ERDDAP   Easier access to scientific data

Brought to you by NOAA NMFS SFSC ERD

ERDDAP > Information

The Problems that ERDDAP Tries To Solve

• The datasets of interest are hard to find because they are at many different web sites.
   
• Each site requires a different protocol to request the data
  (for example, HTTP GET, XML, SOAP+XML, DAP, WCS, WFS, SOS, or an HTML form).
   
• Each site returns the data in a different format
  (for example, XML, SOAP+XML, DAP binary data stream, ASCII text, HDF 4, HDF 5, NetCDF, ...)
  and it isn't the common file format that you want.
   
• Data from different sites is hard to compare because the dates+times are expressed in different formats
  (for example, "Jan 2, 1985", "02-JAN-1985", "1/2/85", "2/1/85", "1985-01-02", or days since has Jan 1, 1980, or ...).

ERDDAP's Solutions

• ERDDAP is a web service that aggregates data from diverse sources and offers a simple, consistent way to access the data.
   
• ERDDAP can serve local or remote data.
  Datasets in ERDDAP can be local (on the same computer) or remote (accessed via the web).
  See the list of types of data sources that ERDDAP can access.
   
• ERDDAP offers different ways to search for datasets of interest.
  For example, full text search and search by category.
   
• ERDDAP lets you make requests in a standardized way, regardless of the data source's request protocol.
  • ERDDAP makes gridded data available via the DAP hyperslab protocol and WMS.
  • ERDDAP makes tabular data available via the DAP constraint protocol.
  ERDDAP also provides Data Access Forms (web pages) which help humans create the DAP requests.
  OPeNDAP's DAP is the recommended IOOS DMAC data transport mechanism a NASA EOSDIS standard. (DAP is great!)
  ERDDAP translates the your request from the DAP format to the data source's request format and converts the response to one of the simple data structures.
  Then ERDDAP reformats the data in the common file format of your choice (for example, .html table, ESRI .asc, Google Earth .kml, .mat, .nc, .csv, .tsv, .json, .xhtml, .png) and sends the file to you.
  See the list of grid file types and the list of table file types.
  Other protocols for requesting the data (for example, WCS, WFS, and SOS) may be added/supported in the future.
  ERDDAP is structured for these additions and there don't seem to be any impediments.
   
• ERDDAP uses just two basic data structures.
  • Since it is difficult for human clients and computer clients to deal with a large number of dataset structures, ERDDAP uses just two basic data structures:
    • a gridded data structure (for example, for satellite data and model data) and
    • a tabular data structure (for example, for in-situ buoy, station, and trajectory data).
  • Certainly, not all data can be expressed in these structures, but much of it can.
    Tables, in particular, are very flexible data structures (look at the success of relational database programs).
  • This makes data queries easier to construct.
  • This makes data responses have a simple structure, which makes it easier to serve the data in a wider variety of standard file types (which often just support simple data structures).
  • This, in turn, makes it very easy for us (or anyone) to write client software which works with all ERDDAP datasets.
  • This makes it easier to compare data from different sources.
  • And even if this sounds odd to you, most ERDDAP clients will never notice --
    they will simply see that all of the datasets have a nice simple structure
    and they will be thankful that they can get data from a wide variety of sources returned in a wide variety of file formats.
  • Other data structures could be supported in the future if called for and if we find a good approach to working with them.
     
• ERDDAP can serve many types of scientific data. It isn't just for oceanographic data.
  ERDDAP is a Data Access Program that was written at NOAA NMFS SFSC ERD.
  The ERDDAP server at ERD serves oceanographic data, but ERDDAP (the program) can serve any gridded or tabular data.
   
• ERDDAP requests can be made in user units.
  Although requests for gridded data in ERDDAP can be made with array indices (following the DAP specification),
  requests can also be in user units (for example, degrees east), using a parentheses notation.
   
• ERDDAP sends results in common data file formats.
  The results can be returned in any of several common data file formats
  (for example, .html table, ESRI .asc, Google Earth .kml, .mat, .nc, .csv, .tsv, .json, .xhtml),
  instead of just the original format or just the DAP transfer format (which has no standard file manifestation).
  These files are created on-the-fly.
  Since there are few internal data structures, it is easy to add additional file-type drivers.
  See the list of grid file types and the list of table file types.
   
