Interpreting NDVI results

[LauraCockrell]

I am looking at a 2005 landsat image, and I used the IAW to calculate the NDVI... easy-peasy! But everything I have read about the NDVI's says that the resulting calculations should be in a range from -1.0 to +1.0 with negative number signifying vegetation loss and the positive values signifying vegetation gain. I am not seeing this. When I create an attribute table from the NDVI raster, the values are on a 0-256 scale. How do I create the NDVI values for the raster in the -1.0 to +1.0 range?

[EricRice]

Hi Laura,

On the IAW options dialog there is an NDVI tab.  Check the box that says, "Scientific Output".  This will give you the values you are looking for. 

Best,
Eric

[LauraCockrell]

Eric-
In the upper left-hand corner of the IAW is the Image Analysis Options. When I open that, the first tab is the NDVI tab. All it says in that box is Red Band: 3 and Infrared Band: 4
Is this an option only on certain licenses?
Thank you for your help
~Laura

[EricRice]

Sorry, I assumed you were using 10.1.  In 10.0 that option did not exist. In 10.0 you will have to use the raster calculator and the appropriate band math to arrive at a floating point raster between -1.0 - 1.0.

NDVI = ((IR - R)/(IR + R))

Best,
Eric

[EricRice]

But everything I have read about the NDVI's says that the resulting calculations should be in a range from -1.0 to +1.0 with negative number signifying vegetation loss and the positive values signifying vegetation gain.

I suppose if you differenced two NDVI's you can tell if you lost or gained vegetation, but the index itself doesn't tell you this.  Below is an excerpt from our documenation.

"This index outputs values between -1.0 and 1.0, mostly representing  greenness, where any negative values are mainly generated from clouds,  water, and snow, and values near zero are mainly generated from rock and  bare soil. Very low values of NDVI (0.1 and below) correspond to barren  areas of rock, sand, or snow. Moderate values represent shrub and  grassland (0.2 to 0.3), while high values indicate temperate and  tropical rainforests (0.6 to 0.8)"

Best,
Eric

[LauraCockrell]

Eric-
Thanks for the advice, I ran the raster calculator to run the NDVI and it maps it out with the values -1 to +1 values. However, I want to get those pixel values, and I don't know how to approach that now! Obviously by changing the original raster to a float to work out the raster calculation, the output NDVI is in float and I can't build an attribute table. Is there another way to find the pixel values of the NDVI equation? Specifically, I ran the NDVI over a large area, and I want to find the NDVI values of the individual habitats within the larger raster. So I guess the mean value of the wetland as a whole is all I really need, maybe there is an easy way to derive mean values of the selected pixels?
Thanks!
~Laura

[EricRice]

If you have a wetland polygon you can use it as a zone in the Zonal Statistics as Table tool, which would give you the mean value (among other statistics) from the NDVI layer you created.

Best,
Eric