top Overview

Datasets Variables Constraints Output Output Options Previous Output Define variable About

The Live Access Server (LAS) provides a web based user interface to geo-referenced data. Data of various types (gridded and in-situ) and stored in various formats are presented to the user in a single interface. For general about LAS including application download, refer to the LAS Home Page at http://ferret.pmel.noaa.gov/Ferret/LAS/ferret_LAS.html The main features of LAS allow the user to select a dataset select one ore more variables specify constraints including output view, output products, axis ranges and SQL style queries (where applicable) view graphical and textual output or download data modify output options review past plots define new variables Each of these tasks is contained on a separate page with a simple user interface that guides the user through the necessary steps. In addition, LAS allows you to search for datasets and variables interpolate data, specifying points that are not on the data grid perform comparisons between two variables from different datasets

top Datasets page(s)

This page contains a list of all available categories or datasets on this Live Access Server.

How to use this page:

• Click on a category name to bring up a list of subcategories or datasets.
• Click on a dataset name to bring up the Variables page associated with that data set.
• Click on the information icon to view information about the data set.

Terms:

category

A category is a collection of subcategories or datasets. The person setting up LAS can create whatever categories and subcategories they want to organize their data.

dataset

A dataset is a collection of variables. The person setting up LAS can create whatever datasets they want, provided that variables in a dataset can always be compared with other variables in the same dataset.

 

 

top Variables page

This page contains a list of all available variables within a dataset. The chosen dataset is displayed in the gray area at the top of this page.

How to use this page:

• Select one or more variables and then click on Next > to proceed to the Constraints page.
• You may click on the links in the gray bar at the top of this page to return to the Datasets page.

  Typically you will select only a single variable for visualizations but some visualization types (i.e. property-property plots) require two. Multiple selections are appropriate for downloads and overlay plots.

Terms:

variable

A variable is any named item in a dataset that may be plotted or downloaded. Some variables exist as fields in a database or a file while others may be computed on-the-fly from 'file variables'. This difference should be invisible to the user.

Top Constraints page

This is most feature rich page in the LAS user interface. After selection of a dataset and variable (displayed in the gray border at the top) LAS knows enough to present this page. Each of the selection tools on this page contains a list of options that are specific to the current dataset/variable. You should not be surprised if the available options differ when you select another variable. Typically, however, many variables within a dataset will share most options. The options displayed are also dependent upon the selected view. Thus, choosing a different view will cause the page to refresh as different views have different output products and axis ranges.

How to use this page:

• Click on a page name in the navigation bar on the left to go to that page.
• Select a view -- this will reset various options as appropriate for that view.
• Adjust any of the selectors to specify your desired output product, region and constraints.
• Use checkboxes to apply any available constraints to your data query.
• Click on Next > to send your request and proceed to the Output page.

How to use the map applet:

• Click on "(Don't) Use map applet" to (disable) enable the interactive map applet for region selection.
• Click and drag in the applet to select a region on the globe (1D or 2D depending on the view).
• Move handles on the map tool to reshape the region.
• Grab the center handle to reposition the region; bump against the sides to scroll the map.
• Click on "Zoom In" or "Zoom Out" to resize the underlying map.
• Type coordinates into the text fields to the right of the map to get precise values.

Axis values and interpolation:

The text boxes duplicate the functionality available in the selectors, you don't need to enter anything in them. However, the text boxes allow you to enter values in between grid points on the associated axes of gridded data. By default the nearest grid location on the associated axes will be used to access the data. For gridded data, the Output Options page allows you to choose between nearest grid point and linear interpolation to create data at a location in between grid points. (Note: whenever a text box contains a value different from its associated selector, the value in the text box is used for interpolation.)

