# The HD(CP)\({}^{2}\) Data Archive for Atmospheric Measurement Data
## 3 The HD(CP)\({}^{2}\) Observation Data Product Standard
Good quality metadata are an asset [17]. For this reason, every dataset which is published by an HD(CP)\({}^{2}\) data server must be associated with metadata in an appropriate form (see Section 5.2). The benefit of semantically annotating data according to some well-established vocabulary is obvious. The controlled vocabulary will aid in searching and finding data, making it more'shareable' with other researchers. Most common standards such as some ISO standards might seem to be ideal for our needs, but unfortunately for atmospheric measurement data there is no standard that fits well. The standard ISO 191 xx family for geographical information systems (mainly ISO 19115-1:2014 [18]) with its sophisticated structure and large number of entities is far too complex for the small set of elements that suits the project needs and is almost a contradiction to our 'easy-to-use' approach. The same can be said of the INSPIRE Directive (2007) [19]. On the other hand, the Dublin Core (NISOZ3985) [20] and the DataCite Metadata Scheme (3.1) [21] are too general. Although the latter provides metadata fields to specify the geographical region where the data were gathered (i.e., geoLocation), we needed to define the exact position and orientation, especially the height, of every single measuring instrument, or combinations of instruments. That is why we decided to create our own metadata scheme. We designed about 30 metadata descriptive elements, most of them with controlled vocabulary, adapting some well-established attributes in the climate community from the NetCDF header. The data type-specific elements reflecting the needs of our climate project and the global attributes are compliant with current metadata standards such as the Dublin Core, the DataCite Metadata Scheme, the ISO19115-1 Standard and the INSPIRE Directive. The HD(CP)\({}^{2}\) Metadata Scheme provides mandatory, optional and automatically generated attributes. For instance, HD(CP)\({}^{2}\) metadata must include at least the following global attributes: title of the data set, institution and name of the data producer, the name and e-mail address of a contact person, the instrument source, the information about the conventions used, the processing date and the HD(CP)\({}^{2}\) license policy. The fields for comments and the history of the data are optional. \begin{table}
\begin{tabular}{c l} \hline \hline
**Part** & **Description** \\ \hline \textless{kkk}\textgreater{} & kind of measurement type (supersites, full domain observation, campaign, etc.) \\ \textless{ssss}\textgreater{} & abbreviation of supersite or owner institute of the instrument or distributor of data \\ \textless{instnm}\textgreater{} & instrument or synergy product, retrieval algorithm, + numbering (starting with 00) \\ \textless{dn}\textgreater{} & HD(CP)\({}^{2}\) level of data post-processing (starting with 1) \\ \textless{var}\textgreater{} & observation variable name \\ \textless{vnn}\textgreater{} & version of dataset (starting with 00) \\ YYYMMDD & date of measurement (UTC): year (YYYY), month (MM), day (DD), and starting time of \\ hhmmss & measurement (UTC): hour (hh), minutes (mm), seconds (ss) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of the HD(CP)\({}^{2}\) data file names and description of each file name part. The HOPS document specifies the metadata standard to describe and index the archived data in a consistent way. The metadata files should be encoded in the eXtensible Markup Language [22]. XML provides a common way of describing particular types of a document structure, which is why it was chosen as the metadata specification language. Data producers have to generate a separated file with detailed information about measurement and instruments, guided by our established vocabulary for metadata. For example, they have to specify keywords by the HD(CP)\({}^{2}\) taxonomy for later search and accurate retrieval results.