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# Comments on A Trail Runner's Blog: Knowing the Distance - An Interview with Dr. David Dreyfuss

Updated: 2018-02-20T11:36:28.344-08:00

Good stuff here. Would Dr. Dreyfuss be willing to ...

2006-07-31T14:02:00.000-07:00

Good stuff here. Would Dr. Dreyfuss be willing to share his experience how a fixed circumference measuring wheel that is calibrated on a flat road course does on a rough mountainous trail? Assuming the wheel is kept on the ground, with the many undulations over rocks and roots and waterbars, etc, can we expect the wheel to overestimate the distance by 1%, 2%?

Also, how long are his calibration courses and how does he get a representative section of trail? It would seem that trail courses have wildly varying types of terrain, from smooth to very rocky, and using only one calibration course might not be representative for the whole trail.

Thanks.

Sorry if came off as mean, Phil. It just seems li...

2006-04-11T07:51:00.000-07:00

Sorry if came off as mean, Phil. It just seems like you were flaming the good Doctor with your arrogance. It also appears that you still miss the point. Your system is less accurate than his. We are measuring DISTANCE here. No one really cares where the Trail is (as long as you can find it). The question here is How Long is it?

Let's look at the facts:

1. You can measure very accurately every 1/2 second.

2. And giving you the benefit of doubt, let's just say that you are able to set up a correction benchmark ahead of time and your accuracy is 1 cm (I doubt that you get this in real time).

3. And lets say (since you said it was fast) that you do it at a walking pace of 3 miles per hour. Three (3) mi/hr = 4.4 ft/sec so you have a very accurate measurement every 2.2 ft.

4. In order for your system to be more accurate than Dr. Dreyfuss (0.02% error) you must measure your two points between a section of trail that does not deviate (in elevation or curvature) over 0.00044 ft (0.02% = 0.002 * 2.2 ft = 0.00044 ft).

5. With a MAXIMUM deviation of 0.00044 feet allowed in the trail from point to point, this works out to be a curve with a MINIMUM radius of about 11,000 feet. i.e. your trail cannot curve (or change elevation) with a radius of less than 11,000 feet. A highway designed for 70 mph typically has a minimum radius of about 1,800 feet for example. So your system won't be accurate enough for an Interstate Highway, much less a trail.

6. "Post processing" has little if any benefit since the algorithm involved still interpolates between the two points but offers no additional actual measurements and therefore is only "a better guess" than a straight line. Overall your PP may provide a small decrease in error. Let's say 10-20%. Way below the estimated 2,000% change you must have to make you system viable.

Sorry I posted anonymously but I just didn't want to take the time to fill out the form (I will try and do that today). Please send your regards to your buddy Mark. And please let me know if you need help with the math on this proof.

See you at the next race.

Best regards,
Tim

My data are based on a track point plotted every s...

2006-04-10T21:55:00.000-07:00

My data are based on a track point plotted every second and then making the same track again in reverse. The track points generally set out at one per footfall. They are later differentally corrected. Did b_upright@hotmail.com (since he was too humble to post his name) miss this, since I stated that the Thales was post-processible? It looks like he didn't read much of my other data on my website, either, but by his tone I don't think he was very interested in comparing data. Those of you who are, feel free to respond. Thanks!

Phil

P.S. If you don't like anonymous posters clogging the blog with insults, let Mr. b_upright know.

Mr. String Bass Player guy (Phil) is truly the BES...

2006-04-10T14:02:00.000-07:00

Mr. String Bass Player guy (Phil) is truly the BEST mapper in the world. Apparently he has been able to get "better" accuracy than Dr Dreyfuss' wheel with his Thales GPS. No setup for differential correction (because that would take too long). Thales website states "sub meter" real time accuracy. So let's say that Mr Bass Player guy can actually fix enough points along his trail to "sub meter" accuracy. What does he have? A trail that is accurately measured IN LOCATION not the distance or length. All GPS software use interpolation for the distance measurement inducing inaccuracies between the known fixed points. This is best case. Worst case is the trail twists and turns between points inducing more error. It would seem you would need something that follows the ground accurately... like maybe a wheel... to measure with a lower amount of error in DISTANCE (Which is what we are trying to measure anyway) Looks like Mr Bass Player has some catching up to do!

