I am getting very low reflectance values (mostly < 0.1) in Landsat 8 bands 2-7 on different scenes after rescaling from Digital Numbers (DN) to Reflectance. All scenes are in a tropical landscape with a lot of vegetation, secondary and old grown forests, agriculture (rice paddy) and urban areas & infrastructure. I see no reason that the majority of reflectance values should be that low.

I am using the formula for TOA planetary reflectance with correction for solar angle (provided on http://landsat.usgs.gov/Landsat8_Using_Product.php).

For example:

The mean DN value in Band 4 (red band) of my scene is 6937. Putting that into the formula for planetary reflectance

ρλ = ρλ'/cos(solar_zenith_angle)


ρλ' = Mρ * Qcal + Aρ


Qcal = 6937 (DN)
Ap = -0.100000 (REFLECTANCE_ADD_BAND_4)

gives me

(0.00002×6937-0.1)/cos(90-41.71087310) = 0.058

Is it possible, that the Ap value (-0.1) has to be "positive" (0.1) in the formula? If it was positive, my reflectance values would become more reasonable:

(0.00002×6937+0.1)/cos(90-41.71087310) = 0.36

For a comparison I also tried to use i.landsat.toar in GRASS 7 but I also get very low reflectance values (TOA, uncorrected) there. Here the mean reflectance values for each band after i.landsat.toar:

B2: 0.0898574
B3: 0.0747076
B4: 0.0582399
B5: 0.252395
B6: 0.160692
B7: 0.0808918

I am having this problem regularly also with other scenes from different locations.

1 Answer 1


The standard formula is correct. You should not turn +Ap into -Ap.

A value of 6500 in the red band is often seen over water, where a reflectance of 0.02 makes pretty good sense - over clear water, that may actually be too high. Furthermore, seeing DN values as high as 40000 are not that uncommon, and with your suggested formula, that would give a reflectance of 1.04, which is clearly wrong.

Furthermore, when looking at average reflectance values, make sure that you aren't considering the no-data areas, where the DN value is 0 as part of your statistics.

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