DODS also includes a way to sample data that makes writing a
constraint expression somewhat easier. Append .html to the URL,
and you get a form that directs you to add information to sample the
data at a URL:
Sending a URL ending in .html returns a form like this:
Near the top of the page, you'll see a box entitled "Data URL". At this point, if you've been following along, it should look pretty familiar. If you're just jumping in, it's the DODS URL connected to the data we're interested in, but unsampled.
Moving down the page, there is a list of "Global Attributes", which is really just for your perusal. At this point, there's not much to be done with this, but it is often helpful information.
The important part of the page is the "Variables" section. For each variable in the dataset, you'll see the data description (e.g. "Array of 32 bit Reals [lat = 0..179]"), a checkbox, a text input box, and a list of the variable's attributes. If you click on the checkbox, you'll see the variable's array bounds appear in the text box, and you'll see that variable appear in a constraint expression appended to the Data URL at the top of the page. If you edit the array bounds in the text box, hitting "enter" will place your edits in the Data URL box.
In the oh-so-unlikely event you dare try all this without your documentation vade mecum along, there's a Show Help button up near the top of the page. Clicking there will show you instructions about how to proceed.
NOTE: You'll see a "stride" mentioned. This is another way to
subsample a DODS array or Grid. Asking for lat[0:4] gets you the first
five members of the lat array. Adding a stride value allows
you to skip array values. Asking for lat[0:2:10] gets you
every second array value between 0 and
10: 0, 2, 4, 6, 8, 10.
Move on down the variable list, editing your request, and experiment with adding and changing variable requests.
When you have a request you'd like to make, look at the buttons at the top of the page.
The DODS Data Access Form interface works for Sequence data as well as Grids. However, since Sequence constraint expressions look different than Grid expressions, the form looks slightly different, too. You can see below that the variable selection boxes allow you to enter relational expressions for each variable. Beside that, however, the function is exactly the same.
NOTE: Not all DODS servers support all the DODS functionality. There are a few non-standard DODS servers out there in the world that only support the bare minimum required. That minimum is to respond to queries for the DDS, DAS, and (binary) data. The ASCII data and the web access form are optional add-ons that are not required for the basic DODS function.