This week’s guided exercise will use EO data to compare seasonal changes in the atmosphere and on land.

For this exercise, we will use the World View tool from NASA, and VegScape explorer from USDA.

Follow the steps below and then join the discussion to compare results and discuss ideas with other learners. Once you feel confident with the tool, you can explore other locations and data sets of your choice.

You may find it helpful to open the exercise in a separate window.

Exercise 1

This first exercise will compare changes in ozone levels in the atmosphere over different seasons.

1. Go to the World View tool. (When you first visit World View you will be offered a tour of its features. You can take this tour if you wish to, but it is not necessary for this exercise).

2. On the left hand side of the screen make sure that under ‘Base Layers’, only the ‘Corrected Reflectance (True Colour) Terra/MODIS’ later is selected. If not, then click on the ‘Add Layers’ plus symbol and add this layer.

3. Next you will need to add another layer to the ‘Overlays’ menu. Click on the ‘Add Layers’ plus symbol and then click on ‘All’. Scroll down until you find the layer called ‘Ozone Aura/MLS’, click on it and select ‘46 hPa, Day’ (this is about two thirds of the way down the menu). You can now see that the layer that have now been added to ‘Overlays’.

4. On the time slider at the bottom of the screen, select January 1st 2015. You should now be able to see the data displayed for this date. Red areas indicate higher ozone levels whilst blue areas indicate lower ozone levels.

5. Take a snapshot of a region of this data set using the camera symbol in the top right-hand corner. Click on the camera icon and an area selection tool will appear on the map. Adjust the area and change the resolution in the drop down menu to ‘5 km’ to keep the file size (shown under ‘Raw Size’) down to a manageable level. Click Download. The image will appear in a new tab. (You can then save this image to your desktop or pictures folder).

6. Now move the time slider to 1st October 2014, you should now see the data displayed for this date. Take another snapshot of the same region as you selected for the previous snapshot, and compare the two ozone images. Make an assessment of whether ozone levels have increased or decreased in different regions of the atmosphere.

7. Describe (in the discussion area below) the changes you have found.

Exercise 2

This exercise will compare vegetation using NDVI (Normalised Difference Vegetation Index).

1. Go to VegScape. On the left hand side of the screen select the ‘Products’ tab

2. In the ‘Type’ drop down menu, select ‘NDVI’ and set the ‘Period’ to ‘weekly’.

3. On the icon menu at the top of the page, select the small map of the USA (in the colours of the US flag). A pop-up box will appear. Click on State, and select California from the drop down menu. Then click on County and select Tulare from the drop down menu. Click submit, and the map will zoom in on the Tulare region of California.

4. Next, in the icon menu at the top of the page, hover over the last icon (of a graph), which should bring up the words ‘Generate NDVI profile at pixel’. Click on this icon.

5. Move your mouse pointer over the map. The pointer should now appear as a cross. Put the cross over an area of southern Tulare which is yellow, and click. A new pop-box should appear.

6. In the pop-up box, select ‘Yearly nevi profile(s) vs Historical median’. Set ‘Begin Year’ as 2010 and ‘Begin Week’ as 01. Set ‘End Year’ as 2014 and ‘End Week’ as 52. Click Submit.

7. After a few moments, a chart will appear, showing the changing vegetation index over the full year for the years 2010 to 2014. Also on this chart, in light blue, will be a plot of the historical median value. Southern California, and particularly Tulare County, has recently experienced its worst drought in generations. You can use this chart to see how the amount of vegetation, as measured through EO techniques, has changed as the drought intensifies in 2013/14. You can see for example that in the early months of 2014, the NDVI value for significantly lower than the historical median.

You can use the tools above to further explore seasonal comparisons. For example in World View you could select the overlay for forests and mangrove, or sea surface temperature, and compare changes over time.