What is the generic name of the electronic hand-held device that measures the relative height of two different points (altitude, etc.)?
As an example, suppose I'm climbing a tree. I turn on the device, put it in my pocket, and once I've climbed 15 feet from the ground, the device should indicate "15 feet" on its display. When I climb further up to 20 feet, the device should indicate "20 feet" on its display.
I used to measure different levels using a Dumpy level and total station "distant measurer-laser." I need a different kind of device that can take levels more quickly but I don't know what this device is called or how to select a particular model. In addition to the name of the device, what properties of the device should I be looking at in order to take measurements more quickly?