Chapter 8 — Question 1

Should radiometric corrections be performed before or after geometric corrections, and
why?

Answer

Radiometric corrections, in this case, refers to the conversion from Digital Num-
bers (DNs) to Top of the atmosphere (TOA) radiances, and eventually the cal-
culation of surface radiances from these TOA radiances. Geometric correction
refers to georeferencing and eventually geocoding. It is desirable to perform
the adequate radiometric correction sequence for the intended application be-
fore

a geometric transformation that modifies the original value of the pixel. In

other words, georeferencing could be performed before radiometric correction,
but not geocoding procedures that involve pixel resampling. The mathemati-
cal algorithms in some resampling methods used in the geocoding process (e.g.
bilinear or cubic convolution) can modify the original pixel value of the image
in a linear way. The relationships between DN and radiances might not be li-
near (mainly in the thermal range), so it is physically inaccurate to convert to
radiances from a distorted (linearly converted) DN value.

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