BMP180 pressure sensor question

Hi, I'm trying to display relative altitude (is relative the right term?? ) in serial terminal, as in, if I stand halfway-up the mountain, and press reset, my altitude will display zero. If I climb the mountain, the altitude will increase. If I descend, the altitude will become a negative value (-).

I'm using the bmp180 sensor, but I'm having difficulty making this happen. I can find my local barometric pressure value from the internet, and use it to calculate the local altitude (then increase the value or decrease the value as I walk around).

However, starting from a zero value would be my ideal. Can anyone help? Sorry if I'm being a dumbass.

I'm using this tutorial: http://www.loveelectronics.co.uk/Tutorials/20/bmp180-arduino-library-tutorial

Why not just do the following:

Go to the place where you want to define your base (relative zero altitude).
Turn on your device. Make an altitude measurement. Then use that measurement as your base_altitude.
To calculate the difference just use relative_altitude = base_altitude + current_altitude.

Did I understand you correctly?