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On my previous question on ASTER I need some more information on ASTER for time series analysis and I need to research on building NDVI because when I ran them for each ASTER. Most of them it came out values of -1 and 1. Only one of them I had the values of between -0 and 0.

How or where do I get the information for each value ? What do the values mean ?

Do I need to convert the NDVI to vector to display them or just leave it as it is ?

I also would like to see a map of a NDVI samples. I have searched on Google however I found a few of them.

My project would be mostly likely a crop on a specific area south of Albuquerque, NM USA.

PolyGeo
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PROBERT
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4 Answers4

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Pending there are no errors in your data or calculations, values for NDVI will always fall between -1 and 1. NDVI values are calculated from reflectance, which is the fraction of radiation that is reflected by a given surface. It sounds like your NDVI raster might be in integer format, which means you'll have rounding errors (e.g. raster displays '1' instead of '0.14'). Try converting your raster to floating point.

Radar
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I should first mention that if your ASTER data are not calibrated to reflectance, you are not actually calculating NDVI correctly. Make sure you have the appropriate calibrations in place and are indeed using relectance values rather than DN, radiance or pixel brightness values. Very often products are already served-up to reflectance.

The actual values indicate the health of living plants. High values near 1 indicate healthy green vegetation, while values approaching 0 indicate stressed or non-chlorophyll objects. You will also note that water has very low values due to the absorptance characteristics of the NIR stectrum.

You need to make sure that your NDVI values are either stored in float format (which will store -/+ decimal values) or stretch the values to 0-255 (unsigned 8-bit). If you are calculating NDVI in Erdas, the procedure for stretching is very simple:

Raster > Unsupervised > NDVI > (fill out necessary parameters) > I/O options > stretch to unsigned 8-bit

The benefit of stretching your dataset is primarily to reduce the size of the raster dataset to improve rendering and processing performance. For example, if you are going to perform further digital image processing using your NDVI. Keeping NDVI values ranging from -1 to 1 is appropriate for mapping purposes.

Aaron
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  • Aaron, They were already float point with 32 bit. Sounded like this one may have reflectance. How do I know or see if there is a reflectance on the ASTER ? – PROBERT Apr 14 '14 at 17:50
  • You will have to track down the source of the data and look at the metadata--specifically the processing level. – Aaron Apr 14 '14 at 18:14
  • This ASTER came from the USGS EarthExplorer. It doesn't show the Reflectance information. – PROBERT Apr 15 '14 at 22:23
  • Aaron, so I have this ASTER on my ArcMap and i hovered the pixel and showed me its number in 71.65558 for Red, 76.46397 for Green and 62.063999 for Blue. Does it mean I need to change to 8 bit to remove the decimal ? – PROBERT Apr 16 '14 at 14:42
  • PROBERT, I wish that I could provide more details, however, I am unsure what you are trying to do. Generally, you do not want want to change the ASTER pixel values to integer prior to calculating NDVI. I would simply keep both in double or float format. You can calculate an index resembling NDVI without atmospheric correction, although you cannot assume NDVI values are consistent from ASTER tile to tile without the atmospheric correction. – Aaron Apr 17 '14 at 03:04
  • Aaron, It's ok I have finally figured it out. I used ENVI instead of ERDAS because there were something was not right. I also spoken to another RS Specialist and said to rescale it in ERDAS but again something was not right too. But everything is taken care of. Thanks again Aaron. – PROBERT Apr 17 '14 at 04:15
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As mentioned by Aaron, if you want to analyse a time series of NDVI data, you need to make sure that it is computed in a consistent way (based on the reflectance values and not on the digit numbers). You can find more information here

If you do so, NDVI will be between -1 and +1, and the larger the value the more "green leaves" you are likely to observe (I say green leaves because some healthy fields with lots of flowers, like tulps, could have negative NDVI). The well hydrated parenchym of the leaves reflects the NIR while the red is used by the photosynthesis.

If your input values are in reflectance, it is often admitted to use 0.3 as a minimum vegetation cover value and NDVI saturated around 0.8. Bare soils will be close to zero and water is usually negative because its absorbance increases with the wavelenght (this is why the colours when you dive are bluish).

that being said, you need to use your raster calculator making sure that you have float output in ArcGIS:

float(nir - red)/(nir + red)

with ENVI, from the Toolbox you need to select Spectral > Vegetation > NDVI

radouxju
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  1. Try converting your raster data set to points,use tool (get values to point) which will give you value of NDVI of every pixel to points. Then again you can convert to raster. 

                      OR

  2. Try 'calculate Statistics' in Raster tools.

希里什
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