Image
Standardization Frequently Asked Questions
1)
When I run the .gmd file in Imagine’s Spatial Modeler, I
get an error indicating that the input image could not be opened. This error most commonly occurs because the .gmd file does not contain a system path to your input image. The easiest way to fix this is to open the .gmd file in Spatial Modeler, select the input image filename, then save the .gmd file. This will save the correct system path to you input image. Note you will have to do the same for the output image. 2)
Why do all three models include a multiplication factor of
400, and how does it affect the data? All
three models convert DN values to reflectance, which is a ratio of calculated
radiance at the sensor / total irradiance at that wavelength and has values
ranging between 0.0 and 1.0.
Multiplying by 400 stretches this range of values across the potential
range of 1-255 available in 8-bit data.
The stretched range of values are rounded and output as unsigned 8-bit
data in order to create images with smaller file sizes. 3)
Why do I see “spikes” and “gaps” in the histogram for
standardized imagery? The spikes and gaps are due to the stretching and rounding that occurs during the calculation. While it looks unusual it does not change the relative distribution of the data (see figure below).
4)
Why would you not want to include TAU in a COST
standardization? TAUz
in the Chavez (1996) method for atmospheric correction estimates the outgoing
path of radiance affected by the atmosphere.
Some use the COStheta (solar zenith angle) expressed as Tests
using COStheta as a substitute for TAUz at the RSGIS Lab have indicated that
COStheta overestimates TAUz, or the amount of outgoing radiance affected by
the atmosphere. This seems to be
especially true when the following conditions are present: 1) Imagery
captured on especially clear days when TAUz is small (common in the western
US), 2) when the solar zenith angle is high (low solar angle), and 3) for ETM
scenes in more northerly rows (latitudes) where solar zenith angle is higher. 5)
What is the recommended method for choosing Dark Object DN
values? While
there are several ways to choose an appropriate Dark Object DN, the following
outlines the method being used by the RSGIS Lab at USU. a)
Load the imagery with DN values. b)
Open ImageInfo in Imagine. c)
Under Edit Menu, calculate statistics with an X and Y skip factor of
1, and set Ignore Value to 0. d)
Display and examine the histogram.
The DN choice for the Dark Object will be the lowest value at the base
of the slope of the histogram. If the
slope is gradual (i.e. contains very few (< 100 pixels of low DN values)
these should be ignored, and the DN value chosen where slope of the histogram
begins to increase more dramatically.
In the figure below, an appropriate DN value for the Dark Object would
be 39.
6)
What Solar Spectral Irradiance values do you use for
Landsat 5 TM scenes?
7)
What Solar Spectral Irradiance values do you use and where
are they published for Landsat 7 ETM+ scenes? Table
1 presents the Solar Spectral Irradiance values used by the tools on this
website. This table can be found in
Chapter 11- Data Products of the Landsat 7 Science Data Users Handbook
located at: http://ltpwww.gsfc.nasa.gov/IAS/handbook/handbook_htmls/chapter11/chapter11.html
Table 1. Spectral Solar Irradiance Values (Landsat
7 Handbook) References: Chavez, P.S. Jr., 1996, Image-based atmospheric corrections—revisited and revised. Photogrammetric Engineering and Remote Sensing 62(9): 1025-1036. Landsat 7 Science Data Users Handbook, NASA, WWW URL located at: http://ltpwww.gsfc.nasa.gov/IAS/handbook/handbook_toc.html FAQ Last Updated: 07/08/03
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