How to enforce positive reflectance values?

[nichess]

Hi,

for my student research project, I like to create an "at surface" - NDVI. Therefore I did:

1 _ Atmospheric correction with ATCOR 2D of all ETM+ Bands (1 - 7)

2 _ Deleting Band 6

3 _ DN to Reflectance with the Landsat7 Reflectance Tool of Erdas 2011.

Now I discover some negative reflectance values in the imagery. The values are arround -0,01.

I tried to enforce positive reflectance values by creating a model in Erdas model maker. For every condition I used a seperate function:

CONDITIONAL { (Band1 <= 0 ) 0 }

CONDITIONAL { (Band2 <= 0 ) 0 }

CONDITIONAL { (Band3 <= 0 ) 0 }

...

But after running the model, everything is 0.

It would be of great help, if you could point me to the right direction.

Thanks!

Nicoyonic