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Right-click on your 'CCMA.obs' ODB Database icon and select examine from the icon menu. This will start the ODB Examiner application that displays the meta-data of the database. By default you should see the Tables tab of the interface showing the ODB table hierarchy. This hierarchy only exists only for the ODB-1 format (so we can see that our 'CCMA.obs' icon represents an ODB-1 database). By clicking on a table name the columns belonging to the selected table are displayed in the right hand side of the interface. There are two more tabs in the interface: the Columns tab showing the list of all the available columns and the SET Variables tab providing the users with the list of pre-defined ODB variables. Explore these tabs and then close the ODB Examiner.

Now right click and examine icon 'AMSUA.odb'. It points to an ODB database (just like icon 'AIREP.odb'). Since there is no table hierarchy to be shown the list of all the available columns is displayed by default (Columns tab).

The data values stored in the ODB can be also inspected in the ODB Examiner (this feature does not work for ODB-1). Just click on the Data tab to see the data values. If there are too many values to be shown the ODB Examiner displays the data items in blocks. The actual size of the data blocks (in terms of rows) can be seen at the bottom of the interface next to the data block navigation buttons. Data blocks were introduced to reduce memory usage since the ODB Examiner has to keep in memory all the values shown in the Data tab.

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In this exercise we will retrieve and plot the brightness temperature values for channel 5 from our 'AMSUA.odb' database. Please open folder 'tb' inside folder 'odb_tutorial_4.1' to start the work.

The ODB Visualiser Icon

The simplest way to plot ODB data in Metview is to use the ODB Visualiser icon. It performs the query, defines which ODB columns should be interpreted as latitude, longitude and value(s) and specifies the plot type (symbol or wind plotting), as well. You can find this icon in the Modules (Plotting) icon drawer.

Create a new ODB Visualiser icon (right-click in the desktop when no icons are selected and use the New icon ... menu) and rename it 'tb_plot'.

First, open its editor and set ODB_PLOT_TYPE to 'Geo Points' to indicate that we want to plot the values on a map.

Second, drop your 'AMSUA.odb' database icon into the ODB_DATA field. This specifies the database for which the query will be performed.

Third, we need to specify the ODB/SQL query and the way the columns are interpreted to generate the plot. We want to perform the following query:

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In the ODB Visualiser interface this query cannot be typed in directly but has to be split into the following individual items:

• ODB_LATITUDE_VARIABLEOdb Latitude Variable: specifies the name of the column holding the latitude data in the SELECT statement (here lat@hdr).
• ODB_LONGITUDE_VARIABLEOdb Longitude Variable: specifies the name of the column holding the longitude data in the SELECT statement (here lon@hdr)
• ODB_VALUE_VARIABLEOdb Value Variable: specifies the name of the column holding the value data in the SELECT statement (here obsvalue@body)
• ODB_WHEREOdb Where: specifies the WHERE statement. In our example it is as follows:
  
  

vertco_reference_1@body = 5 

Last, we have to specify the units of the geographical co-ordinates (here lat@hdr and lon@hdr) in the Odb Coordinates Unit field. It is necessary since Metview requires geographical co-ordinates in degrees, but there is no general way to find out their units in an ODB database. Instead an explicit declaration is needed from the users. Our database stores co-ordinates in degrees. So, to correctly interpret our co-ordinate values Odb Coordinates Unit should be set to 'Degrees' (which is the default value so we do not need to change it).

Having finished the modifications your icon editor should look like this:

Running the Query

Save your ODB Visualiser icon (Apply) then right-click and execute to run the query. Within a few seconds the icon should turn green indicating that the retrieval was successful and has been cached. Now your icon behaves exactly like an ODB Database icon. Right-click examine to look at its content. You can see that the resulting ODB contains only three columns: lat@hdr, lon@hdr, obsvalue@body. By clicking on the Data tab you can even see the data values.

Visualising the Output

Right-click and visualise the icon to plot the retrieved data (please note that you can directly visualise the icon by skipping the execute step). This will bring up the Metview Display Window using the default visualisation assigned to symbol plotting. By default the data values are plotted to the map. Unfortunately, it is not the desired visualisation in our case (we cannot even see the satellite tracks) so we will further customise the plot.

We will change the plot by using markers instead of numbers and change the colour, as well. Let's create a new Symbol Plotting icon (it can be found in the Visual Definitions icon drawer, you may need to scroll the drawers to the right).

Rename it 'symbol' then edit it, by setting the following parameters:

Now drop this icon into the plot to see the effect of the changes.

We used the Symbol Table Mode in our icon and set it to 'Advanced' which enabled us to automatically define intervals with a separate maker type, colour and size. These settings work in a similar way as in the Contouring icon.
Our palette was automatically generated from a colour wheel by interpolating in clockwise direction between Symbol Advanced Table Min Level Colour and Symbol Advanced Table Max Level Colour.
The identifiers of the available symbol markers are summarised in the table below. Please note that the rendering speed of the markers can be significantly different and using a simpler symbol (in terms of rendering) can greatly reduce the plotting time. For example, the usage of marker 3 (plus sign) can result in much faster plot generation than that of marker 15 (filled circle).

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