AR.Drone 2.0 altitude from navdata is showing wrong value

edit

I am having a problem. This issue is very important to me. If anyone helps me in this, i can complete my project.

The problem is: When my ardrone is flying, I will be able to get altitude value from /ardrone/navdata/altd and then when the ardrone moves and when we find it above a table of height x and then normally the altitude of the drone should be previous - x, but this is not happening. Why??

Example : when its flying, let's say the altitude be 800 mm and then, table height be 300 mm so when the ardrone is above the table, then altitude should be 800-300 = 500mm but it is not working such a way.