Relative Altimeter
A Relative Altimeter is a type of altimeter that measures the vehicle's altitude, relative to ground. Normally, an altimeter would measure the distance above sea-level. However, for some creations, such as hovercraft, a normal altimeter would be of little use. Thus, the Relative Altimeter is used in these situations.
Construction is simple. It is comprised of a Gyrometer, a Hemi servo, and a Distance Sensor. Depicted below:
Also note the properties of the Hemi Servo.
In order for these properties to cause it to function correctly, it may be required that you rotate the Hemi Servo. In order to know how much to turn it, temporarily put the number 90 into the top offset property. Then rotate the Hemi Servo's weld until the Distance Sensor's arrow faces the same direction as you would read the Gyrometer's printed words from. Like so:
Congratulations, if you've made it this far, your Relative Altimeter is now complete. If you refer back to the tuning screen, the output of the Distance Sensor now serves as your vehicle's altitude relative to the nearest object below it, which will almost always be ground. Though note, this system is very simple, yet has a fatal flaw. It only functions while the Gyrometer faces upward. There are means to correct for this though, such as using 2 Relative Altimeters, one on the top of the vehicle and the other on the bottom, then switching between the two based on whether or not the gyrometer is facing up or down. However, those are beyond the scope of this tutorial.