|
top Overview
|
|
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. The main features of LAS allow the user to
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
|
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:
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 Server-side map inteface
This page replaces the Constraints page and presents the user with a clickable map that does not depend on a Java applet.
How to use this page:
- Click on the map image to begin selecting a region.
The result of the click depends on the current map selection tool.
- cross hair
- The result of the first click of a two-click operation.
Drawing a box, an x-line or a y-line requires two clicks to define the region.
- XY-Box
- If the view requires an XY region to be selected, the first
click draws cross hair on the map as shown above.
The second click draws
a box using the second click as an oposite corner of the box. For example
if you click south and east of the cross hair, a box will be drawn with the second
click as the "lower right" corner of the box.
If the second click is north and east of the cross hair, the box will be drawn
using the second click as the "upper left" corner of the box.
- X-Line
- If the view requires an X-line to be selected, the first
click draws a cross hair on the map as shown above.
The second click will
draw a a line at a fixed latitude located at the average latitude of the two clicks.
The line will extend from the crosshair in the direction of the click.
- Y-Line
- If the view requires a Y-line to be selected, the first
click draws a cross hair on the map as shown above.
The second click will
draw a a line at a fixed longitude located at the average longitude of the two clicks.
The line will extend from the crosshair in the direction of the click.
- Point
- If the view requires a single point to be selected, the first
click draws a small box on the map surrounding the point as shown below. Only one
click is required to specify the location of the point.
- X-Line in Comparison Mode
- When the view requires an X-line to be selected while
using comparison mode the user can select a different X-line for each
of the two variables being compared. The east-west extents of both
lines are determined by the east-west extends of the line selected
for the first variable (the yellow line). The first
click draws a cross hair on the map as shown above.
If the Var 1 radio button is selected when the map is clicked, the second click will
draw a a line at a fixed latitude located at the average latitude of the two clicks.
The line will extend from the crosshair in the direction of the click.
If the Var 2 radio button is selected when the map is clicked,
only one click is required to determine
the north-south location of the line (white line) for the second variable.
- Y-Line in Comparison Mode
- When the view requires a Y-line to be selected while
using comparison mode the user can select a different Y-line for each
of the two variables being compared. The north-south extents of both
lines are determined by the north-south extends of the line selected
for the first variable (the yellow line). The first
click draws a cross hair on the map as shown above.
If the Var 1 radio button is selected when the map is clicked, the second click will
draw a a line at a fixed longitude located at the average longitude of the two clicks.
The line will extend from the crosshair in the direction of the click.
If the Var 2 radio button is selected when the map is clicked,
only one click is required to determine
the east-west location of the line (white line) for the second variable.
- Point in Comparison mode
- If the view requires a single point to be selected while
using comparison mode the user can select a different point for each of the two
variables being compared. A single
click draws a small box on the map surrounding the point as shown below. Only one
click is required to specify the location of the point. If the Var 1 radio
button is selected when the map is clicked the location of the point
for the first variable is
drawn (the yellow box). If the Var 2 radio button is selected when the
map is clicked the location of the point for the second variable
is drawn (the white box).
- Pan buttons
- Selecting one of the pan buttons will move the eye
observing the map in direction of the arrow. The map will only pan in
the direction of a modulo axis or if the map has been zoomed and there is
an area of the map that is not visible in direction of the pan.
- Zoom buttons
- Selecting one of the zoom buttons zoom the map
in or out. The map will not zoom out if the entire map is already being
displayed.
- Reset button
- Selecting the reset button zooms the map to
a view appropriate for the geographic extents of the selected variable and
selects the entire domain for the selected variable. If you ever get lost
or don't like the look of the map, try the reset button.
- Help button
- Selecting the help button displays this page.
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
Experimental!
This page will be reworked with improved functionality in a future release.
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
Variable page
This page looks similar to the Constraints
page but allows you to define a new variable that is based on the currently
selected one. Various analysis functions (average, maximum, minimum, sum and
variance) may be applied to one ore more axes. This 'user' variable will be
compressed along the specified axis or axes. Derived variables that can be
defined in this manner might include temporal variance of SST, zonally averaged
atmospheric CO2, vertically summed biomass, etc.
Note 1: The axis range for all compressed axes
is remembered when you define a variable. Thus, if you want to look at time
series that are spatial averages you will have to define a new variable for
each XY region of interest.
Note 2: You cannot compress all the axes at once.
LAS does not currently deal with variables that consist of a single value.
How to use this page:
- Select an analysis function.
- Give your new variable a memorable name.
- Choose the axes on which to apply the analysis
function.
- Optionally apply a mask.
- Click on Next > to
create your variable or Cancel > to return
to the Constraints page
Terms:
- user variable
- Any variable created by the user from pre-existing
variables. The ability to create user variables depends upon the functionalitiy
available in the back end analysis package. The Ferret
package is used as the default back end and has very powerful capabilities
for defining variables that are functions of existing variables.
- compressed
- An axis is compressed when a function applied to it
returns a single value instead of an axis of values. A function like SUM
compresses an axis whereas a function like SIN
does not. If you start with a variable that has X, Y and T axes and apply
SUM to both the X and Y axes you will end up with a spatially averaged
time series.
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
The NOMADS LAS is 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.
|
