If you plot an elevation profile using topo software, it has the correct general shape but there is a lot of "grass" or noise in the plot. When the software calculates the total elevation gain, it includes all the little ups and downs that are not really there.
Here's what I think is going on. Roads are stored as a series of points, every so many feet along the route. When the elevation profile is calculated, the software plots straight lines between the points. If the road is curving around a hill, the straight line goes "uphill" and then "downhill" between the points, resulting in a bump that is not really there. In addition, the topo software does not take account of bridges and road cuts into hillsides. Most roads have less up-and-down than the land they are built on.
That screws up the net feet-of-climb calculation and also makes the elevation profile graph look messy. GPS altitude data doesn't have this "grass" since it is based on actual readings on the actual road.
To illustrate, here are three elevation profiles of Graton Road, between Hwy 116 and Bohemian Hwy in Occidental. The first was generated by National Geographic Topo! Version 3.4.2 using route points hand-entered by Frank Hamlin. The second profile was output by DeLorme Topo USA version 3.0 using an auto-generated route. The third was done with my homebrew Perl program using GPS data from Lou Salz.
Topo! reported an elevation gain of 782 feet for the route. Topo USA claimed 783 feet. The homebrew program using GPS data calculated 671 feet. I believe the GPS data is the most accurate.
I have found that total distance is also more accurate with GPS data than with a mapping program. The reason is similar to the altitude errors. The straight lines plotted between road points "short cut" the curves, which can result in a significant mileage error on curvy roads. GPS data actually has a similar problem, but the points are much closer together (at least at bicycle speeds) so it doesn't seem to cause significant error.
Several people have commented to me that GPS data is not suitable for elevation profiles because, while the absolute accuracy is pretty good, the differential accuracy (relative difference between points) is poor. In fact, exactly the opposite is true. The absolute accuracy can be off by as much as 10-20 meters (30-60 feet), but the differential accuracy is typically a few feet. There is a technique called "Differential GPS" that takes advantage of this fact to achieve improved accuracy. Special-purpose differential GPS units made for surveying can achieve accuracies of a few inches. The WAAS system, which nowadays is supported by most GPS units, uses a similar principle to broadcast error-correction data in a grid over a wide area.
That said, I have to admit that the elevation profiles generated by my Magellan Meridian Platinum GPS (without internal altimeter) have more noise in the elevation plots than I would expect, with occasional blips up to 50 feet or so. It seems to be correlated with poor signal reception conditions from the GPS satellites.
While hysteresis is a "good thing" the consensus among most commentators is that 30 feet is too much of a good thing. On a trip with lots of little ups and downs the Avocet 50 tends to under-report the total climb. The altimeter in the Cateye AT100 bicycle computer does not include hysteresis (reportedly because of a patent on the concept held by Avocet). The consensus is that the Cateye consistently over-reports total climb.
So 30 feet of hysteresis is too much and 0 feet is not enough. I chose 10 feet of hysteresis as a compromise between over- and under-reporting total elevation gain. That is the figure used to calculate the elevation gain figures listed with all the profile graphs.
- Alan Bloom