Let's take our CLUE altimeter for a hike and see how it does. Here's a map of the hike with several locations of known altitude indicated.
We will use point 1, Snow Lake, as our calibration point. When we get there, we'll stop and set the altimeter to the known altitude at that location (4016 ft). Then, we'll check the altitude again at points 2 and 3 and see how it compares.
Here we go:
At Snow Lake the altimeter was set to 4019 ft. The lake surface is 4016 ft, but we are standing with the altimeter several feet above that. :)
Note how the STD altitude is off by a couple hundred feet.
The trail at Gem Lake is 10's of feet above the actual lake surface of 4857 ft. So the altimeter reading here of 4872 ft. seems OK.
That's the summit of Wright Mountain in the background - our next stop.
Standing on the summit of Wright Mountain we get a reading of 5436 ft. That's excellent agreement.
Oh, but we're holding the altimeter above the summit. OK...let's put it down on the actual summit.
Even better. The CLUE based altimeter is agreeing within a few feet! 5432 ft.
Note again how the STD based altitude is off by hundreds of feet.
And there you go. That's a perfect example of how a pressure based altimeter can be used to provide surprisingly accurate results. It also hopefully illustrates how far off the altitude reading can be if proper calibration is not done.