Finding Accurate Base Position

Thanks bide for your answer.

Is there a need for absolute accuracy?
In terms of accuracy, who or what are you needing to be absolutely accurate in relation to?
Is the relative +/-2cm accuracy of RTK between base and rover good enough?
Why is averaging a single-mode base position for 30min not good enough for your project?

I’m not sure if a fixed inaccurate base position affects the consistency of rover positions while the rover is in same place or not . If no , I don’t care about absolute accuracy.
I need the rover to be consistently accurate in relation to the base.
My application is localization of a car in a google map in a 1Km range.
Unfortunately Google map isn’t good enough (streets aren’t accurate enough).
So I want to create a map with accurate street edges using reach GPS and then localize the car within the map.

An accuracy of 5 cm of the rover is good enough for my application. Only I wanna be sure that every time I’m in a specific position in reality , The gps give the same latitude , longitude with an error less than 5 cm.
also I don’t need a very frequent gps message , 0.5 Hz Frequency is OK to me.

Here are some of my results , It’s not consistent at all . Although the status is fix , Being in the same position twice gives a different lat & long with a distance more than 2 meters between them.

Also , I’m confused , Why the maximum antenna height of base in base mode settings is only 6.5 meters ?
I’m placing the antenna on the roof of a building which is 7.15 m.