Ah, you say it is a graduated filter. Now that makes sense!
I mean I have a skylight filter on my lens all the time. And only the other day did I take one photo with and one without, and that filter hardly makes any difference at all. But then that filter is not a graduated filter... so it does not filter the light from the sky and leaves the light from the ground untouched. I think I should really think towards more filters ... but even after over a year of having lost that polariser of mine have I not had the chance to replace that one (and polarisers are a bit more on the costly side ... uh-oh!)...
As to your photo here, yes, I guess your filter sure worked and saved you from having to create an elaborate HDR

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