Is an altimeter useless?

After plenty of distractions about calibrating and pressing on the case, I
am still left with puzzlement about my GPSMap 60cs.

I looked at my local topo map carefully, and decided that the porch rail
outside my office is at exactly 1085 feet. I don't think it matters
whether I am correct.

So, I set my GPSMap 60cs down on the porch rail. I let it sit for about
fifteen minutes, powered on. Then I calibrated the altimeter to 1085 feet.
As I pressed the button, I saw a spike, but it settled back.
I left it sitting for one hour. I have an altitude plot that is gradually
rising. My altitude is now 1118 feet.

How is the altimeter of any value?

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Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5
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