BAND RATIOS Today, we begin to speak of the relationships between two+ bands.

BUT WHY? To highlight certain features To highlight certain features To “correct” for topography To “correct” for topography

EFFECT OF TOPOGRAPHY ON SCATTERPLOTS Grassy fields Bare ground Water/ shade Flat terrain Terrain with topography Terrain without topography Thanks to Robin Weeks

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

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

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

NORMALIZED DIFFERENCE VEGETATION INDEX NDVI = (NIR – Red)/(NIR + Red) NDVI = (NIR – Red)/(NIR + Red) Ranges from -1 to 1 Ranges from -1 to 1 Never (rarely?) divide by zero Never (rarely?) divide by zero Indicates amount of vegetation, distinguishes veg from soil, minimizes topographic effects, etc. Indicates amount of vegetation, distinguishes veg from soil, minimizes topographic effects, etc. A good index! A good index! Does not eliminate atmospheric effects! 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) E.g., Soil Adjusted Vegetation Index (SAVI) Uses a soil background “fudge factor” 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.

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

SCATTERPLOTS On to ERDAS to calculate a NDVI. On to ERDAS to calculate a NDVI. And look at scatterplots. And look at scatterplots. 1.Classifier Icon 2.Select Signature Editor 3.In signature editor Select Feature 4.Then Create Feature Space Layers 5.Input Raster Layer (navigate to source file) ( the output image automatically is entered in the Output window) 6.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) 7.Tick output to viewer 8.The plots will appear on the screen - See more at: Two-Wavebands-Scatterplot#sthash.4dLo6Gkw.dpuf Two-Wavebands-Scatterplot#sthash.4dLo6Gkw.dpuf

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

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

BACK TO ERDAS TO…. Look at some of the canned ratios. Look at some of the canned ratios.