Today, we begin to speak of the relationships between two+ bands. Band ratios Today, we begin to speak of the relationships between two+ bands.

But why? To highlight certain features To “correct” for topography

How? Simplest ratios are simply one band divided by another. Though they get a bit trickier in reality. The trick is truly knowing how different materials reflect – and using that to your advantage. Let’s start with vegetation.

Vegetation Ratios Well, what bands tell us things about vegetation?

How can we use this with digital imagery? Many vegetation indices are based on accentuating the DIFFERENCE between red and NIR reflectance in image pixels Big Difference Small Difference

Ratio-based Vegetation Indices RVI = NIR/Red Simplest ratio-based index is called the Simple Ratio (SR) or Ratio Vegetation Index (RVI) High for vegetation Low for soil, ice, water, etc. Indicates the amount of vegetation Reduces the effects of atmosphere and topography – because the ratios should be the same regardless of sun intensity!

Problem with SR Division by zero Wide range of possible values depending on amount of red reflectance These problems addressed by development of the NDVI

Normalized Difference Vegetation Index NDVI = (NIR – Red)/(NIR + Red) Ranges from -1 to 1 Never (rarely?) divide by zero Indicates amount of vegetation, distinguishes veg from soil, minimizes topographic effects, etc. A good index! Does not eliminate atmospheric effects!

But… what if your field area has a lot of exposed dirt/rock?

Indices get “tuned” to try to reduce these problems. E.g., Soil Adjusted Vegetation Index (SAVI) Uses a soil background “fudge factor” SAVI = [(NIR – Red)/(NIR + Red + L)] * (1 + L) L is a soil fudge factor that varies from 0 to 1 depending on the soil. Often set to 0.5 as a default. http://wiki.landscapetoolbox.org/doku.php/remote_sensing_methods:soil- adjusted_vegetation_index

OK First, let’s goto ERDAS and look at Ellensburg in NDVI (raster, unsupervised, indices) Then, scatter plots!

Scatterplots Raster – supervised – signature editor Select Signature Editor In signature editor Select Feature Then Create Feature Space Layers Input Raster Layer (navigate to source file) ( the output image automatically is entered in the Output window) In the Create Feature Space Image window click on the layers you wish to plot in the Feature Space Layers window (to do this click on the tabs on the right under FS Image holding the shift key down to select the spectral plots you wish to generate) Tick output to viewer The plots will appear on the screen - See more at: http://learningzone.rspsoc.org.uk/index.php/Learning-Materials/Image-Processing-for-ERDAS/1.3.-Two-Wavebands-Scatterplot#sthash.4dLo6Gkw.dpuf And look at scatterplots. On to ERDAS to calculate a NDVI.

Class exercise 1 A flood index along the Nile River in Egypt. Which bands do you choose? Assume Landsat 8

Class exercise 2 Mineral mapping in Australia Assume you can see, in roughly equal portions, rocks, soils, and scrubby vegetation.

Back to ERDAS to…. Look at some of the canned ratios. Raster – unsupervised - indices