I'm not sure how much difference that would make in our kind of craft, Nev.
And I'm not sure whether the act of calibrating in situ for current Area QNH, say, or to a known ground elevation before take-off would obviate that problem.
Be that as it may, these kind of apps aim to exploit the precision barometric instrument that's built into most modern phones, and I guess, tablets.
This particular one from Radiant allows you to Bluetooth to a separate 'Sensor Tag' which provides temperature and humidity data (which'd have the same problem, depending on where you placed the tag.)
From their website:
"An external BlueTooth sensor is optional, and opens up more capabilities like true temperature / humidity based Density Altitude. (SensorTag from Texas Instruments -- $29.00 ( Part # CC2650STK)."