• ERDDAP standardizes the variable names and units for longitude, latitude, altitude, and time in the results.
  To facilitate comparisons of data from different datasets,
  the requests and results in ERDDAP use standardized space/time axis units:
  • longitude is always in degrees_east.
  • latitude is always in degrees_north.
  • altitude is always in meters with positive=up.
  • time when formatted as a number is always in "seconds since 1970-01-01T00:00:00Z" (which is UDUNITS-compatible) and,
    when formatted as a string, is formatted according to the ISO 8601:2004 "extended" format standard (YYYY-MM-DDThh:mm:ssZ, for example, "1985-01-02T00:00:00Z").
    Also, to avoid time zone and daylight savings time confusion, time values are always converted to the UTC time zone.
  This makes it easy to specify constraints in requests without having to worry about
  the altitude data format (are positive values up or down? in meters or fathoms?)
  or the time data format (a nightmarish realm of possible formats and time zones).
  This makes the results from different data sources easy to compare.
   
• ERDDAP lets you request .png and .pdf image files with graphs and maps of the data in addition to the actual data.
  Three special uses of these images are:
  • Web page authors can embed a graph with the latest data in a web page using an HTML <img> tag.
  • Anyone can use ERDDAP's Slide Sorter to build a personal web page that displays graphs with the latest data (or other images or HTML content), each in its own, draggable slide.
  • Anyone can use or make Google Gadgets to display images with the latest data on their iGoogle home page.
     
• ERDDAP adds metadata.
  Many data sources have little or no metadata (for example, CF metadata) describing the data.
  ERDDAP lets (and encourages) the administrator to describe metadata which will be added to datasets and their variables on-the-fly.
  See the addAttributes section of the directions for administrators.
   
• ERDDAP has REST- and ROA-style links to make its services available to computer programs.
  These features can be used to build another web service on top of ERDDAP (making ERDDAP do all the work!).
  ERDDAP is not intended to be a high-level data exploration/graphing service.
  Instead, ERDDAP is intended to provide services for such web sites and programs.
  So if you don't like ERDDAP's interface, we encourage you to build your own, and use ERDDAP as the foundation.
  Read more about the Computer Program interface.
   
• ERDDAP processes data in chunks.
  To save memory (a big issue) and make responses start sooner, ERDDAP processes data requests in chunks --
  repeatedly getting a chunk of data from the source, cleaning it up (for example, adding metadata), and sending that to the client.
  For many data sources, this means that the first chunk of data (for example, from the first sensor)
  gets to the client in seconds instead of minutes (for example, after data from the last sensor has been retrieved),
  reassuring the client that the data is coming (albeit slowly).
  From a memory standpoint, this allows numerous large requests (each larger than available memory) to be handled simultaneously.
   
• ERDDAP has a modular structure.
  ERDDAP is structured so that it is easy to add different components
  (for example, a class to request data from a SOS server and store it as a table).
  The new component then gains all the features and capabilities of the parent
  (for example, support for DAP requests and the ability to save the data in several common file formats).
   

Is ERDDAP a solution to everyone's data distribution / data access problems?
No. ERDDAP tries to find a sweet spot that is a really good solution to most (80%?)
of the data distribution problems that we confronted.
ERDDAP can get data from most types of data servers.
ERDDAP can distribute data to most types of data clients and to common file formats.

It isn't that the remaining datasets (20%?) (e.g., model data using a cubed sphere projection) aren't important.
It's just that we haven't yet found a simple organizing principle to deal with these diverse and more complex datasets.
Groups of researchers working with more complex data structures often already have specialized data servers
and specialized client software which are customized to their community's needs.
ERDDAP, as a general program, doesn't try to compete with these specialized data servers.
They are customized to the needs of their community and do a great job.

You can Set Up Your Own ERDDAP Server and serve your own data.

Contact - If you have questions, suggestions, or comments about ERDDAP in general (not this specific ERDDAP installation),
please email bob dot simons at noaa dot gov .
 
ERDDAP Version 1.12