I am studying nightlights data and trying to model cross-sectional convergence in nightlights in India. However data has some stray light corrections, which has made some of the measurements below zero. Another point being, that the value of 0 has a representative meaning.
Hence any conventional way of measuring growth rates in literature is not applicable here.
In simple words: Nightlights are measured on a relative scale from 0 to 63. The corrections for cloud cover and ephemeral lights makes some of the data points negative. So:
It is analogous to ask what is the percentage growth in nightlights if it increases from - 1 DN to 0 DN
Even more problematic question is what is the percentage growth in nightlights if it increases from 0 DN to 2 DN.
I have already thought of adding an arbitrary but reasonable positive number to data points, to bring everything into a positive domain. But this is a very crude and not an elegant way to do this.