R help - Apr 2011 - Using a variable in a covariance structure and as a predictor - is this OK?

Dear list,

I have the model below which i am using to account for spatial autocorrelation:

exponential <-corExp(form = ~ Longitude + Latitude)

explanation_mod_all <-
gls(Lower_PD~Area+Elevation+Temperature+Preceipitation+Agriculture+Urban+Human.footprint+Population,
correlation = exponential)

I want to see if Latitude and Longitude are significant, is it statistically and
methodologically correct to add them to the model i.e

explanation_mod_all <-
gls(Lower_PD~Area+Elevation+Temperature+Preceipitation+Agriculture+Urban+Human.footprint+Population+Longitude+Latitude,
correlation = exponential) ?

Thanks

Chris