Step 4 - Interpreting the image.

Step 4. Interpret the image.

Let's see what's going on with those colors in your NDVI composite.

We've assigned April and November NDVI values to color channels, so let's look at what colors will result from various combinations of high or low NDVI in each month. The table below gives several possible patterns of high or low NDVI in April and November. Remember that we assigned November NDVI to both red and blue channels, so the first and third numbers are always the same in the Example RGB code column. The Color column shows the color produced by the example RGB code. Finally, an example of the kind of land cover that would show this seasonal pattern of change in greenness is given in the last column.

April NDVI	November NDVI	Example RGB code (November, April, November)	Color	Kind of land cover with this pattern
High	High	250, 250, 250		Golf courses (and large, well-watered lawns), wetlands, and possibly herbaceous riparian vegetation (that is, vegetation that grows next to streams and other fresh water bodies).
Moderate	Moderate	150,150,150		Orchards, possibly woody riparian vegetation.
High	Low	25,250,25		Natural grasslands
Moderate	Low	25,150,25		Woody, perennial natural vegetation, such as shrublands and forests.
Low	High	250,25,250		Irrigated landscaping in housing developments (lots of mature landscaping, larger yards).
Low	Moderate	150,25,150		Irrigated landscaping in housing developments (woody vegetation, greater amounts of rooftop and pavement compared with green vegetation).
Low	Low	25,25,25		Un-vegetated areas.

You can use these colors as a guide to try to pick out areas with each pattern of NDVI in April and November. There will be lots more shades of gray, green, and purple on the map than these, so as you're looking over the map there are a couple of basic principles to keep in mind:

If the April and November greenness are the same, you will get a shade of gray (at the extremes, NDVI of 0,0,0 gives black, and 255,255,255 gives white, and any set of three identical numbers will give a shade of gray).
If the April NDVI is higher than November, you will get a greenish color. The lower the numbers the darker the green will be. The more different the April NDVI is from the November NDVI, the more intense the green color will be.
If the November NDVI is higher than April, you will get a shade of purple. The lower the numbers the darker the purple will be. The more different the November NDVI is from the April NDVI, the more intense the purple color will be.

Now that you have the basic pattern in mind, look for examples of each. Specifically, see if you can find:

Golf course - is it white, as the table above says it should be?
Undeveloped vegetation - is it a shade of green, as the table above says it should be?
Agricultural fields - what color(s) are they? Why might fields that are right next to each other have really different colors?
Housing development - why might vegetation be even greener in November than in April in a housing area? Hint: when do people regularly water their yards?

Remember that you can swipe the NDVI composite to reveal the high-definition Imagery layer to double-check what you're looking at.

That's it for this activity. If you're done early...

Pan and zoom around and use the high resolution Imagery layer to find some undeveloped grasslands, and some undeveloped shrubby vegetation. See if you can see the differences in NDVI between grasslands and the woody shrub vegetation. Are grasslands more seasonal than shrubby areas?