Doug Hormann
Patrick,
The problem you've described is in the software, not the GPS unit. The software - doesn't matter which one - does not use the elevation data from your GPS unit; instead it uses only the route data and then compares it with a elevation database embedded in the software to derive a ballpark elevation curve. The resolution of this database is what's causing your elevation curves to vary so much.
In order to make the software package use as little memory as possible, the company has to degrade the resolution of the database. Imagine a database that has a discrete datapoint for each square meter on your map. While this would make for a nicely accurate elevation curve, it would also result in a database with a million cells for each square kilometer! Multiply that by the roughly 252,000 square kilometers in the State of Oregon and you have a spreadsheet with 252,000,000,000 (that's Billion with a B)cells!
To make the database more manageable, they software might use a database that averages the elevation in each 20 meter square. This would reduce our one kilometer spreadsheet to one containing only 2,500 cells. Still this would be huge (628,000,000 cells anyone?) if extrapolated to a map covering the entire state, so some software packages will reduce the resolution even further.
This inaccuracy is tranferred to your elevation profile and accounts for the huge discrepancies.
An additional problem with using a GPS for elevation is that unless you have a lock on five or more satellites, you will be seeing some pretty inaccurate readings on your GPS. Many units are including a built in barometric altimeter for just this reason.
Doug (GPS Diety) Hormann
>From: Patrick wilder
>Date: 2008/04/28 Mon PM 10:40:11 CDT
>To: obra@list.obra.org
>Subject: [OBRA Chat] Garmin Elevation Discrepancies
>Lately I've noticed considerable discrepancies in total elevation data lately. Anyone else having this problem? Solutions? I've added all the updates from the Garmin website. I'm using an Edge 305.
>
>~Patrick
>
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