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samrae
(Stranger )
06/22/12 07:38 AM
NDVI - preprocessing Report this article as Inappropriate to us !!!Login to Reply

Hi, I would like to know if any pre-processing is required when creating an NDVI?  I have tried using SPOT and LANDSAT data and using the NDVI function, but the results seem low for some of the areas (e.g. 0.25 for forested areas)? Does this seem strange, and what values should water read?





LColson
(Stranger )
07/05/12 07:20 AM
Re: NDVI - preprocessing new [re: samrae]Report this article as Inappropriate to us !!!Login to Reply

NDVI ranges from -1 to +1. Green vegetation is closer to +1. Sometimes it is converted to digital numbers (e.g., 8-bit 0-255), but that is not as accurate. If you are using ENVI, you could run the simple Landsat calibration and then calculate NDVI, but the amount of pre-processing depends upon the atmospheric conditions of the scene and the degree of accuracy you need. I'd check out a few journal articles for more advice.





pbeaty
(Stranger )
11/29/12 04:49 AM
Re: NDVI - preprocessing new [re: samrae]Report this article as Inappropriate to us !!!Login to Reply


Depends on what you are doing with the NDVIs, but the formula has a mild normalization of the data when the atmospheric conditions are somewhat similar when comparing two different images of similar times in the region’s phenological cycle. There is some imperfection as we have to go through the atmosphere twice (light down to the ground, and them back up to the sensor), all the while the near infrared light trying to get past the water in the atmosphere (which absorbs NIR light), and other elements in the atmosphere). In general:


 


-1 to 0 value = no chlorophyll activity in the leaf’s spongy mesophyll (no photosynthesis activity)



0 to 1 values = chlorophyll activity in the leaf’s spongy mesophyll (photosynthesis occurring)






DrGuy
(Stranger )
12/05/12 07:07 AM
Re: NDVI - preprocessing new [re: pbeaty]Report this article as Inappropriate to us !!!Login to Reply

A few notes on NDVI ranges:


Very few materials and soils in a scene will have negative NDVI values, and if they do, they will likely only be slightly negative.  The only thing offhand that might have significant extent and be (slightly) negative would be water, as it would likely be more reflective in the red vs. the NIR.  However, most soils and vegetation (whether live or senescent) will be more reflective in the NIR than the red portions of the spectrum.  I have a paper where I analyzed the NDVI ranges of a number of soils- see https://www.soils.org/publications/sssaj/articles/73/5/1545


Most natural surfaces, excepting water, will have NDVI ranging between 0-0.98.  The median value of surface soil layers that I tested using Brown et al. (2006)'s spectral library (http://www.sciencedirect.com/science/article/pii/S0016706105001564) had NDVI =  0.15, but the total range was between 0.02 and 0.38.


As for preprocessing- it is generally best to convert your data from DNs to surface reflectance.  If you're using Landsat and have access to a Linux machine, then I recommend using LEDAPS (http://ledaps.nascom.nasa.gov/) for this, as it is free and does a good job.  LEDAPS is based upon 6S code, also freely available from the University of Maryland.  You can also use proprietary MODTRAN or MODTRAN-based methods such as ATCOR or FLAASH to do this, provided you have the necessary licenses.  I highly advise against using QUAC in ENVI unless you have hyperspectral imagery.


If you need a "quick-and-dirty" calibration then I suggest converting from DN to top-of-atmosphere reflectance (in ENVI this would be Landsat Calibration to Reflectance), just be aware that any aerosols and high altitude clouds will be biasing your data.





DrGuy
(Stranger )
12/05/12 07:12 AM
Re: NDVI - preprocessing new [re: DrGuy]Report this article as Inappropriate to us !!!Login to Reply

Another thing- you may want to try to "Google Truth" your study area in Google Earth to see if your forest is sparse or sense tree cover and if you're seeing a lot of dry sensecent vegetation or soil.  Also don't forget to look at seasonality- you may want to look at MODIS NDVI time series trends of the area. Also, be aware the land cover data sets, like any classification, are not 100% accurate.





Xone
(Stranger )
02/08/13 11:14 PM
Re: NDVI - preprocessing new [re: LColson]Report this article as Inappropriate to us !!!Login to Reply

You might try to "Google Truth" the research area within Yahoo World to find out should your do can be sparse or perhaps impression sapling deal with in case you happen to be viewing lots of dry sensecent plants or perhaps dirt. If you want some sort of "quick-and-dirty" calibration then i advocate changing via DN to top-of-atmosphere reflectance.


 


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jesspinto
(Stranger )
03/23/13 02:30 AM
Re: NDVI - preprocessing new [re: samrae]Report this article as Inappropriate to us !!!Login to Reply

A PDF on creating a NDVI  http: // www(dot)globe(dot)unh(dot)edu/MultiSpec/NDVI(dot)pdf


 


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