TOMS Data Example

This example shows how to use ASCII files and how to concatenate a daily 2 dimensional data set into a 3-D time series of maps of total ozone from the TOMS instrument. We will edit the datamanager.txt file and use the ASCII schema to tell WebWinds how to interpret these TOMS files.

First we need to obtain the data from the TOMS Website. From this site, select "Ozone data-ASCII" format, and a date and press "request". You will need to save the resulting file on your local disk. If you are interested in April, 1998, as we were, you could instead jump directly to the 1998 ASCII data directory on the FTP server. If you have an FTP tool, you might want to use it to download a large number of daily files. We've chosen the 30 files for April.

Next, you need to concatenate these daily files into one large file, which we call ga9804.ept. This file can be put wherever you like, but see the discussion of paths and for more information. On Unix machines, you can use the "cat" command. Any text editor can also be used to place these files one after the other. You will notice that they are structured so that data vary fastest in longitude (for a single latitude starting at -89.5). The data vary next with latitude and since we have concatenated these files together, the resultant file varies slowest in time. There are 288 longitude points (-179.375 to 179.375), 180 latitude points (-89.5 to 89.5) and 30 time points (days). Also, in each of the original daily files, data values appear after 3 header lines and each data line has a blank space at the beginning followed by 25- 3 digit numbers with no spaces in between. There are 11 such lines followed by a similar line but with only 13-3 digit numbers. Then the whole process repeats. To read this file, the following text must be added to the appropriate (e.g. in the "TOMS MONTHLY" folder in the "Earth" folder) folder in the datamanager.txt file:

DataSource "April, 1998 TOMS"
file "ga9804.ept"
format "Ascii"
schema
"start:"                                
"3/"
"180(11(x,25I3/),x,13I3/)"
"goto start"
endschema
inputorder "1-23,123"
converter "Linear"
meta "Time" "days" axis 3 entries 30 range 1.0 30.0
meta "Longitude" "degrees" axis 1 entries 288 range -179.375 179.375
meta "latitude" "degrees" axis 2 entries 180 range 89.5 -89.5
meta "ozone" "du" axis 0 entries 256 range 0.0 595.0
colorhint "cutten"

The keywords between "schema" and "endschema" define the method for reading the data. "start" signifies the beginning of a repetition or loop and "goto start" is the end of the loop. "3/" means skip 3 lines. The line beginning with "180" reads the data according to the recipe discussed above. The "inputorder" keyword specifies that the most rapidly varying dimension (longitude) will be displayed in the "1" or horizontal direction, the next rapidly varying dimension (latitude) will be displayed in the "2" or vertical direction and the last dimension (time) will be displayed in 'slices'. Since the latitudinal ("2") variation is 'reversed' from what we would normally see in a map of the earth, "inputorder " shows a "-2" which forces the variation to be inverted. Labels for the axes are defined with the "meta" keywords. Axis 0 identifies the properties of the dependent variable, total column ozone in this case.

With the above keywords in your datamanager.txt file, you are ready to start WebWinds.

From the "File" menu, select "Open". From the resulting Data Directory window, double click "Earth", then "TOMS MONTHLY". Under "Data Source Descriptions", double click "April, 1998 TOMS". Next click the "Load" button in the object window that appears and watch the progress bar at the top turn green. You should then have the following Data Object

Next, select "Image" from the "Tools" menu on the Desktop. You can also select a "Slider" from the same "Tools" menu. Once the "Status" of the TOMS object is "Loaded", drag it into the image, then drag it into the slider. Next, link the slider into the image and click the "Magnify" button on the image to get an image similar to the one below. Next, move the (time) slider back and forth. You may (depending on your computer) have to turn tracking off (click the "track" button). You will notice 2 items of interest: the column ozone varies from day-to-day in a way that mimics the weather patterns, especially at high latitudes. This is a well known effect. What is less well known is the source of the features that remain stationary as you move the slider. If these features look vaguely familiar, try selecting "coast 180" from the "Overlays" menu on the image. The resulting image looks like:

Notice that one feature correlates perfectly with the mountains along the west coast of South America.

There are a number of other tools that can be used with this data object. Select "Profile" from the "Tools" menu on the Desktop. Next, drag the image into the profile. Select "Draw profile line" from the image "Menu" button and move the cursor to the image window. While holding the mouse button down, move the mouse across the image. A line will appear on the image which shows the area where the TOMS ozone are depicted in the profile:

Notice that all windows update if the slider is moved.

A number of other tools can be used here including Value View and Track Pixel. When you exit from this WebWinds session, remember to save your work.

WebWinds Home / Oct 5, 2001