This page is an expanding set of tips that will help improve the appearance and usefulness of visualization outputs from Giovanni (and also other applications using Giovanni data, such as MS Excel spreadsheets). The tips will soon be accompanied with instructional videos.
Plotting Tips Table of Contents
An animation consists of successive data maps over time for each increment of time, day or month. Each map is referred to as a frame (like in a movie) of the animation. For any data parameters that have a dynamic color palette (a color palette where the minimum and maximum values for the color scale are determined based on the full range of values in the area selected for the data map), the minimum and maximum values will likely chance for each individual frame of an animation. This is due to the fact that the range of values for each increment of time are likely to change. This particular feature of Giovanni sometimes makes it difficult to visually compare the frames of an animation to observe where and by how much the values are changing, because the color scale is changing.
To have one color scale for all the frames of an animation, simply follow the steps below.
1. Create the animation using the dynamic color palette. It may be useful to watch the animation to determine the best range of values to use.
2. Select the “Custom” option in the Color Bar field.
3. Provide a value for the Parameter Min Value (this is the minimum value for the color scale).
4. Provide a value for the Parameter Max Value (this is the maximum value for the color scale).
5. Click “Generate Visualization” or “Submit Refinements” as appropriate.
The annotated image below of the Edit Preferences pane shows how to do this. Click to see it larger.
Below is a comparison of precipitation over the continental United States for the period April 2009 - July 2009, showing the difference between the dynamic color palette and the custom color palette, which used the values shown above. Each of these images was a frame in the four-month animation. The dynamic (variable) palette is shown at left, the custom (static) palette is shown on the right.
For time-series plotting in Giovanni, a dynamic Y-axis, the dependent variable, is always used. (The X-axis is time.) This means that the minimum and maximum values are determined based on the range of values in the time-series. If time-series for different time periods are created, this can make comparing these time-series difficult or confusing. Giovanni has the capability of customizing the Y-axis to have a user-defined range of values.
To customize the Y-axis values for a time-series, simply follow the steps below.
1. Select the “Time Series” option under “Select Visualization”.
2. Click “Edit Preferences”.
3. Provide a value for the minimum value of the Y-axis in the “Y-Axis Minimum” field
4. Provide a value for the maximum value of the Y-axis in the “Y-Axis Maximum” field.
5. Click “Generate Visualization”.
The same steps can be followed after generating a time-series first, and will appear below the map. Provide the minimum and maximum values and then click “Submit Refinements”. Note that once you have provided a Y-axis minimum and maximum value, the same values will be used for any time-series that are created subsequently for different time periods.
The image below illustrates the steps described above. (Click on this image to see it larger). Below the image is a comparison of annual time-series of Florida precipitation for the years 2008 and 2009, showing how the Y-axis customization provides a better way to view the data.
Florida annual precipitation in 2008 and 2009 with dynamic Y-axis (different minimum and maximum values)
Florida annual precipitation in 2008 and 2009 with dynamic Y-axis (identical minimum and maximum values)
The easiest way to make an area of interest with the Giovanni area selection map is just to click on the map and drag to select the area. However, for smaller areas, it is easier to use a smaller area than the global map and then use that selected area to select the area of interest. This can be done with the “Zoom” tool, which has an icon that looks like a magnifying glass with a plus sign (+) in the middle.
To choose the area using the Zoom tool, follow the steps below.
1. Click on the Zoom tool icon.
2. Click and drag on the area selection map. As soon as the mouse button is released, the map will zoom in to the selected area.
3. Now click on the area selection tool icon (the red box with the plus sign (+) in the lower right corner). Click and drag on the map to choose the exact area of interest.
Another way of using the color palette is to see detail in the data on a lat/lon data map. One of the main types of data that this will be useful for is ocean radiometry (“ocean color”) data, which uses a predefined color palette for many parameters, such as chlorophyll-a. However, in many areas of the ocean, particularly the open ocean, the pre-defined palette may not show detail for low concentrations, because it has values ranging up to very high chlorophyll concentrations.
To adjust the color palette to see detail, simply follow the steps below.
1. Select an area of interest and a parameter, and plot the area with the default plotting parameters. This may be either a pre-defined or dynamic color palette.
2. If most of the area is all one color or one or two colors – which for low concentrations of ocean chlorophyll will be magenta and blue – then choose a much smaller range of values for the color palette. In areas of the ocean where the chlorophyll concentration is very low, a range from 0.08 to 0.5 could be a good choice. If there are higher values, they will all appear as orange and red, but the detail will still appear for the low range of values.
3. Select the “Custom” option in the Color Bar field.
4. Provide a value for the Min Value (this is the minimum value for the color scale).
5. Provide a value for the Max Value (this is the maximum value for the color scale).
6. Click “Generate Visualization” or “Submit Refinements” as appropriate.
Note that for chlorophyll-a concentration, the pre-defined scale is actually a logarithmic scale. The customized color scale will be a linear color scale. It may take a few tries to come up with the best range of values to display the data.
If several years of time-series data generated by Giovanni are imported into an Excel spreadsheet (see the video “How to put Giovanni time-series data into an Excel spreadsheet” on the DICCE Resources Page
), then it is possible to create seasonal or monthly time-series for specific months or seasons.
To do this, follow the steps below.
1. If the data has been imported following the instructions in the video, it is then possible to copy the values for a month, and the month label, from the adjacent fields in the spreadsheet. Then paste them into a new pair of columns.
2. Do the same thing for each pair, the month and the value for the month, into successive rows.
3. The new pair of columns will then consist of the values for the same month in successive years, such as March 2000, March 2001, March 2002, etc.
4. Then plot the values as an X-Y plot using the Excel function.
For seasons, create a function in Excel that averages cell values. Then select the three cells for a season (such as the values for April 2001, May 2001, and June 2001) and let the AVG Excel function in the cell calculate the average value. Put a label in the adjacent cell, such as “Spring 2001”. Do the same thing for each set of values for the same season of each successive year. This will create seasonal averages from the time-series values.