I want to calculate the maximum height of something. Is there a way I can build some sort of on-board altitude meter, rather than using trigonometry to calculate the height..

Precise measurement of the acceleration due to gravity would work, since it varies inversely with distance from the center of mass of the earth. Barometric pressure could also be used.

Aircraft use altimeters (re: air pressure) and radar-altimeters (speak for themselves). I understand these work quite well.

Therefore tie an aircraft to the top, problem solved.

there are such devices that are only tiny, they work on barametric pressure, it`s like 2 bowl shaped copper discs sealed at a known pressure with a hysteresis strain guage attatched. a reading is taken at ground level and recorded digitaly as the baseline, at the device gains altitude the gas sealed in the copper discs expands and changes the resistance of the strain guage, that`s then recorded, it can be used also at a predetermined altitude to make a circuit, either used to deploy a chute or take a picture etc... I`ve seen them used in High Power model rocketrey, they`re quite expensive, so maybe you could borrow one? or make one?

Just a thought

Why not use trig? Using anything else introduces lots of sources of error, altimeters measure the altitude above mean sea level, which varies by a few meters, and then you need to know the height of the ground above mean sea level. Not too bad if you do it by the sea I spose.

alternatively

 

Doppler Effect!, either Microwave like the speed meters the police or road cameras use, as do these electronic tape measures (ultra-sonic) IR could also be used.

all you`de need then would be little oscilator at a fixed frequency flashing a lazer or IR LED at a known frequency, and a little reciever at base. a measurement of frequency drift will tell you the speed. a Speed/Time calc should give you the distance

Use a laser distance measuring thingy.

Actualy, Blike, can you tell us a bit more about what you`re trying to do here? is it a moving object or stationary like a building?

I only assumed moving as you stated "Onboard"?

any clues as to what it is please

the method for a rocket wouldn`t be the same as for a hot air ballon or an airoplane. either of which you could use a little Garmin GPS as I do.

we could do with a bit more info as to what it is you are constructing. if it is something like a plane, then you have to be very careful that you don't screw up barometric readings by virtue of the fact that the plane is moving.