GPS Distances

[Nick]

I'm wondering what everyone things of the accuracy of measured distances, particularly using a GPS. I've noticed over the years that many trail descriptions are short of what a GPS records the mileage. Granted, sometimes the track bounces around a bit, like in narrow canyons, but generally the tracks look quite accurate. I've kind of chalked it up to the possibility that people writing guides 10 or 20 years ago were likely just measuring distance on a paper map and not getting all the small bends in the trail, hence the inaccuracy.

So now I'm in a situation where I really need to make sure the data I'm presenting is accurate. Think of it like I'm writing a book, but I'm not. I basically have a job where I'm providing trail guides to a local business that plans to publish them to a very large audience.

So with that said, take a trail like Lake Blanche. The huge official sign at the trailhead says 2.8 miles one way. Other places around the web say it's 3 or 3.1. My GPS tracks on the other hand show at least 3.2 one way. And that is if I get into the software and make sure to only count from parking lot to the edge of the lake. Other people's GPS tracks tell a similar story. I'm of the opinion that a good GPS track should be the gold standard. What do you all think? Do I stand by my data even though the government-issued information at the trailhead says otherwise? This is not just a Lake Blanche issue either. It seems to be pretty a widespread problem, but usually the gov resource is overestimating mileage.

 

[Jammer]

I think you are correct that GPS logs are more accurate and tend to be slightly longer. Here are the reasons I've considered:

1) GPS is more accurate measuring each slight bend in the trail.
2) GPS commonly measures VERTICAL distance as well which actually adds slight distance to the horizontal total.
3) People tend to walk more than just the trail (eg back-and-forth to take pictures, rest stops, etc.)

My GPS will lose signal every now and then and give me some bonus mileage while re-locking on correct location.

In the end the differences usually aren't that big of a deal -- maybe a consideration for longer hikes.

- Jamal

 

[John Fowler]

It has also been my experience that my GPS tracks come out longer than the trail description. I think Jammer's comments are appropriate. But, remember that the GPS takes readings at discrete time intervals. This interval might be user-settable on some devices. If the interval is too long, the GPS will underestimate distances on winding routes due to cutting off the tangents. Unless you are going very fast, or unless the interval between measurements is excessive, this shouldn't be a problem though.

 

[John Fowler]

An interesting test is to turn on your GPS and tracking and just set it out in the yard for a few hours. I bet it will say that you have gained and lost several hundred feet in elevation and moved at least some small distance.

 

[Bob]

Depends on how short of time / distance your GPSr is recording. Creating a point every 100' is more accurate than recording a point every 500'.

I have always found mine to record a little longer than map measured.

 

[John Fowler]

An interesting test is to turn on your GPS and tracking and just set it out in the yard for a few hours. I bet it will say that you have gained and lost several hundred feet in elevation and moved at least some small distance.

Well, I was wrong! I turned on my Garmin Colorado300 and set it outside on a rock for an hour with tracking on, and it did exactly the right thing: said it had moved 1 foot. I was surprised because I had done this same test several years ago with an older model and got much different readings.

 

[Aldaron]

was surprised because I had done this same test several years ago with an older model and got much different readings.

Did you do it before they turned off Selective Availability in 2000?

Older GPS devices often didn't lock onto as many satellites, either, allowing for more errors. I've experienced the same thing as you did a long time ago, but I haven't noticed it lately. Either that's a good thing, or I just don't pay attention anymore!

@Nick, I used to be friends with a wilderness ranger, and I know that at least sometimes he measured trails with a measuring wheel. I don't know if that's what they usually use for official trail distances, but if it is, I can imagine that would give some really bad results on rocky trails.

I think you and I even talked a little about this out in the Winds this summer.

I haven't piped in on your question because I have absolutely no scientific or objective support for my opinion. That having been said, my subjective observational experiences have made me rely on the GPS distances more than published or signed distances.

 

[uintahiker]

@NIck- If you are providing the trail guides in electronic form so that users can download the tracks to their own devices, it makes a whole lot more sense to go with the distance from your GPS. You probably have a little bit of error built in by any step or two this way or that to get a better photo etc. The end users will also experience the same thing as they inevitably will make variations of the same moves. Then in the writeup, mention that the trailhead sign says x, but actual GPS distances are X. If the end user can download your track, it makes the most sense to have your report and the map be the same, regardless of signs at the trailhead.