Terms:

view

A view is most easily understood in terms of the plot that you would like to see. If you would like a map showing a variable on longitude and latitude axes then your view is "longitude-latitude". If you would instead like to see a plot showing the time history of a variable at some point in space then the view is "time". Views that are one dimensional (like "time") and two dimensional (like "longitude-latitude") typically have graphical output products (plots) available. Views which are three or four dimensional typically have only file output products available.

output product

An output product is returned to the user after all appropriate portions of the LAS request have been specified. Typical products include various plots in either .GIF or PostScript output, netCDF files, and tables of numbers in various ASCII formats. The available product choices depend upon both the variable chosen and the view selected. Not all output products are available with all views.

region

A region is a specification of longitude, latitude, depth/height and time. Some variables (i.e. annual averages, surface fields) may not contain all four of these axes. LAS presents the user with the appropriate user interface elements for selecting positions or ranges along the pertinent axes. Text fields next to selectors allow the user to input precise locations. Values in the text fields will always override values shown in the selectors.

constraints

Some data sets support SQL style queries. In such cases this page may show additional constraints to be used to further subset the data beyond lon-lat-depth-time. The available constraints are entirely dependent upon the dataset chosen.

 

Top Output page

This page displays the graphical or textual output requested from the Constraints page.

How to use this page:

• Click on a page name in the navigation bar on the left to go to that page.
• Click on "View in a new window" to bring up a separate page for this output.

Top Output Options page

Many output products will have options associated with them. These can range from named color palettes to contour line levels to gridding algorithms. To choose an entirely different output product you should go to the Constraints page. It is the combination of variable/view/output product on the Constraints page that specifies which options are shown on this page.

How to use this page:

• Click on a page name in the navigation bar on the left to go to that page.
• Click on the icon to get information about an option.
• Select a value and click on Next > to create an output plot using your chosen options.

Terms:

option

A parameter that may be set by the user to modify the default output plot or data. Typically, the LAS installer chooses reasonable default options for most variables and output products. But the end user has the power to override various options that affect graphics and occasionally algorithms. You can always apply the default options by clicking Next > from the Constraints page, bypassing the Output Options page.

Top Previous Output page

This page contains a list of all graphical products generated during your session with Live Access Server. The names contain dataset, variable, product and region information to help you identify old products.

How to use this page:

• Click on a page name in the navigation bar on the left to go to that page.
• Click on a link to bring up a new window with your chosen output.

Top Define variables page (new in LAS v6.1)

This page allows you to define new variables based upon existing variables within a dataset. This capability is provided by the Ferret analysis package that provides analysis and visualization in the default LAS release. More information about the exact deatils of the analysis transformations can be found at the Ferret site. Once created, variables will be found on the dataset page of the associated source variables. This variable will disappear when you quit your browser or your session times out.

How to use this page:

• Select from among the available analysis types.
• Give your new variable an appropriate name.
• Choose the axes on which you want your analysis preformed.
• You may apply a land or ocean mask (based on etopo60)
• For the axes you selected, pick the region to be included in the analysis.
• Click Next > to proceed to the Constraints page or Cancel > if you change your mind.

Top Search

LAS has a simple search capability that lets you query the LAS database. Just type some text into the entry field in the upper right hand corner of LAS and click . You will be presented with a list of all datasets and variables that match your query string.

 

Top Interpolation

LAS is typically set up with Ferret as its back end data analysis package. Ferret has the ability to interpolate gridded data from its source grid onto another destination grid. By default, LAS has this functionality turned off. If you specify a value on an axis that is not at a gridpoint Ferret will use the nearest gridpoint on that axis when generating the output product. Through the Output Options page the user can turn interpolation on in which case Ferret will use linear interpolation to calculate data values at precisely the location the user specified.

Top Comparison

By default, LAS starts out in "single data set" mode. This mode allows you to create graphical output products and download data using variables within a single dataset. Graphical output products in this mode are limited to a single variable. [The exception being property-property plots for in-situ data.] You can create multi-variable graphical products by switching to "comparison" mode.

To switch to comparison mode you need to click on the "compare two" tab at the top of the left hand navigation box.

When you are in comparison mode you will be presented with a new left-hand navigation box which allows you to go to Datasets and Variables pages for either variable 1 or variable 2. The pages function in exactly the same manner as in single data set mode. The Constraints page is the only page that appears different in comparison mode. The changes occur in the region selection area where dual selectors allow you to specify separate regions for the two variables. Note that the choice of view limits which axes will have dual selectors: e.g. both variables will share the same latitude-longitude region for a latitude-longitude view.

In order to difference (subtract) data on potentially different grids the back end data analysis package, Ferret, must regrid variable 2 onto the grid of variable 1. Thus the ordering of variables will have an effect not only on the sign but potentially also on the grid resolution of the result.