A Simple Data Visualisation
When you first start Metview, you will see something like this:
XXXX
This sort of window is called a Metview desktop.
Copy a GRIB data file into your Metview directory (~/metview); if you are attending the training course at ECMWF, then you can instead type the following command in a terminal window:
cp ~trx/mv_data/t1000.grb ~/metview
Otherwise, download the file here XXXX.
You should now see a new GRIB icon in your Metview window:
XXX
If it does not appear immediately, press F5 (or View | Reload).
When you right-click on an icon, a context-sensitive menu appears. XXXX
Choose examine to quickly see the structure of the GRIB file. XXXX It contains multiple fields, each listed in the left-hand panel of the GRIB Examiner. We will look more closely at this tool later, but for now close it.
To visualise this data, right-click on its icon and select visualise. You will now see the Display Window. Its toolbars can all be moved, docked, undocked and hidden to suit your preferences.
XXXXX
Antialiasing
To the right of the zoom buttons should be the Antialias button. When active, a smoothing is applied to the lines in the plot – it is worth doing although it comes at the cost of a small amount of plotting speed. This setting will be remembered the next time you visualise data. Note that the antialiasing is not carried through to the various export image formats (see later) – it is active only in the interactive window.
Zooming in a Plot
The diagram to the left shows the Zoom toolbar at the top of the Display Window. Click the Zoom button to enter ‘zoom mode’. Now you can select an area by dragging with the left mouse button. You can zoom in as many times as you like. In order to ‘undo’ or ‘redo’ a zoom, click the Zoom out or Zoom in buttons respectively. The Zoom stack provides quick visual access to the current zoom history. Notice that when a new area is selected, the contours are recalculated - you see more detail as you zoom into a smaller area; you may also see more detailed coastlines.
Using the Magnifier
The Magnifier button in the toolbar toggles the magnifier tool on and off. Unlike Zoom, this is a purely graphical enlargement of the plot. It is usedmainly to inspect small text such as contour labels. The magnifying glass can be moved and resized using the mouse, and the magnification scale on its left-hand side can also be adjusted.
Animation Steps
The Frames tab to the right of the plot shows the set of fields contained in the GRIB file. You can move between fields by clicking within this tab, by using the animation control buttons or by using the cursor keys. Note that each plot is computed only when you select a field. Generated plots are cached, indicated in the Frames tab through shading. This can quicken their rendering when the plots are complex. Note that modifying the plot in any way (such as zooming) clears the cache.
Layer Meta-data
There are three tabs in this panel - Frames, Layers and Data. We will look at Layers soon, but for now select the Data tab.
This reveals a page of meta-data for the current layer, including a histogram.
We will investigate these features in more detail later, but for now close the Display Window.
Creating and Editing an Icon
Let’s customise the coastline plotting attributes.
First, create a new Coastlines icon. You can right-click within the Metview desktop to obtain a context menu from where the option Create new icon is available (shortcut: CTRL-N).
XXX
XXXX
This brings up a dialogue from where you can find the Coastlines icon; either double-click the icon, or drag it onto the desktop to create a new instance. Close the dialogue.
Edit the newly-created icon by either double-clicking on it or else right-click, edit (double-clicking an icon performs the edit action for most icon types). This brings up the icon editor for coastline plotting. All user-selectable parameters for plotting coastlines are here. Set the following parameters:
| Map Coastline Thickness | 2 |
| Map Coastline Land Shade | On |
| Map Coastline Land Shade Colour | Cream |
For colour-based parameters, there are two small arrows - the one on the right reveals a drop-down list of predefined colours (use this one); then one on the left reveals an advanced colour selection tool.
XXXX
After making these changes, click the Ok button to save and exit the editor.
Visualise the data again, and drag your new Coastlines icon into the Display Window.
Your Coastlines icon can be dragged into any plot, and later we’ll see how to store useful icons so that they can be easily accessed from anywhere.
Changing the Map Projection and Storing the Area
Metview's default map projection is Cylindrical. However, meteorologists often use other projections when plotting data.
Create a new Geographic View icon and rename it to polar_europe by clicking on its name and editing the text. Edit the icon and change the following parameter:
| Map Projection | Polar Stereographic |
Save the changes and visualise the icon. Drop the GRIB data icon into the Display Window to see it on the new map. It is also possible to visualise the GRIB icon and then drop the Geographic View icon into the plot to achieve the same effect. Have a look at some of the other projections on offer, then go back to polar stereographic. Now we want to set the area used in the view. Although we can interactively zoom into smaller areas in the Display Window, we now want to store a particular area so that we can use exactly the same one again and again.Set the Map Area Definition to Corners and click on the Geography Tool button next to the Area parameter (shown in the picture above).
XXX
This tool helps you define a region.
XXX
Use the Zoom tools to enlarge the European area and use the Area tool to select a region over Europe. Click Ok to save your selection - your choices will now be updated in the Geographic View editor. Click Apply in the Geographical View editor to save everything. Plot your data in this view to confirm that the area and projection are as desired.
Modifying Layers
Now look at the Layers tab again. Drag the shaded Coastlines layer so that it is above the t1000.grb layer – a quick way to mask out the sea points! Imagine looking down through the layers from the top to the bottom in order to understand how they work. You can also select the Coastlines layer and change its transparency value. You can also toggle layers on and off using the checkboxes next to them. Note that these adjustments are not carried through to the various export image formats (see later).
Future versions of Metview will incorporate more advanced plot-editing facilities available directly from the Layers tab. You can close the Display Window again.
XXX