Some time ago, I was looking for info on ways to get a reading as accurate as possible for hills I ride. With altimeters, there were two options - one that worked off barometric pressure and one that used GPS satellites. A third option, which I now use, was to get the Delorme Topo USA map software.

Of course, the question with the map was how accurate its figures were. As it happens, I just found a guy to ride with sometimes who has an altimeter as part of his bike computer. This offers an opportunity to make periodic comparisons. So far, it seems his altimeter always gives higher figures, but I don't think he calibrated it properly the first time and I know he has not recalibrated it in a week.

Nevertheless, I can look for the DIFFERENCE in altitude on a climb and see if that matches the map. They come pretty close - probably close enough - but there is one thing I'm wondering about since I don't know the details of the calculation process.

Suppose it is erroneously set to be 100 feet too high. Would this 100 foot error be consistent (allowing for built-in error tolerances) for any altitude? I mean, if I'm at sea level, it would show I am at 100 feet. If I then climed to 1000 feet, would it theoretically show 1100 feet or does the barometric pressure it is based on change different amounts at different altitudes?

I have a Polar 725. Depending on the weather, the altituge of my house ranges several hundred feet. But the relative changes in altitude remain very constant. Some complain that a barometric altimeter can shift during the ride. I've only seen this once. On a 4 hour metric, I had the beginning and ending points (i.e., the same point) end up different by 10 vertical feet. That's really not much shift over 4 hours.

Yeah, I know the weather will affect things a lot since, virtually by definition, that means a change in pressure. I do have some concern for the changes during a ride. For example, I sometimes do a 5 mile ride up Tantalus (on Oahu). You are heading into the mountains where it often rains while a very short distance away, its not raining. But since I am on an island, I figure I can check various places multiple times and et a pretty accurate average to make up for that.

What I was mostly concerned about was whether the pressure built up at the same rate, regardless of the starting point. That is, if you climb 500 feet starting from sea level, the formula would say the pressure should decrease by "x". I do not know if you START at 500 feet and climb an addition 500 if the pressure decreases by "x" again or by a different factor. If it is still "x", then if you just want to know the change in altitude, it wouldn't matter if you started it with an accurate starting point. But if the rate does change, it could matter.

Excellent point. I can say that when the pressure changes enough to change my "apparent" altitude at my start point by several hundred feet, the relative changes stay constant. Thus it would appears that the system is linear within say a 1000 foot range.

That's good news. I could care less what my highest altitude is - I just want to know the difference in altitude. Not that it isn't nice to have both accurate, but one is definitely more important than the other.

Okay, related question for those with altimeters (or GPS units for comparison sake): On different rides, how much variation are you finding for the DIFFERENCE in altitudes? For example, if you climb 500 feet, does it sometimes read as low as 475 and as high as 525 or what?

Topo maps for me ...