Heat islands have been known about for quite some time, and you would *think* this would been considered by those claiming human influenced global warming. But as far as I know, it wasn't & it would require a fair amount of study to come up with a accurate modifier.
The simple solution would be to take only rural areas which have remain rural over the last 100 hundreds or so. I believe this has already been done, and it does not show a rise or decline in global temperature- not enough data points to give accurate tread above error margin. And of course the satellite readings don't show a trend of warming or cooling- above expected error margins.
Nevertheless, from tree records and other sources, we know we are in a warming tread- since the "Little Ice Age".