NDVI

Description

The NDVI task is computes the Normalized Differential Vegetation Index (NDVI) from the visible channels of an AVHRR scene (channels 1 and 2). The NDVI result has been proven to be a good monitor of vegetation changes.

Parameters

Name	Type	Length
Input: AVHRR Visible Layer(s): Input AVHRR visible channel(s) or layer(s)	Raster port	0 - 2
Output: NDVI Layer: Output NDVI channel or layer *	Raster port	1 - 1
InputText: AVHRR Text Layer(s): AVHRR calibration/orbit text segment	Text port	0 - 1

* Required parameter

Parameter descriptions

Input: AVHRR Visible Layer(s): Input AVHRR visible channel(s) or layer(s)

Specifies the AVHRR visible channels (channels 1 and 2). If this parameter is not specified, it defaults to 1 and 2.

Output: NDVI Layer: Output NDVI channel or layer

Specifies the image channel to receive the output NDVI values. To ensure the preservation of NDVI values, this channel must be 16-bit or 32-bit real.

InputText: AVHRR Text Layer(s): AVHRR calibration/orbit text segment

Specifies the text segment (140) that contains AVHRR calibration and orbital data. The image year and day-of-year values are extracted from this text segment. If this parameter is not specified, NDVI searches for the first text segment in the input file.

Details

NDVI computes the Normalized Differential Vegetation Index (NDVI) from visible channels of an AVHRR scene (DBIC) in the input file. Ensure that the first channel specified refers to data from the first visible band in the AVHRR scene, and that the second channel refers to the second visible band. If these are reversed, or if other data is used, the resulting NDVI result will be meaningless.

The output NDVI value is written to the specified output channel (DBOC) in the input file. This channel must be of 16-bit or 32-bit real type to avoid truncating the NDVI result.

The formula used for calculation incorporates both time-dependent radiometric calibration and the NDVI computation itself. Normally, the user should use FIMPORT to import the raw AVHRR data. An extra (6th) channel can then be added to the resulting PCIDSK file if desired. NDVI can then be used to compute the NDVI channel. The NDVI task itself is not sensitive to image geometry, and so can be run before or after geometric correction of an AVHRR scene.

The radiometric calibration that is performed requires various coefficients, the values of which change with the satellite, image date, and channel (1 or 2). In particular, NDVI has been set up with coefficients which are valid only for NOAA-14 and AVHRR imagery from 1997. The NDVI task, however, is implemented as an EASI script, allowing users to easily modify the coefficients and even the equations as desired. The script is $PCIHOME/pro/NDVI.EAS. One of the purposes of this script is to allow users to make modifications where appropriate, because radiometric calibration coefficients may vary over time.

Note that NDVI.EAS reads the input file's AVHRR text segment to extract the image year and day-of-year. These quantities are used to compute the number of days since the launch of the satellite for radiometric calibration. If this information is missing from the AVHRR text segment, the user must manually edit the text segment and enter the information before running NDVI; to do so, use the TEXWRIT and TEXREAD programs. When FIMPORT or MIAVHRR is used to import raw AVHRR data, an AVHRR text segment is automatically created.

Algorithm

Radiometric calibration

Radiometric calibration refers to the process of calibrating digital signal level counts to apparent sensor radiance. The apparent sensor radiance quantity is used in the computation of the NDVI channel. Radiometric calibration is performed on each pixel, with no dependence on location within the image. The equation for computing apparent sensor radiance is:

Lstar(i) = (D(i) - O(i,t)) / G(i,t)

where:

i: channel number (1 or 2)
t: number of days since the launch of the satellite
D(i): raw digital signal level (counts)
O(i,t): calibration offset coefficient (counts)
G(i,t): calibration gain coefficient (counts/W m^-2 sr^-1 um^-1)
Lstar(i): apparent sensor radiance (W m^-2 sr^-1 um^-1)

The O(i,t) and G(i,t) coefficients can be computed as follows:

O(i,t) = C * t + D

where:

A is in units of counts/radiance/day
B is in units of counts/radiance
C is in units of counts/day
D is in units of counts

The values of the A, B, C, and D coefficients vary with the satellite, channel number (1 or 2), and year.

Note: The NDVI task does not actually have a radiometric calibration step whereby apparent sensor radiances are generated as (intermediate) channel values Rather, the radiometric calibration computations are embedded directly within the NDVI computation process. This is done to avoid the loss of precision inherent in writing radiance values into channels of integer type.

Radiometric calibration is performed for channels 1 and 2 only. The thermal AVHRR channels (3, 4, and 5) are not processed at all.

Normalized Differential Vegetation Index (NDVI)

The NDVI channel has been proven to be a good monitor of vegetation changes.

The steps for computing NDVI are as follows:

Radiometric calibration

Assuming that channels 1 and 2 contain raw digital signal level counts, apply the radiometric calibration procedure described earlier to obtain apparent sensor radiances (Lstar(i)).
Compute apparent sensor reflectance

Given apparent sensor radiance as input, compute the apparent sensor reflectance (RHOstar(i)) as follows:
```
RHOstar(i) = [(PI * Ds^2) / (E0(i) * cos(THETAs))] * Lstar(i)
```
where:
- PI: 3.14159...
- Ds: Earth-Sun Distance in A.U. (approximately 1)
- E0: exo-atmospheric solar irradiance (W/m^2/str/um)
  
  = 1605.4 for channel 1 (NOAA-14)
  
  = 1028.7 for channel 2 (NOAA-14)
- THETAs = solar zenith angle (degrees)
- RHOstar(i) = apparent sensor reflectance (%)
Compute NDVI

Given apparent sensor reflectance as input, compute the NDVI channel as follows:
```
NDVI = (RHOstar(2) - RHOstar(1)) / (RHOstar(2) + RHOstar(1))
```
NDVI values range from -1 to +1.
Scale the NDVI values

The NDVI values are scaled to the range, 0 to 20000, using the following equation:
```
Scaled NDVI = (1 + NDVI) * 10000
```
This scaling is used to approximate the GeoComp NDVI pseudo-channel.

In NDVI.EAS, steps 2 and 3 are combined into a single step using the following equation:

NDVI = (E0(1)*Lstar(2)-E0(2)*Lstar(1))/(E0(1)*Lstar(2)+E0(2)*Lstar(1))

Notice that the dependence on the solar zenith angle and the Earth-Sun distance is removed by combining these two steps into a single equation.

User-modifiable values

NDVI has been configured for NOAA-14 and AVHRR imagery acquired in 1997. For other satellites, and/or other image years, users must modify the following quantities in NDVI.EAS:

the launch year and launch day of the satellite
the A, B, C, and D coefficients for channels 1 and 2. These are the radiometric calibration coefficients required to compute the time-dependent gain and offset for both channels 1 and 2.
the E0 (exo-atmospheric solar irradiance) values for channels 1 and 2.

References

Teillet, P.M., "An Algorithm for the Radiometric and Atmospheric Correction of AVHRR Data in the Solar Reflective Channels", Remote Sensing of the Environment 41:185-195, 1992.

Environments	PYTHON :: EASI :: MODELER
Batch Mode	Yes
Quick links	Description :: Parameters :: Parameter descriptions :: Details :: Algorithm :: References