A quick thought experiment:
One would rightly be considered an extremist if they held that their property rights superseded the liberty of others to the point that human ownership were viable. I believe polls would reflect that extremism as I doubt few currently hold that view.
But would the same individual be an extremist in 1850? I believe so. There are timeless truths and a belief in human ownership runs counter to them. However, polls, had they been possible, would not have reflected that extremism in 1850. The result is that our extremist slave holder was able to hide their extremism behind the skirt of extremist polls.
Today we see the same dynamic.
Instead of one's property rights superseding the rights of another to the point of their enslavement, a group of Americans believe one's privacy rights supersede the rights of another to the point of their death. Like the slaveholders of a previous era this group hides its extremism behind the skirt of extremist polls.
Polls are clearly not the tool by which we may divine extremism.
Extremism can, therefore, only be measured against a fundamental principle.
To conclude, it is arguably the case that extremists are sometimes those who turn their backs on fundamental principles so as to run with the herd rather than those few who choose stand with truth.
(edited for clarity)