Accurate Altitude measurement

You can measure altitude level accurately with your mobile without internet connection. Just download this free digital altitude meter App which Turns your phone into an altimeter! (An Accurate Altitude measurement). An app you always need to measure the pressure and height of buildings above sea level.Use this Altitude measurement tool to measure building heights. Height of buildings from ground level to top floor may be cannot measure with a GPS because of inaccurate altitude units.

Question

Is possible Measuring my altitude above sea level accurately ?

I live in a building which sits very high near the top of a hill. I estimate the ground level of the building to be around 700 feet, I live on the 13th floor, so my altitude is approximately 800 ~ 850 feet. I would like to get an accurate measurement (correct to +- 10 feet).

How I did the measurement ?

Measure the ground level altitude at a park with digital altitude meter

Estimate the height of each floor

Usin