Altimeter

[olseric]

radar altimeter that always gives the correct distance above ground.

That probably works fine if your approach takes you over the ocean and you're landing at an airfield that is at sea level, but most airport
environments aren't flat...especially in the FS world with many airports being "plateaued" due to the flattening.

Your best bet: Grab the charts for the airport and read the field elevation. Set and use the barometric altimeter.