* EXAMPLES INCLUDED BELOW *

IMPORTANT: YOU CAN NOW USE FREE QGIS (VERSION 3; click to download from this website) TO DEVELOP A RASTER ATTRIBUTE TABLE TO DETERMINE THE FREQUENCY 50 VALUE AND USE THAT VALUE AS OPPOSED TO THE BIN 5 VALUE, IF YOU PREFER (DESCRIBED BELOW).

Landsat 8 & Sentinel-2 Frequency 50 Attribute Table Scatter Method and

Landsat 8 QGIS Histogram Bin 5 Scatter Method (Sentinel-2 has a separate scatter method for QGIS)

The Landsat 8 & Sentinel-2 Frequency 50 scatter reflectance method was developed by GIS Ag Maps for ArcGIS users, or other GIS software users where the program can produce a raster attribute table - the method is based on the raster attribute table. The Bin 5 scatter method is designed for free QGIS software users because QGIS cannot produce a raster attribute table (if a raster attribute can be produced in QGIS, simply start using the Frequency 50 method); the Bin 5 method produces a scatter reflectance value similar to the ArcGIS Frequency 50 method. (QGIS software can be downloaded from this website here.) Both methods are quite systematic. The Frequency 50 & Bin 5 scatter amounts are usually not right at the base of the histogram, but are very close. It is an advantage that both methods produce similar surface reflectance, because GIS work can be done in tandem with the different software. After the reflectance is calculated from the Frequency 50 or Bin 5 value, we recommend deducting .008 (similar to the .01 Chavez [1996] deduction amount, but based on research here with Landsat 8 and Sentinel-2) to calculate starting scatter for Landsat 8 or Sentinel-2 band 4 (red) and then using the Relative Scatter Calculator for other bands. By deducting .008, the scatter value will have .008 surface reflectance (0.8 percent) as opposed to zero reflectance. (The .008 deduction is based on research here with Landsat 8 data that shows the average and median difference between the Frequency 50 and Lowest Valid Value reflectance for 34 images throughout the year [10 of 12 months; included a significant amount of a variety of surfaces, including agriculture, mountain, desert, and water] were both .008.) Results comparing Landsat 8 and Sentinel-2 Frequency 50 surface reflectance to USGS Landsat 8 Algorithm surface reflectance are listed on this website (scroll to bottom of page).

The Frequency 50 scatter value is visually deciphered from the ArcGIS attribute table as the value where the frequency is precisely 50 or the closest lower frequency to 50 (see examples below). The Frequency 50 attribute table method applies to Landsat 8 and Sentinel-2. The red band (Landsat 8, band 4) is recommended as the starting scatter. There may be a small (but negligible) amount of ambiguity when selecting a starting scatter amount with this method - but any difference is negligible and insignificant. A suitable value will be obvious in nearly all cases. THE METHOD IS EASIER UNDERSTOOD BY EXAMPLES BELOW. (Sentinel-2 example are shown separately at bottom of page.)

The Bin 5 scatter value is established from the QGIS histogram processed as shown in tutorial (this preliminary processing is simple but is critical [need to apply the Translate tool to convert zero values to no data and produce Int16 data]). The Bin 5 scatter method selects the lower value (left side of bin) from the lowest bin with at least a frequency of 5 that does not have a bin to the right of it (greater value bin) that has a frequency less than 5 (keeps value off statistical tail; see second example below). (It is important to keep in mind that QGIS histogram do not represent all values and frequencies are generalize, but the histogram is suitable for this application.) THE METHOD IS EASIER UNDERSTOOD BY EXAMPLES BELOW.

BOTH METHODS ARE DESIGNED TO KEEP VALUES OFF THE SIGNIFICANT LANDSAT 8 STATISTICAL TAILS

LANDSAT 8 EXAMPLES (SENTINEL-2 EXAMPLES SHOWN AFTER)

Landsat 8 imagery is downloaded in digital number (DNs) format. As explained in tutorial, DNs are converted to reflectance by first applying following equation: ([DN x .00002] - 0.1); then divide result by the cosine of the solar zenith. The solar zenith can be calculated by inputting the Sun Elevation from the .MTL file into the Cosine of Solar Elevation Calculator on the top menu.

ArcGIS Attribute Table Frequency 50 Value is 6002; QGIS Histogram Bin 5 Value is 6009 (the value is not 5983 because the frequency is less than 5).

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 6327; QGIS Histogram Bin 5 Value is 6286 (the value is not lower because there would be a bin with a frequency less than 5 greater than it [to the right of it]).

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 6114; QGIS Histogram Bin 5 Value is 6082 (the value is not 6030 because there would be a bin with a frequency less than 5 greater than it [to the right of it]).

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 5970; QGIS Histogram Bin 5 Value is 5969 (the value is not less because both lower bins have a frequency less than 5).

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 6019; QGIS Histogram Bin 5 Value is 6051 (the value is not less because both lower bins have a frequency less than 5).

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 6116; QGIS Histogram Bin 5 Value is 6119 (lowest bin is used because it has a frequency ≥ 5).

 

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 6704; QGIS Histogram Bin 5 Value is 6696 (the value is not 6670 because the frequency is less than 5).

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 6214; QGIS Histogram Bin 5 Value is 6229 (the value is not 6203 because the frequency is less than 5).

 

 

 

 

 

 

 

 

ArcGIS Attribute Table Frequency 50 Value is 6004; QGIS Histogram Bin 5 Value is 6002 (the value is not 5976 because the frequency is less than 5).

 

 

 

 

 

 

 

 

 

Sentinel-2 Frequency 50 Attribute Table Scatter Method

AS IS THE CASE WITH LANDSAT 8, WE RECOMMEND THE SENTINEL-2 FREQUENCY 50 REFLECTANCE VALUE SHOULD HAVE .008 DEDUCTED FROM IT TO CALCULATE STARTING SCATTER.  

Important: Sentinel-2 pixel values represent Top of Atmosphere (TOA) reflectance units x 10,000 (for example, a pixel value of 5000 = 0.5000 TOA reflectance units [5000/10000], a pixel value of 250 = .0250 reflectance, a pixel value of 3482 = 0.3482 reflectance, and so on).

The graphic below shows actual low portions of Sentinel-2 band 4 (red) attribute tables from ArcGIS (the lower values to left represent exceptionally clear atmospheres). Frequency 50 values from left to right are: 179, 147, 145, 188, 236, 223, 295, 287, and 269. (The second and third tables from the left represent extremely clear atmospheres.)

 

 

References

Chavez, P.S., Jr. 1996. Image-based atmospheric corrections–revisited and improved. Photogrammetric Engineering and Remote Sensing 62(9): pp.1025-1036.

Chavez, P.S., Jr. 1988. An improved dark-object subtraction technique for atmospheric scattering correction of multispectral data. Remote Sensing of Environment 24: pp.459-479.