2005-08-13T21:47:00.000-07:00

I assume all of your comments are with the understanding that truly post-processible GPS has been out of reach for the consumer (and most RDs). That is not the case. I have recently measured the Wasatch 100 and the Katcina Mosa 100K Mountain Challenge with a Thales PP GPS unit that sells for the ridiculously low amount of \$1800. Plotted over a DEM the tracks from this unit on the treacherous Wasatch trails (which beat the snot out of my Rolatape) yields a repeatability of <0.02%. A wheel cannot touch that. Wheels are doomed.
Katcina Mosa 100K, by the way, is the most accurately measured trail race in the country, having been differentially surveyed. Looks like the Redwood race has a bit of catching up to do. ;-)
BTW, I play string bass with the Mormon Tabernacle Choir. What is with lower strings and anal retentiveness?
Cheers,
Phil Lowry
www.phillowry.com

Certifying a marathon distance is a day or two’s w...

2005-02-18T07:24:00.000-08:00

Certifying a marathon distance is a day or two’s work depending on ease of access and such. It takes some time, certainly, but so do many of the other organizational tasks that go into putting on an event. And if you think that’s a lot of work, you should try preparing for a major orienteering event! An “A” meet is generally conducted on a newly drawn map (so that no competitors have an unfair familiarity advantage), which can take a few man-weeks to prepare!

I’ve never done a 100-miler, but the process wouldn’t be any different. If you need to get it done fast, you might use multiple measurement teams doing different segments, but one or two people could do the whole thing over a few days.

—David Dreyfuss

Actually no, we don’t use GPS for altitude, except...

2005-02-18T07:08:00.000-08:00

Actually no, we don’t use GPS for altitude, except possibly as a starting reference to determine the offset for barometric altimeter measurement if we don’t have anything better. The accuracy of GPS altitude measurements is often significantly worse than latitude/longitude. Now, as it happens, the barometric altimeter that I use is built into a hand-held GPS unit, so you might mistakenly think I’m using GPS if you see me out measuring!

—David Dreyfuss

While I understand your desire to have “degrees of...

2005-02-18T07:01:00.000-08:00

While I understand your desire to have “degrees of difficulty” posted for races, it's a concept that is difficult to implement in practice. There are, of course, a number of factors to consider. Some are clearly a property of the particular route: average altitude, total climb, steepest climb, number of steep climbs, trail surface, distance. Others are a property of the race day: wind, temperature, rain, sun, mud. And different runners respond differently to these challenges. Some adapt easily to cold or heat. Others may be more or less sure-footed on rough terrain. Some cope better with steep slopes or long uphill sections or may be better adapted to the altitude, or be better able to speed up on downhill sections. I think as race organizers, the best we can do is to provide good information about the particular event. Even a composite number like “total climb” is kind of slippery, because it depends on how fine-grained the elevation data you use is. And, of course, race promoters are always going to play up the “attractive” features of an event and play down things which might discourage entrants. One thing I can suggest, though, is to learn to look carefully at the scale of an elevation profile. Profiles may look very similar at a glance, but if one has 10 times the vertical scale range, you know you’re in for more climbing. And look for elevation changes over moderate distances—say how much climb over a mile. Most people can cope with short climbs without slowing down much, but a long steady climb is more of a challenge.

—David Dreyfuss

Man that sounds like a lot of work. How long does ...

2005-02-17T19:46:00.000-08:00

Man that sounds like a lot of work. How long does it take to certify the distance of a marathon? Have you ever done a 100-miler?

But you still use the GPS for altitude measurement...

2005-02-17T19:45:00.000-08:00

But you still use the GPS for altitude measurements, right?

I wish there was a way to assign a "degree of diff...

2005-02-15T13:12:00.000-08:00

I wish there was a way to assign a "degree of difficulty" to a trail. How can you look at two 1/2 marathons and know which is the most difficult? Accurate distance and altitude change is one thing, but "steepest ascent", " trail condition", etc., can also factor in. I've run 10ks much harder than some 25ks, and I've never got the hang of looking at the maps to understand it.