# Landsat 8 reflectance values very low

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)
``````

where

`ρλ' = Mρ * Qcal + Aρ`

and

``````Mp = 2.0000E-05 (REFLECTANCE_MULT_BAND_4)
Qcal = 6937 (DN)
``````

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.

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