Mt. Whitney Portal Store & Hostel Forums General Mt. Whitney Portal Store Message Board GPS distance ?

Our group just completed the day-hike last friday. I have been using the Garmin 60csx for a year now, and it is always right on the money as far as distances of hikes go. After doing the Main Whitney trail, my GPS reported about 19.8 miles. I thought it was 10.7 each way, and I was just curious if anyone had recorded a shorter round-trip like I did. Thanks.

I think most evey map, or book has the MMWT @ 22miles round trip. In my experience, a gps will vary slightly on mileage on almost every hike. The gps is constantly tracking different satellites, and your accuracy is changing between 8-50 feet. I notice that the longer the hike I am on the more it is off. I have also found it is more accurate on the way down from a peak. I think it works better if you are constantly on the move.

I have measured 1-way distances as small as 10.1 mi and as large as 10.9. Looking at the traces afterward, I found that the short ones had breaks where the GPS lost reception. The straight line distances calculated in such spots are inevitably short ("the shortest distance between two points..."). GPS can also overestimate distances. Mine recorded 4+ miles over a mile or so this summer during a storm.

Distance accuracy depends alot on the sampling rate your gps is set at - all else being equal, the higher the rate the more accurate it will be. I think gps underestimates distances when there are alot of turns, such as in the case of 97 switchbacks. In order for the distance to be measured accurately in the case of switchbacks, a measurement would have to be taken exactly at each turn. Of course if you have the gps sampling rate set high enough this becomes less of an issue.

Also, an occasional loss of signal can easily add up to a mile or two over a 22 mile hike due to the resulting low effective sampling rate. As Alan indicates, you can examine your gps track and any signal losses can be detected.

Another possible source of inaccuracy is whether the GPS takes into account elevation differences. If it just measures distance assuming constant elevation (which I believe most do), it won't include the extra distance due to elevation change. This could be significant in the mountains...

Steve

The difference between distance measured horizontally and distance measured on a slope is very small in hiking. For example, the Whitney trail gains about 6000' in 11 miles. That's an average slope of 10%. On a slope of 10%, a horozontal distance of 1 mile translates to a distance along the slope of 1.005 miles. The difference is half a percent! The bottom line is that elevation change does not make much difference in distance travelled.

True, on a 10% grade the difference is not that much. On a 100% grade (45 degree angle), the difference is 41%. But you won't find that grade on the MMWT. But I agree this isn't the main source of error on the MMWT, just another thing to consider. More important for off-trail routes though, like the MR.

Steve

You don't hike up a 45 degree grade!

The MR gains 6000' in about 4.5 miles, which is a 25% grade. On such a grade, one travels 1.03 miles along the slope for each horizontal mile. That's a 3% error.

That's an extreme grade for hiking I agree that things change as one approaches vertical walls.

Thanks foe the info guys. I actually was looking at the trip summary page and not the trip log page which has the actual distance logged. The trip summary page I reset a little into the hike. So it turns out that the 60csx reported 21.6 miles, which is pretty close to the actual value.