Landsat 8 Lowest Valid Value (Dark Object/Scatter) Attribute Table Method & Examples
Landsat 8 Lowest Valid Value Attribute Table Method
The Landsat 8 Lowest Valid Value Attribute Table Method was the original scatter deduction method developed by GIS Ag Maps to establish a Landsat 8 atmospheric scatter value (low value; dark object; haze). The method uses the lowest DN for an entire Landsat scene as long as there is not a break of values ≥ 100 beyond it (greater than it in value; see examples below) in the low end of the histogram - the method may or may not establish the very lowest value in the histogram as scatter (easier understood by looking at examples below). Based on a typical solar zenith cosine value, a break of 100 corresponds to approximately .0025 reflectance units (a quarter percent); it is more likely that a DN value associated with a break such as this is affected by something nonphysical and is erroneous (a break of over 900 is shown in a histogram example below). Using the Lowest Valid Value method establishes a limit on how much a low value can be detached from higher values (it has never been a rule in dark object subtraction [DOS] that the very lowest value should necessarily be used). We recommend that Lowest Valid Value reflectance for the red band be used as the basis (the starting scatter) for relative scatter for the other bands (blue, green, and near infrared); by using the Relative Scatter Calculator on this website. This GIS Ag Maps method has been cited in Remote Sensing of Environment and retrieved good results when applied to NDVI.
Below are green band Lowest Valid Value examples (though we recommend the red band to be used as starting scatter for relative scatter). The Lowest Valid Values from left to right are: 6824 (lowest), 6595 (second lowest; break of over 900 to lowest value), 6972 (lowest), 6942 (second lowest; break of over 300 to lowest value), 6780 (lowest), and 6914 (third lowest; break of over 200 to second lowest value). Based on a typical solar zenith cosine value, a break of 100 corresponds to approximately .0025 reflectance units (a quarter percent).
(Top value in attribute tables is lowest in entire Landsat 8 scene; examples below are from Landsat 8 green band.)
* It is important to note that a gap of 100 DNs corresponds to different gaps in TOA reflectance because TOA reflectance is partially based on the cosine of the solar zenith angle - as the cosine becomes smaller, a gap of 100 DNs will correspond to more reflectance units. In an extreme example, a cosine of .50 will result in a 100 DN gap that is .00189 larger than a cosine of .95 (.00400 compared to .00211). A more common range of cosine values of .70 to .90 results in a different of .00063. * If you want to establish the Lowest Valid Value based on a gap in reflectance units, use the value of 0.0025. (For the Tutorial imagery, the starting value would be the same whether using DNs or TOA reflectance.)
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.