Guillaume at 11:47, 10 March 2009

2009-03-10T11:47:50Z

Guillaume at 10:43, 10 March 2009

2009-03-10T10:43:33Z

Guillaume: 1 revision

2008-12-03T10:39:00Z

1 revision

192.168.20.177 at 11:53, 11 January 2008

2008-01-11T11:53:21Z

New page

== Definitions ==

Below lie the different concepts associated with [[OpenDAP]].

=== [[DAP]] ===

is an [http://en.wikipedia.org/wiki/Acronym acronym] for '''D'''ata '''A'''ccess '''P'''rotocol. It's a [http://en.wikipedia.org/wiki/Protocol_%28computing%29 protocol] for accessing and extracting information from large dataset files through the internet.

=== [[DAP]] [http://en.wikipedia.org/wiki/Server_%28computing%29 server] ===

is a suite of [http://en.wikipedia.org/wiki/Common_Gateway_Interface CGI] [http://en.wikipedia.org/wiki/Scripting_language scripts] and [http://en.wikipedia.org/wiki/Executable executables] which serve large dataset files. An http [http://en.wikipedia.org/wiki/Daemon_%28computer_software%29 daemon] [http://en.wikipedia.org/wiki/Aka a.k.a] [http://en.wikipedia.org/wiki/Httpd httpd], such as [http://www.apache.org/ Apache], working on a given port (typically port 80) is required.

=== [[DAP]] handler ===

is a program that deals with a specific file type (ex: [http://www.unidata.ucar.edu/software/netcdf/ netcdf], [http://hdf.ncsa.uiuc.edu/ HDF], [http://www1.whoi.edu/ JGOFS], FreeForm etc ...). It requires a [[DAP]] [http://en.wikipedia.org/wiki/Server_%28computing%29 server]. An [http://hdf.ncsa.uiuc.edu/ HDF5] handler is on the way (as of April 2006).

=== [[DAP]] [http://en.wikipedia.org/wiki/Client_%28computing%29 client] ===

is a program that can communicate with a [[DAP]] [http://en.wikipedia.org/wiki/Server_%28computing%29 server]. Reknowned clients are [http://www.epic.noaa.gov/java/ncBrowse/ ncBrowse] , [http://ferret.wrc.noaa.gov/Ferret/ Ferret], [http://www.opendap.org/download/matlab_gui.html matlab]. There's even a command line [http://en.wikipedia.org/wiki/Client_%28computing%29 client] ''geturl'' that comes with the [[DODS]] package.

=== [[DAP]] interface ===

is an solution for users to access, browse and retrieve data from a data repository. For example, [[LAS]] is a DAP interface.

=== [[OpenDAP]] ===

is an open source project for a network data access protocol. [[OpenDAP]] is also known as [[DODS]], '''D'''istributed '''O'''cean '''D'''atasets '''S'''ystem. It provides a package encapsulating a [[DAP]] server with file handlers such as [http://www.unidata.ucar.edu/software/netcdf/ netcdf] or [http://hdf.ncsa.uiuc.edu/ HDF]. The official website is found at http://www.opendap.org.

=== [[DODS]] [http://en.wikipedia.org/wiki/URL URL] ===

A [[DODS]] [http://en.wikipedia.org/wiki/URL URL] is the http address that allows to browse and retrieve data from a [[DAP]] [http://en.wikipedia.org/wiki/Server_%28computing%29 server]. A [[DODS]] [http://en.wikipedia.org/wiki/URL URL] can append a query string, indicating the variable(s), the time and the space window. It can also query for the metadata of the file.

== End user's guide ==

This is a local user's guide. These are the official [http://www.opendap.org/user/guide-html/guide.html user's guide] and [http://www.opendap.org/user/quick-html/quick.html quick start guide].

=== Quick start ===

==== From [http://www.epic.noaa.gov/java/ncBrowse/ ncBrowse] ====

# Download and install the latest version of the [http://java.sun.com/j2se/ Java platform] from [http://java.sun.com/ Sun].
# Download and install the latest version of [http://www.epic.noaa.gov/java/ncBrowse/ ncBrowse].
# Run [http://www.epic.noaa.gov/java/ncBrowse/ ncBrowse].
# Go to ''file --> OpenDAP'', write down the [http://en.wikipedia.org/wiki/Url URL] to the desired file http://data.mohid.com/opendap/nph-dods/mercator-ist/Portugal/20061213_Portugal_WaterProperties.nc, then hit ''Accept''.
# You can now browse at will the content of the remote file.

