DAP Data Request Form Help Resource

The Basics

The Data Request Form allows for visual inspection of the contents within a remote dataset of interest, whose URL is shown in the Data URL Box. The Form does not download any data until the user selects a “Download Encoding” and clikc on the “Get Data” button.

To arrive at this Form you can paste into any browser the URL of a dataset known to be hosted by Hyrax or any other partner OPeNDAP server. For example consider the dataset

http://test.opendap.org/dap/data/nc/coads_climatology.nc

Glossary

Actions

To download dataset, using the web browser. There are two possible and necessary Actions:

Download Encoding: Type of file format you want your dataset to be downloaded into. Options are: NetCDF4, NetCDF3, CSV, DAP4 binary, Source Data file.
Get Data: This button triggers the download to your local machine. You need to select one of the available download encoding options before.

Data URL

This is the URL that specifies the data. You may edit this by hand adding a DAP constraint expression to the end of the URL (see the OPeNDAP User’s Guide for information on constraint expressions), or can be created interactively by clicking on one or more of the variables (see below).

There are two buttons below the URL:

Copy Encoded Data URL: When using constraint expressions (like in the example above), certain characters need to be escaped first. This option is useful when client API being used does not know how to do that. In the example above (image), selecting the encoding option will result in the following url (NOTE: You need to first select a Download Encoding option from Data URL , in the case below netCDF4 was chosen):


http://test.opendap.org/opendap/data/nc/coads_climatology.nc.dap.nc4?dap4.ce=/SST%5B0:1:11%5D%5B0:1:89%5D%5B0:1:179%5D

Copy Raw Data URL: This is equivalent to copying as is, the URL. The only difference is that to use this button you need to select an Download Encoding options from Data URL. If the client API (e.g. Pydap) knows to encode characters such as “[” and “]”, you do not need to encode the URL. In fact encoding may result in errors. Using the same example above, the Copy Raw Data URL (using NetCDF4 as download encoding) yields:


http://test.opendap.org/opendap/data/nc/coads_climatology.nc.dap.nc4?dap4.ce=/SST[0:1:11][0:1:89][0:1:179]

Global attributes

The self-describing information about the dataset creation. For example, how the data was generated, type of numerical implementation, missing data, shipboard measurements, date, DOI if dataset is associated with a publication, etc.

Global dimensions

Specifies the dimensions and dimension names used in the dataset. Global dimensions are (usually) 1-dimensional arrays.

Variable attributes

Contains specific information about each variable in the dataset. These appear below each variable name. Some commonly used attributes are offsets, scale factors and fill values are usually defined for each variable, and must follow CF-conventions.

Variables

All variables contained in the dataset. By default, when all variables are unchecked (unselected checkbox to the left of each variable name) then all variables are included in any download of the dataset in the event of clicking Get Data button(see Actions above). If one or more variables are checked, only the selected variables will be downloaded. In addition, the boxes underneath each variable name allows for indicial subsetting of said variable along one or more dimensions. This means, when you click on the Get data button, only the subset of data associated with the selected variables will be retrieved.

Interactive URL builder

The Form can function as an interactive URL builder when only a subset of the original dataset is needed.

To select a variable, click on the checkbox to its left.
To constrain a variable that you’ve selected, edit the Index information that appears in the text boxes below the variable. When entering values for selections on String variables, you may need to quote those values.
To directly download the data in any of various common formats, click on one of the buttons labeled Get DATA. You need to Choose One of the file encoding formats (see Actions), e.g. NETCDF3, NetCDF4, Binary, etc. Then press the button Get Data.
The downloaded file can be read by any compatible OPeNDAP client.

The URL displayed in the Data URL field may include Constraint Expressions. Depending on the specific DAP implementation (DAP2 vs DAP4) these may look slightly different. Considering the example above, the resulting URL implements a DAP4 constraint expression that only selects SST from the dataset, all the spatial domain but only a single time unit:

http://test.opendap.org/opendap/data/nc/coads_climatology.nc?dap4.ce=/SST[0][0:1:89][0:1:179]

This new updated URL can be used by the various clients with a compatible DAP implementation such as PyDAP, Matlab, Ferret, Xarray, etc.

NOTE: When clicking on the data URL above, the URL displayed on your browser will have a .dmr, this is the dataset metadata response. Without the .dmr included in the data URL, clicking it would trigger a download of the selected dataset.

For more on DAP Protocols, see OPeNDAP’s Official Documentation.