Knowing the Distance - An Interview with Dr. David Dreyfuss

How do you know that a trail marathon will equal exactly 26 miles and 385 yards? Few things are as frustrating as clocking a PR, only to realize that everyone clocked a PR thanks to the course being two miles too short. But then there are folks like Redwood Trails, who will start many races by saying “you MUST start at this post, otherwise you'll be 90 feet short”. Curious to how they can get so accurate on a varied terrain, I tracked down the man behind the method, Dr. David Dreyfuss, who addresses “accurate measurement” like a true PhD from MIT, helping dispel rumors of GPS accuracy, and explaining what it really takes to measure a course to the very last inch.

(Dr. David Dreyfuuss, out on the trail)

Brief background on who you are and how you got into the trail business.

DD: Well, I've been a lot of things and worn a lot of different hats over the years. Roughly, I still think of myself as an experimental physicist, though I have a doctorate in Gas Dynamics (MIT, 1980) and have spent much of my recent professional career developing digital printers--laser printers and the like. I came out to California to try my hand at a start-up ventures in new color printing technology, helping launch Google Answers, and rebuilt the website and e-commerce business for Aqua Safaris, a Santa Cruz SCUBA shop. I'm also a board member and edit the quarterly national newsletter for the Viola da Gamba Society of America - and yes, I also occasionally perform on the instrument.

So how did you find Redwood Trails?

DD: I started my relationship with Redwood Trails by responding to an ad for aid station staff. I've never been much of a competitive runner; I can probably count the number of races I've entered on one hand, and I'm not particularly fast. However, I do have a lot of useful skills related to trail running. I have thousands of miles of trail and wilderness travel experience, hiking, backpacking and recreational running. I spent three summers leading backpacking trips as a Ranger at Philmont Scout Ranch. I also have a lot of experience orienteering, which is to trail running roughly as trail running is to road running--it's off-trail running through the woods with map-and-compass navigation between control points. So I already knew many of the trails that Redwood Trails uses for events, and the basics of scouting, navigating and marking a race route are kind of second nature.

It would seem with the advancement in GPS that measuring trails these days is pretty easy.

DD: GPS (Global Positioning System) has revolutionized many aspects of navigation, mapping, surveying, and position determination. A $100 handheld receiver can tell you your location in three dimensions (let's say, latitude, longitude and elevation) to about 10' accuracy, at least some of the time. With slightly more expensive equipment and some patience (to average out some randomness that exists in the measurement due to things like atmospheric effects, and to allow time for the satellites to move around a bit), you can locate a single position at least of couple of orders of magnitude better--good enough to match or exceed the accuracy of typical surveyors' instrumentation.

But while GPS continues to get better, it is still an imperfect technology. To get the 10' accuracy, you need what is called "differential" GPS--simultaneous measurement of a known location so that some sources of inaccuracy can be subtracted out. Right now, differential GPS is readily available in the US via a couple of extra satellites that continuously broadcast the measured location of a set of fixed ground stations, which in turn have well-known absolute locations. Unlike most of the GPS satellites, which are in relatively low polar orbits, these two extras are in geostationary orbits near the eastern and western horizon. Unfortunately, that means they are often out of sight behind a ridge. Without their assistance, the accuracy drops to more like 30' at best. And, although nominal accuracy is the same for horizontal (lattitude and longitude) and vertical (elevation) positioning accuracy, practical experience shows that the elevation accuracy and repeatability is usually worse than the horizontal accuracy).

There are about 24 satellites in the system right now. In principle, you can get a position fix as long as you can see at least three of them at any given time. With an unobstructed horizon, you can typically see at least ten, but if your view of the sky is partially obstructed by hills or even dense trees, you can easily lose sight of most of them. Accuracy drops as the number of visible satellites goes down, and also gets worse if the visible satellites are clustered too close together. Airplanes and boats can navigate quite accurately using GPS, because they almost always have a good view of the sky down to the horizon. Car navigation systems generally cheat by assuming that you must be on their known network of mapped roads and using dead reckoning by direction change and wheel rotation when GPS signal is lost (occasionally generating some amusing errors, when their best efforts fail!). In any case, such cheating is not useful if your goal is measurement instead of navigation! Trail users in mountainous and wooded terrain find GPS to be useful, but only intermittently. Tests on numerous Redwood Trails routes have shown that there are always portions of any interesting run where accuracy is poor, and occasional complete loss of usable signal is very common in most locations.


