A Look At Elevation Statistics From the Barometric Altimeter
There are two sets of data below.  One was done on Wednesday evening June 7, 2006 and the second one on Thursday evening June 8, 2006.  The first one was done on foot with many stops to mark waypoints.  The second one was done on a bike with only a few short stops (no waypoints). The data was collected with a Garmin GPSmap 76S.

The route follows a creek downhill; crosses and returns back up the other side.  The route had a dip under a road that would add about another 15 feet to the loop otherwise the route was fairly consistently down to one end and up coming back.  So based on the map, the route should have a gain and loss of approximately 115 feet (6080 - 5980 + 15).

The first lines come from Nat'l. Geographic's TOPO! regional program, version 2.7.7.  This program doesn't use the track elevations; the gain/loss and max/min values come from the USGS DEM data for 7.5 minute (1:24000) maps.  The first gain/loss set (DEM Data) is from the basic profile.  The second set is from the magnified profile.  The observed values are from the 7.5 minute map with 20 foot contours as viewed in TOPO!.
Walked:
705
Trackpoints		 Distance
(miles)

 Gain
(feet)

  Loss
(feet)

 Max. Elev.
 (feet)  

Min. Elev.
 (feet)  

Difference
Duration
(hh:mm:ss)
 Avg. Speed
(miles/hr)
Observed


6080 5980
 100
Track Len.
 4.67






DEM Data
107
 107
 6082
 5975
 107
Magnified
 4.67
 225
 225













GPSmap 76S
 4.69
 532.4
 550
 6122
 5983
 139
1:24:10 3.3
GPS Utility
 4.666
 2393.8
 2368.6
 6127
 5983.5
 143.5
1:24:03 3.331
Biked:
186
Trackpoints 		Distance
(miles)

 Gain
(feet)

  Loss
(feet)

 Max. Elev.
 (feet)  

Min. Elev.
 (feet)  

Difference
 Duration
(hh:mm:ss)
 Avg. Speed
(miles/hr)
Track Len. 4.60






DEM Data
107
 108
 6083
 5975
 108
Magnified 4.60
 222
 223













GPSmap 76S 4.64
 86.5
 82.9
 6123
 6027
 96
 0:21:21 13.1
GPS Utility 4.619
 179.8
 153.0
 6120.7
 6027.6
 93.1
0:21:27 12.92
Comments:

I think the Nat'l. Geographic's TOPO! program tends to slightly under calculate the gain/loss for the basic profile while the magnified profile tends to be too high (based on more observations than noted here).

GPS Utility (version 1.5) apparently just totals up all the elevation differences between track points with no filtering.

The values obtained by biking are close to the low end of accuracy limits and I wonder if they would be consistently low with the same test setup. It is much closer to reality than the values obtained while walking apparently because the route was traveled fast enough to keep the receiver from adding much "noise" into the gain/loss. It seems like the receiver could check the difference between the high and low and make sure the gain or loss is at least as much as the difference as appropriate.

My casual observations on many hikes ranging from about 8 to 18 miles long with 1,000 ft. to 4,000 ft. of elevation gain has been that the recorded gain tends to be approximately 10% to 20% high. Hikes that are fairly flat tend to have elevation gains that are 2, 3, 4 or more times the actual elevation gain. If you stand still the gain and loss will start climbing as "noise" is accumulated so the error steadily climbs. Or in other words, keep moving as fast as you can to keep the error down. If you go too fast the accumulated gain/loss may be less than the actual value.

Get to know how your altimeter is working because reality may not be very close to what your receiver is telling you.
 
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