Originally Posted by
EivlEvo
Alright. Im a moron. It utilizes data from a barometer.
GPS satellites are 1 way in the standard constellation of 24. There are however actually 26 satellites as well as WAAS stations that are ground based. These as well as the standard constellation of 24 are capable of learning via uploads. Every time the US department of Defense notes a change in an airport approach or terrain someplace, they don't launch a new satellite.
It is physically impossible to receive a signal from more than 5 satellites at any given time. There is no way you're receiving 12 satellites.
UMD. You come in here and start trashing me by highlighting selective text in my post. This information that was going on isn't unit specific, people are mistaken on how a GPS system operates in general. I initially suggested that the difference in the 2 elevations may be from the altitude calculated from the GPS sats vs. the distance to baro calculation. Everyone else starts going on about algorhythms and how its an error in the program. Its not an error in the program its the inability of a GPS system to be accurate within 1 meter 100% of the time. Im not trying to tell people that they are wrong... Im trying to find out why you all seem to think this unit has a discrepancy in the altitude charts, and then figure out how the unit would actually work. Im not here to be a prick or a spinster or troll, Im here to learn and perhaps even engage in educated discussion and argument. This is what the forum is for.
Im confused about what ur edit is saying exactly. I think we're saying the same thing there. That the GPS systems require alternate databases to acquire correct terrain info. Which is what I had mentioned just then regarding that it is likely that in an older unit the data will be less accurate unless there is a way to update the system.
Also... to Psimet (who always seems to find my posts and argue with me
If you never have to enter the barometer then I would be confused about how its accurate.
The altimeter in an airplane (where altitude is obviously more critical) has to be reset every 10 - 20 minutes. (this is based on the vast distances being covered of course) But the barometric "altimeter" in my watch if not reset won't give an accurate reading within a 6 hour timeframe? Also, even the sensitive altimeter in an aircraft from parking it overnight will need to be reset to show proper altitude. Unless the GPS unit simply uses the barometer to record pressure changes to figure out if its going up or down and then uses the standard pressure loss equals a climb or descent in this many feet of altitude. Which would again point out that this is not an altimeter.
Is there a separate page where you CAN enter the baro, but you are just choosing not to?
I don't think any of us would say it's accurate, but I think everyone would agree that 500 to 1,000 ft is more of a variation than expected.
I have not found a single place to enter in a calibrated elevation or barometric reading. From what I can tell it tries to calibrate the current baro reading from altitude information it receives - I guess - from the sat signal upon power-up. So apparantly my lack of clibrating manually is not by choice. It just doesn't seem to be available.
Keep in mind that we're not using these to land planes.....just to figure out pathetic cycling statistics. I don't really care about the absolute value of my elevation data rather I am interested in knowing if the data is relatively consistant. I now know that it is not, and will not ever be.
FWIW - I loaded the ride I was talking about into Motion Based and got over 1900 ft of climbing - for a roughly 700 ft difference between what the unit recorded and what Motion Based is giving based on survey data.