Recently started to calculate the vegetation coverage, we must first understand the specific meaning of NDVI and calculate it.
Free download of MODIS NDVI product data: http://reverb.echo.nasa.gov/reverb/
The following problems are encountered in the data processing of NDVI products:
Question 1: NDVI is a normalized vegetation index, its value range is -1 to 1 , how to understand?
Method: For land surface coverage, cloud, rain, and snow have higher reflection effects in visible light than near-infrared bands, so NDVI is negative; the NDVI of rocks and bare soil is generally 0; the vegetation coverage is generally greater than 0.
Question 2: Import ENVI for viewing, and found that the image data range before splicing and reprojection is -3000—0.99880, and the image data range after splicing and reprojection is -3000—9988. Did the data value change?
方法 ： ENVI — Basic Tool — Preprocessing — Data-Specific Utilities — View HDF Attribute
(View NDVI band parameters in envi)
It can be found that its valid value range is (-2000—10000), so the data value -3000 is an invalid value, and the other values are multiplied by a factor of 10000 (scale_factor), so the ndvi value did not occur during the splicing and reprojection process Variety. There are also some values, such as 32767, which indicate clouds.
Question 3: According to the above, its fundamental value has not changed, but after all some of the values have been expanded, how to deal with it more reasonably?
Method: ENVI — Basic Tools — Band Math
(Band calculation, remove the negative value)
In the pop-up dialog box Band Math, Enter an expression: (b1 lt 0)*0+(b1 ge 0)*(b1*0.0001).
This formula means: if the value is less than 0, multiply it by 0 to make it 0; at the same time, if the value is greater than or equal to 0, multiply it by a factor of 0.0001.
This is OK. About B1 represents the selected image.
Question 4: How to calculate vegetation coverage through NDVI?
Method: Generally used pixel binary model. Use NDVI value to get vegetation coverage.
公式： fc = (NDVI - NDVIsoil)/( NDVIveg - NDVIsoil)
Take NDVIsoil = NDVImin, NDVIveg = NDVImax.
(1) Get NDVI data
(2) Find out the maximum and minimum values of NDVI images: Right-click in the scroll window — Quick Statistics
For the minimum value, since the vegetation coverage is calculated, the NDVI of the vegetation should be greater than 0, so the minimum value greater than 0 should be selected.
For the maximum value, the maximum value is not 1 (the reflectivity of the red light band is not equal to 0), so the maximum value whose number is greater than 0 and less than 1 should be selected.
Note: The maximum and minimum extraction needs to set the confidence level. If the confidence level is 0, then NDVImin takes the smallest value among all pixels, and NDVImax takes the largest value among all pixels; if the confidence level is 1, then NDVImin takes the pixels near 1% of the cumulative probability of the pixel value in the pixel Value, NDVImax takes the pixel value near 99% (note the cumulative probability of the pixel value: cumulative probability = cumulative number of pixels/total number of pixels), the same is true for others.
Question: How is the confidence level obtained?
(3) Band calculation: Basic Tool — Band match
Enter the calculation formula: float((b1-minb1)/(maxb1-minb1))
Question 5: How to view the "spectral profile" (that is, the spectral profile) of NDVI data?
方法 ： Basic Tool — Layer Stacking
Multi-period data band overlay, then view Z profile