I am playing around with the arducopter 2.6 I was wondering if there was a way to have a fixed altitude. That is whenever I change the flight mode to Alt Hold, the quad comes and settles at that height say for example 2m over the ground. Is this possible without a sonar sensor. Just using the barometer?
Hi, AltHold will keep its current altitude using its baro (and some corrections from accelero/gyro).
You may have something similar when using auto mode / take off command setting the altitude to 2m.
But… the baro may drift over time; also if you have high vibrations, althold may be a bit jumpy.
The thing is, when my throttle is at 50% and I switch to Alt Hold mode, the quad goes up instead of staying stable at the height it currently is in. I immediately switch to Stabilize mode to prevent the quad from flying off. Is this traditional behavior?
What is Arducopter 2.6 ???
@Emin_Bu: Possibly the last software version that the obsolete APM boards could run… (This is a guesstimate not a statement) 
Warning
The APM2.6 board is no longer supported for Copter. From Copter 3.3 firmware (and later) no longer fits on APM boards. The last firmware builds that can be installed (AC v3.2.1) …
http://ardupilot.org/copter/docs/common-apm25-and-26-overview.html
When your copter does not AltHold but goes up it means your throttle-Mid is not set correct…all details are in links,pay attention wich version you load to the board,some older ones do not have all the functions of new firmware…
http://ardupilot.org/copter/docs/altholdmode.html?highlight=althold
http://ardupilot.org/copter/docs/ac_throttlemid.html#ac-throttlemid
it is difficoult to give your advice when you do not know is it 2.6 version of your APM Autopilot board or firmware you loaded onto APM2.5 or 2.6 board
For best results in getting help it is a good idea to post a log file from your flight controller.
As it has all the specs in it, such as board, firmware, parameter settings, as well as logging the flight, it makes it much easier to see what is actually going on.
As @Emin_Bu said, with an APM 2.6 you would be running Copter 3.2.1, being the last supported firmware release.
What you are seeing is indicative of your throttle not being centred for normal Stabilise hover.
Get your settings right for that first adjusting your throttle mid parameter.
Your copter is a little under powered, as you need to have the throttle above 50%.
When you switch to AltHold mode it sees above 50% as an instruction to rise.