Pressure altitudes are referenced to mean sea level. But not corrected for local difference to standard pressure.
You can put in the airport elevation and Islip altimeter setting into for example this calculator:
(FltPlan.com - Pressure and Density Altitude Calculator)
Altimeter setting / local pressure you can find for example here:
(Long Island MacArthur Airport Weather (Ronkonkoma, NY) [KISP/ISP] - FlightAware)
Currently it’s 30.38 inHg
Airport elevation for Islip is 100ft AMSL
That means the current pressure altitude on the ground at Islip is -321 feet.
So currently you can subtract 321 feet from the altitudes in skyview if you want to know the feet AGL.
If the local pressure changes that will also change.
Does that help you? Your question didn’t make too much sense to me.