==== From a web-browser ====
# From a web-browser, compose the address where the [[DAP]] files are being served. ex: http://data.mohid.com/opendap/nph-dods/LASoutput.nc.html or http://www.cdc.noaa.gov/cgi-bin/nph-nc/Datasets/. You can browse the directories to see the available files. [http://www.unidata.ucar.edu/software/netcdf/ netcdf] files are most likely to be found.
# Select a [http://www.unidata.ucar.edu/software/netcdf/ netcdf] file by clicking on it or simply append it's full address in the address bar followed by ''.html'' suffix. ex: http://data.mohid.com/opendap/nph-dods/mercator-ist/Portugal/20061213_Portugal_WaterProperties.nc.html or http://www.cdc.noaa.gov/cgi-bin/nph-nc/Datasets/coads/1degree/global/std/air.mean.nc.html.
# Select the desired variables by clicking on their select boxes. Then write their subdomain limits. For example, putting in latitude ''[12:2:24]'' selects the subdomain ranging from 12ºN to 24ºN with a 2º step. Only integers are allowed.
# Get the data by clicking in ''Get Ascii'' or ''Binary Data Object''. Respectively you will obtain the data in an [http://en.wikipedia.org/wiki/Ascii ASCII] file or in [http://www.unidata.ucar.edu/software/netcdf/ netcdf] format (rewrite the extension ''.nc.dods'' to ''.nc'').

=== Quick ref ===

Here's a list of useful [http://en.wikipedia.org/wiki/Suffix suffixes] for extracting metadata from netcdf files:

*''.help'' generates a web based help document.
*''.html'' generates a web page containing information and action buttons over the netcdf file.
*''.info'' returns the netcdf file metadata.
*''.dds'' returns information about the variables.
*''.das'' returns information about the variable attributes.
*''.ver'' returns the OpenDAP server version.
*''.asc'' returns the netcdf file in [http://en.wikipedia.org/wiki/Ascii ASCII] format.

Here's a quick reference of [http://en.wikipedia.org/wiki/Suffix suffixes] for extracting data from netcdf files:

*''.asc?time'' returns the variable time in ascii format.
*''.asc?time[0:2:6]'' returns the variable time in ascii format but within a subset of stride 2.
*''.dods?time[0:2:6]'' returns the same variable as above but in [http://en.wikipedia.org/wiki/Binary_file binary] format (i.e. netcdf).
*''.dods?time[2:3:8],sst[2:3:8][43:47][215:234],ssh[2:3:8][43:47][215:234]'' returns the time, the SST and the SSH variables for a given domain.
*''.dods?time[2:3:8],sst[2:3:8][43:47][215:234]&pressure>50.0&pressure<100.0'' returns the time and the SST variables for a given domain with the additional constraint that the variable pressure must range between 50.0 and 100.0 units.

== External references ==
*[http://www.opendap.org/ OpenDap home]
*[http://nco.sourceforge.net/ NCO homepage]

[[Category:Technology]]
[[Category:Linux]]
[[Category:Matlab]]

← Older revision		Revision as of 11:47, 10 March 2009
Line 1:		Line 1:
		+	==Available software==
		+
		+	===libdap===
		+	Developer libraries and tools. Provides the command-line '''getdap''' tool that allows to download opendap files.
		+
		+	===libnc-dap===
		+	Provides substitute netcdf libraries, yet opendap''-able''.
		+
		+	===Hyrax===
		+	Is the opendap server. Requires handlers for specific file format such as netcdf, hdf4, hdf5, etc...
		+
		+	===Matlab loaddap===
		+	Is a matlab client which allows to access and read netcdf files from opendap urls. Requires the libdap libraries.
		+
	== Definitions ==		== Definitions ==

@@ Line 72: / Line 72: @@
 *''.dods?time[2:3:8],sst[2:3:8][43:47][215:234],ssh[2:3:8][43:47][215:234]'' returns the time, the SST and the SSH variables for a given domain.
 *''.dods?time[2:3:8],sst[2:3:8][43:47][215:234]&pressure>50.0&pressure<100.0'' returns the time and the SST variables for a given domain with the additional constraint that the variable pressure must range between 50.0 and 100.0 units.
 == External references ==

← Older revision	Revision as of 10:39, 3 December 2008
(No difference)

OpenDAP - Revision history

Guillaume at 11:47, 10 March 2009

Guillaume at 10:43, 10 March 2009

Guillaume: 1 revision

192.168.20.177 at 11:53, 11 January 2008