How to calculate NDVI is a big problem. I am confused very much. There are three methods about this.
Method one
Calculate using function NDVI=(NIR-RED)/ =(NIR+RED)
http://grindgis.com/blog/vegetation-indices-arcgis
Step 1: Calculating Reflectance value from the Satellite data
ρλ’ = M ρQ cal + A ρ
ρλ’ = TOA planetary reflectance, without correction for solar angle. Note that ρλ’ does not contain a correction for the sun angle.
M ρ = Band-specific multiplicative rescaling factor from the metadata (Reflectance_Mult_Band_x, where x is the band number)
A ρ = Band-specific additive rescaling factor from the metadata (Reflectance_Add_Band_x, where x is the band number)
Q cal = Quantized and calibrated standard product pixel values (DN).
Step 2: Correcting the Reflectance value with sun angle
ρλ = ρλ ‘/cos θSZ = ρλ ‘/sin θSE
ρλ = TOA planetary reflectance
θSE = Local sun elevation angle. The scene center sun elevation angle in degrees is provided in the metadata (Sun Elevation).
θSZ = Local solar zenith angle; θSZ = 90° – θSE.
Step 3: Calculating NDVI from 4th and 5thbands
Find NDVI we use the formula of
NDVI=(NIR-RED)/ =(NIR+RED)
We called this result “NDVI1”
Method two
Download it directly from website. https://espa.cr.usgs.gov
We called this result “NDVI2”;
Method three
Download Landsat 8 Surface Reflectance data product on website http://earthexplorer.usgs.gov/.
Use formula of
NDVI=(NIR-RED)/ =(NIR+RED)
We got “NDVI3”.
Bur for the same pixel, three results are different. For example LC80420352015194LGN00, this is the location where grape planted(Geo: 36°27’31.41″N,119°34’48.798″W).
REFLECTANCE_MULT_BAND_n = 2.0000E-05, REFLECTANCE_ADD_BAND_n = -0.100000, sun elevation= 66.80954488.
| Method 1 | Method 2 | Method 3 | |||||
| sr_NDVI | 7263 | cfmask | 0 | ||||
| BQA | 20480 | sr_EVI | 4808 | cfmask_conf | 1 | ||
| B1 | 10695 | sr_band1 | 236 | toa_band1 | 1239 | ||
| B2 | 9765 | sr_band2 | 315 | toa_band2 | 1037 | ||
| B3 | 9160 | sr_band3 | 573 | toa_band3 | 905 | ||
| B4 | 8471 | sr_band4 | 549 | toa_band4 | 755 | ||
| B5 | 20246 | sr_band5 | 3298 | toa_band5 | 3317 | ||
| B6 | 15043 | sr_band6 | 2254 | toa_band6 | 2185 | ||
| B7 | 10680 | sr_band7 | 1341 | toa_band7 | 1236 | ||
| B8 | 8814 | ||||||
| B9 | 5051 | toa_band9 | 11 | ||||
| B10 | 28974 | ||||||
| B11 | 26161 | ||||||
| sr_cloud | 16 | ||||||
| 0.6291 | 0.7263 | 0.7146 | |||||
What’s the different among the three ndvi results?
For the peach orchard, I compared the NDVI2 and NDVI 3 in 2013-2015 using the average value of nine pixels. It seems that NDVI 3 is smaller that NDVI 2.
This might be cause by different version of products.