATA STACK operation is used to request the products to be available for direct download.

Product type ID Description Notes CLS preliminary classification maps

with basic geo-reference informa- tion

Default format is GeoTIFF PROJ-AAAA:NNNNN Re-projected outputs, possibly

with improved geo-reference ac- curacy. Where:

ˆ AAAA can be either EPSG or ESRI, indicat- ing the related standard identification;

ˆ NNNNN is a valid EPSG / ESRI projection code.

Projection codes are defined by established standards [44] e.g., EPSG:32663 for World Equidis- tant Cylindrical. Default format is GeoTIFF.

SAT Original Data (when available) Direct download of original data from which the map has been de- rived.

Table 4.3: ASQuLD service interface: defined product typologies.

The Order operation mitigates the meta-data overhead issue by allowing a simple list of resTitle identifiers, grouped by collection and requested product typology iden- tifiers, as input. As output, for each requested product, the structured set depicted in Figure 4.9 is returned. This is a minimal set of information required for an auto- mated software component to understand and use the provided information: for each collection a set of dataset links is returned, each in turn composed of links to files composing it. Status and identification attributes are added to the dataset elements to allow automatic interpretation of the results. Status can be one of: Complete, In- complete or Empty and it is set depending on which data types have links provided in the dataset versus the types requested (e.g. a Complete dataset may contain just one link to the only requested data type). Available typologies for a given collection are retrieved with the RFQ operation.