Sentinel-2 QGIS Base of Histogram Method Rules & Examples
A raster attribute table cannot by produced in free QGIS as it can in ArcGIS, for example, so the Frequency 50 Attribute Table Scatter method (which is a very simple way of establishing atmospheric scatter) cannot be used in QGIS to establish atmospheric scatter.
However, GIS Ag Maps has developed a method customized for QGIS that retrieves a similar scatter reflectance value as the Frequency 50 value. The Sentinel-2 QGIS Base of Histogram method is a simple and systematic method that uses rules that apply to the QGIS Sentinel-2 histogram after the histogram has been properly processed in QGIS (processing is simple but critical; explained in free tutorial) - the method establishes a scatter value that is similar to the Frequency 50 attribute table method (as can be seen in examples below).
THE SENTINEL-2 QGIS BASE OF HISTOGRAM RULES ARE AS FOLLOWS (band 4 histogram needs to be processed in QGIS as shown in tutorial; easy and simple step):
USE THE LOWEST CONNECTED VALUE FROM THE BAND 4 (RED) HISTOGRAM WITH THE FOLLOWING TWO STIPULATIONS: 1) DO NOT USE ANY VALUE FROM BINS THAT ARE SEPARATED FROM THE REST OF THE HISTOGRAM (AS IS SHOWN IN THE FIRST EXAMPLE BELOW); 2) IF THERE IS MORE THAN ONE BIN AT THE CONNECTED LOW END OF THE HISTOGRAM WITH A FREQUENCY LESS THAN 5, USE THE VALUE AT THE LOW (LEFT) END OF THE HIGHEST SUB-5 FREQUENCY BIN (WHICH IS ALSO SHOWN IN THE FIRST EXAMPLE BELOW).
______________________________________________________________________________
THE SENTINEL-2 QGIS BASE OF HISTOGRAM VALUE IS EASIER EXPLAINED BY THE EXAMPLES BELOW (QGIS HISTOGRAMS IN THE EXAMPLES ARE ZOOMED INTO DIFFERENT SCALES).
IMPORTANT: You need to zoom in to where you can see a frequency of 5 and less (may be more or less than this, depending on the particular histogram, but you need to zoom in a lot). This is an uncommon histogram; usual there is a much simpler end to the histogram as is shown in most of the examples below. We start with histogram that explained the rule effectively.
The band 4 base of histogram scatter is 389, which corresponds to .0389 reflectance (divide Sentinel-2 values by 10,000 to convert to Top of Atmosphere [TOA] reflectance), because you ignore the separated bin and use the left edge of the highest sub-5 frequency bin. The .008 amount is deducted from .0389 for a starting scatter of .0309 for the raster calculator; in other words, that point of the histgram will have a reflectance of .008 (.01 equals 1 percent reflectance).
______________________________________________________________________________
The band 4 base of histogram scatter is 287, which corresponds to .0287 reflectance, because you use the left edge of the highest sub-5 frequency bin. The .008 amount is deducted from .0287 for a starting scatter of .0207 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 300, which corresponds to .0300 reflectance, because you use the left edge of the highest sub-5 frequency bin. The .008 amount is deducted from .0300 for a starting scatter of .0220 for the raster calculator.
______________________________________________________________________________
For the next series of examples, the corresponding ArcGIS attribute table has been included.
The band 4 base of histogram scatter is 244, which corresponds to .0244 reflectance (similar to the Frequency 50 attribute table value of 238), because you use the left edge of the lowest connected bin (there is no sub-5 frequency bin). The .008 amount is deducted from .0244 for a starting scatter of .0164 for the raster calculator. Keep in mind, Sentinel-2 imagery can be acquired in parts of tiles - that is why there are zero values. Apply the same rule whether or not an entire or partial tile has been downloaded (try to download tiles that are at least 1/4 the size of an entire tile).
______________________________________________________________________________
The band 4 base of histogram scatter is 300, which corresponds to .0300 reflectance (same as the Frequency 50 attribute table value of 300), because you use the left edge of the highest sub-5 frequency bin. The .008 amount is deducted from .0300 for a starting scatter of .0220 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 299, which corresponds to .0299 reflectance (similar to the Frequency 50 attribute table value of 292), because you use the left edge of the lowest connected bin (there is no sub-5 frequency bin). The .008 amount is deducted from .0299 for a starting scatter of .0219 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 206, which corresponds to .0206 reflectance (same as the Frequency 50 attribute table value of 206), because you use the left edge of the lowest connected bin (there is no sub-5 frequency bin). The .008 amount is deducted from .0206 for a starting scatter of .0126 for the raster calculator.
______________________________________________________________________________
The examples that follow do not included corresponding attribute table, they show a variety of histograms and the base of the histogram values.
The band 4 base of histogram scatter is 238, which corresponds to .0238 reflectance, because you use the left edge of the lowest connected bin (there is no sub-5 frequency bin). The .008 amount is deducted from .0158 for a starting scatter of .0126 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 142 (exceptional clear mountainous area and rare), which corresponds to .0142 reflectance, because you use the left edge of the lowest connected bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it (as explained neare the top of the page, if there are more than one sub-frequency 5 bins, use the low edge of the highest of the bins). The .008 amount is deducted from .0142 for a starting scatter of .0062 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 149 (exceptional clear mountainous area and rare; same location as previous image), which corresponds to .0149 reflectance, because you use the left edge of the lowest connected bin. The .008 amount is deducted from .0149 for a starting scatter of .0069 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 211, which corresponds to .0211 reflectance, because you use the left edge of the lowest connected bin (there is no sub-5 frequency bin). The .008 amount is deducted from .0211 for a starting scatter of .0131 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 401, which corresponds to .0401 reflectance, because you use the left edge of the lowest connected bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it. The .008 amount is deducted from .0401 for a starting scatter of .0321 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 334, which corresponds to .0334 reflectance, because you use the left edge of the lowest connected bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it. The .008 amount is deducted from .0334 for a starting scatter of .0254 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 246, which corresponds to .0246 reflectance, because you use the left edge of the lowest connected first sub-5 frequency bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it. The .008 amount is deducted from .0246 for a starting scatter of .0166 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 250, which corresponds to .0250 reflectance, because you use the left edge of the lowest connected bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it. The .008 amount is deducted from .0250 for a starting scatter of .0170 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 189, which corresponds to .0189 reflectance, because you use the left edge of the lowest connected bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it. The .008 amount is deducted from .0189 for a starting scatter of .0109 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 260, which corresponds to .0260 reflectance, because you use the left edge of the lowest connected bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it. The .008 amount is deducted from .0260 for a starting scatter of .0180 for the raster calculator.
______________________________________________________________________________
The band 4 base of histogram scatter is 200, which corresponds to .0200 reflectance, because you use the left edge of the lowest connected bin; the lowest bin is a sub-frequency 5 bin, but it is the only sub-frequency 5 bin so you use the edge of it. The .008 amount is deducted from .0200 for a starting scatter of .0120 for the raster calculator.
