USGS MIPS Vratio Documentation

USGS MIPS Vratio Documentation

This program is used to generate a ratio between the red and near-infrared spectral bands with corrections for the sensor gain, offset, and solar exoatmospheric spectral irradiance values (Ei). By correcting for these sensor and spectral band dependent parameters the output vegetation ratio image can be used for comparing with vegetation ratio images that are collected not only at different times, but also collected by different sensors. If the input DN values are ratioed without these corrections the output values can be compared only with other values generated from the same sensor (not even with the same sensor if its gain and offset values have been changed). Caution, this program assumes that either an atmospheric haze correction has already been applied; or that there is no atmospheric effects in the data.

If the assumption is made that the haze/Atmospheric effects have either been corrected or that these two bands are not affected by it then the output ratio values can be compared in a temporal basis, as well as between sensors. Remember, relatively little scattering occurs in the red and near-infrared bands in comparison to the blue and green bands; however, they can be affected by absorption, depending on the water vapor in the air (humidity). If TM data are being used the user can use the program TMHAZE to get the additive component, which is mostly equal to the scattering caused by the path radiance, and include it as the offset value so that this program will automatically correct/subtract it out before computing the vegetation ratio. Currently (6/90) for MSS, SPOT, and AVHRR data the user must use the histogram method to select the dark-object haze value if a correction is desired/needed for these data --- Caution that the dark object subtraction method overcorrects the data at times and uncorrected data may give better results under this condition.

The output ratio created is the following:

  VRATIO = (DNRED - ROFF) / (DNNIR - IROFF) * (IRGAIN / RGAIN) * (IREI / REI)

where

  DNRED is a dn from the red band
  ROFF is an offset for the red band
  RGAIN is a gain value for the red band
  IREI is the solar exoatmospheric irradiance for the red band

  DNNIR is a dn from the infrared band
  IROFF is an offset for the infrared band
  IRGAIN is a gain value for the infrared band
  IREI is the solar exoatmospheric irradiance for the infrared band

The vegetation ratio is created differently for the AVHRR data because the gain and offset coefficents given for it convert the DNs directly to reflectance, rather than radiance. Therefore, the output ratio created for it is as follows:

  VRATIO = (DNRED * RGAIN + ROFF) / (DNNIR * IRGAIN + IROFF)

This is equivalent to the ratio of the spectral radiance of the red band divided by the spectral radiance of the near-infrared band. Notice, the output will differ from a straight ratio of the image DN values. The offset, gain, and solar irradiance (E values) corrections allow for the output values to be compared between sensors. If only the same sensor is to be used and the comparison is in a temporal sense, then these corrections are not necessary; unless the offset and gain values have been changed between the time of the two images being compared.

Also, if zero atmospheric haze is assumed, then this is also equal to the ratio of the reflectance values of these two bands.

Files

This program requires as input three unsigned 8-bit or two 16-bit image files and will generate one unsigned 8-bit image file. The following information is transfered from the input files to the output files(if appropriate):

Image labels
Mosaic labels
Projection labels
History file

Sensor

If the input image was 8-bit data, one the following parameters will need to be selected. Otherwise, the input image data was 16-bit(AVHRR data) and these parameters will not appear.

TM, MSS1, MSS2A, MSS2B, MSS3A, MSS3B, MSS4A, MSS4B, MSS5A, MSS5B, SPOT1A,

SPOT1B, SPOT2A or SPOT2B

These parameters are used to select the sensor that collected the red and infrared bands. The sensors currently available are:

  TM     Landsat 4 or 5 TM

  MSS1   Landsat 1 MSS
  MSS2A  Landsat 2 MSS from 1/22/1975 to 7/16/1975
  MSS2B  Landsat 2 MSS after 7/16/1975
  MSS3A  Landsat 3 MSS from 3/5/1978 to 5/31/1978
  MSS3B  Landsat 3 MSS after 5/31/1978
  MSS4A  Landsat 4 MSS from launch to 3/31/1983
  MSS4B  Landsat 4 MSS after 3/31/1983
  MSS5A  Landsat 5 MSS from launch to 11/9/1984
  MSS5B  Landsat 5 MSS after 11/9/1984

  SPOT1A Spot 1 HRV-1 from launch to 2/15/1989
  SPOT1B Spot 1 HRV-1 after 2/15/1984
  SPOT2A Spot 1 HRV-2 from launch to 2/15/1989 
  SPOT2B Spot 1 HRV-2 after 2/15/1984

  AVHRR Same default values are used for 
        NOAA 6-10. Recall that these values 
        are converted directly to reflectance
        rather than radiance values.

Note that when using the default options for the given sensors because the gain and offset values used are the preflight values, which are often INCORRECT! Whenever possible, the actual values should be extracted from the CCT headers and used as input. There is not a default sensor.

Sensor parameters

IRGAIN, IROFF and IREI: These parameters are used to select the gain, offset and solar exatmospheric spectral irradiance of the near infrared band. IRGAIN is the value used by the sensor to collect the NIR band. The default is the preflight coefficient. IROFF is an offset value used by the sensor to collect the NIR band. The default is the preflight coefficient. IREI Solar exoatmospheric spectral irradiance of the NIR spectral band looked at by the sensor that collected the data. The default is the preflight coefficient. NOTE: The units used for the solar spectral irradiance is not important, what is important is that both the RED and NIR values be given in the same units. This is because it is the ratio of the two that is important, so the units cancel out.
RGAIN, ROFF and REI: These parameters are used to select the gain , offset and solar exoatmospheric spectral irradiance of the red band. RGAIN is used by the sensor to collect the RED band. The default is the preflight coefficient. Remember that these values are just estimates because they have changed. If possible, the user should use the values stored in the header file of the CCT containing the image data. The gain values used to correct/convert the data to spectral radiance must also include the gain value used in the destriping procedure, if one was used. For example, the Landsat TM data in the QCAL format includes gain corrections for both the sensor and striping removal. ROFF is an offset value used by the sensor to collect the RED band. The default is the preflight coefficient. REI is the solar exoatmospheric spectral irradiance of the red spectral band looked at by the sensor that collected the data. The default is the preflight coefficient.
FACT: This parameter is used as a multiplication factor used to convert the data to 8-bit (i.e., keep the output ratio results in 0-255 dn range). The default value is 100.