In the comments below one of the answers to [this question](https://aviation.stackexchange.com/q/56954/19) about the influence of temperature on vertical separation, the question was raised whether ATC ensure 1000ft vertical separation based on geometric difference (true altitude) or pressure altitude difference.

In cold air, the true vertical separation will be less than 1000ft if the difference in altitude indicated on the altimeter is 1000ft. 

[![enter image description here](https://i.stack.imgur.com/RmA3r.png)](https://aviation.stackexchange.com/q/56954/19)

Source: [the other question](https://aviation.stackexchange.com/q/56954/19)

Is there an offical reference that gives the answer?