# Calculating TOA reflectance from Radiance using Erdas Imagine for a Spot2 image

Using the SPOT equation ρ = π L/[E cos (θs) d-²] to calculate TOA reflectance from Radiance.

L was calculated using this equation [L = (X/A) + B]

d was calculated by converting date to julian day then converting to d using http://landsathandbook.gsfc.nasa.gov/pdfs/Landsat_Calibration_Summary_RSE.pdf

To calculate cos (θs), taking the sun elevation from 90 and then converting to radians http://www.mathinary.com/degrees_radians.jsp and then getting the cos of the answer

Example of above equation filled in for a Spot2 image p= (3.14*radian band1)/(1865*(0.98717*0.98717)*cos(70))

This equation works in Erdas Model maker however the resulting image is blank.

I need to know where I'm going wrong using the equation in Erdas

• not sure, but I think Erdas work with radians – radouxju Aug 29 '14 at 18:17
• that'd be my first question: are you using the correct angle units? – Martin F Aug 30 '14 at 18:23
• @martinf converted to radians still having the same problem – Barry Aug 30 '14 at 23:40
• It's not my specialty but I'd suggest providing more details of your equation: a link, descriptions of d, L, etc, and values that seem correct. Other folks should then have more to go on, to help. – Martin F Aug 31 '14 at 5:07
• L was cal using this equation [L = (X/A) + B]. Esun using www2.astrium-geo.com/files/pmedia/public/… d was cal by converting date to julian day then converting using landsathandbook.gsfc.nasa.gov/pdfs/… cos (θs) by taking the sun elevation from 90 and then converting to radians mathinary.com/degrees_radians.jsp and then getting the cos of the answer – Barry Aug 31 '14 at 12:41