ndvi {ASIP}

R Documentation

Normalized Difference Vegetation Index

Description

Normalized Difference Vegetation Index (NDVI). It is the most widely used satellite image derived index emphasizing on vegetation mapping. This function can also be used to obtain the ground emissivity as well.

Usage

ndvi(directory = getwd(), crop = "n", ext2crop = "none",
  op_directory = directory, emissivity = "n")

Arguments

ext2crop, crop, directory	Same as mentioned in arvi.
op_directory	Specify the output directory (within double quotes). By default the input satellite image directory will be selected as the output directory.
emissivity	Assign value "y" to get the emissivity of the surface as a seperate raster file.

Value

File named ndvi_'date of satellite image acqisition'.tif in the input folder

Note

1. NDVI= (r_nir - r_red)/(r_nir + r_red)

where, "r_" denotes TOA reflectance band.

2. Emissivity is calculated according to Van De Griend and Owe (1993).

3. Emissivity values will be absent on pixels with negative NDVI values. This can affect the land surface temperature results (not at-sensor brightness temperature) as well. So, please review the study region and the requirements before using this function.

4. Other important notes are mentioned in custom.eqn.

References

1. Huetet A R and Jackson R D (1987) Suitability of spectral indices for evaluating vegetation characteristics on arid rangelands, Remote sensing of environment, 23(2), pp:213-232. doi: 10.1016/0034-4257(87)90038-1.

2. Van De Griend AA, Owe M (1993) On the relationship between thermal emissivity and the normalized difference vegetation index for natural surfaces. Int J Remote Sens 14:1119–1131. doi: 10.1080/01431169308904400

Examples

library (raster)
library (rgdal)
# Finding the path of the sample satellite image directory.
# User may define paths directly like "/home/ur_folder" or "C:/ur_folder"
path <- system.file ("TM_sample", package = "ASIP")
shapefil <- paste0 (path, "/test.shp")
ndvi (directory = path, crop = "y", ext2crop = shapefil)

---