in my case the altitude is fixed because I have a ground rover.
I can place the openmv in the middle of my rover and I can illuminate the surface with a couple of leds.
SLAM is very interesting but requires a lot of ram that openmv doesn't have, maybe the version 2 could do the job. Also I've seen SLAM implemented with the data collected from laser scanner; don't know how it works with camera.
I've tried to measure ground speed with the accelerometers on the imu but without success: too much drift...
Yaw angle seems correctly calculated; at the moment I'm working with a STM32F3 discovery board.