So, given that the technology is not really there and perhaps never will be what does work?

DD: All the "old-fashioned" measurement techniques! You can choose any number of specific tools depending on your goals and the accuracy you need or want--surveyors' transits, tape measures, odometers, pedometers, etc. If you're "lazy," you can try to estimate distance off a map or by using published or posted trail distances, and reading elevations from a topo map. Many trail events are, in fact, organized based solely on such estimates. Occasionally we find specific trails that have been accurately measured and are accurately mapped and posted. More often, we find that such measurements are approximate at best, and large errors are not uncommon! We encountered one example where an advertised marathon route was short by several miles.

We measure all of the routes we use ourselves. Our preferred instrumentation is a wheel rolling along the ground. Depending on the circumstances, we use either a "measuring wheel" or a bicycle, essentially counting revolutions or fractions of a revolution of the wheel. For elevation, we use a barometric altimeter. When we generate a profile for a new route, we typically record elevation every 500' and both elevation and distance at all major landmarks (trail junctions and so on). This can be done with pencil and paper, but more recently, we usually use a pocket tape recorder with a headset microphone which allows data recording almost without stopping. Barometric altimeters are accurate to a few feet as long as the weather is stable and a reference altitude can be found to zero out the day's weather conditions.

How can you ensure accurate measurements?

DD: As with any measurement, accuracy requires attention to detail. And, of course, "precision" does not guarantee "accuracy"! We use a measuring wheel which reports measured distances in feet, but not surprisingly, we never measure exactly the same number of feet for a given trail route. Trails often have rough surfaces, and it is almost impossible to roll a wheel along exactly the same detailed path on a repeat measurement. The accuracy of a measuring wheel also depends on the constancy of the circumference of the wheel. If the wheel acquires a layer of mud, its diameter increases. It may expand with increasing temperature. It may wear with use. If it has a pneumatic tire (as do most bicycles), the circumference also depends on tire pressure and rider weight. For best accuracy, one needs to check the calibration of the wheel against a known distance on a similar surface before and after the trail measurement.

So, do you use the same method that is required by the USATF to certify a course.

DD: Essentially yes! The USA Track & Field Road Running Technical Council establishes measurement standards and procedures that are used for all events for which distances are certified for record purposes. The process requires the use of a calibrated bicycle with a special counter mounted to the wheel. A calibration course of at least 1000' must be run four times before and after each set of measurements. A course must be measured twice along the shortest possible route (usually not much of an issue for trail runs). Repeatability must be better than 0.08%. A "safety factor" of 1.001 is used to ensure that the actual race distance is AT LEAST the advertised distance. If a new record is set for the distance, the course may need to be recertified for distance, and the safety factor helps ensure that the record will not be disqualified for being set on a "short" course. So next time you think you're running 26.219 miles for a marathon, if it's a certified course, you're likely running at least an extra 138 ft--more of you don't cut all the curves as tight as possible. Or think of it as 5 ft per mile--not all that significant.

We went through this process to certify the Bizz Johnson Marathon so that it could be used as a Boston qualifier. Most of our trail races have no real need for formal certification. Each is a unique race with special topographic challenges, and records are really only meaningful for that particular route. But Redwood Trails tends to be obsessive about accuracy anyway. In most cases we relax our measurement accuracy requirements by skipping the repeated calibration checks and simply double-checking the race-day distance measurement against our previous profile run. But worst case, our distances are still accurate to within better than a hundred feet on a 10K. (If you can feel that difference when you're running, let us know--we'll hire you!) And while the RRTC requires the use of a calibrated bicycle, we usually prefer to use a measuring wheel instead, both because it is allowed on all trails while bicycles are not, and because it can go essentially anywhere you can run or walk (or even climb!). We do use a measuring wheel with a large circumference (six feet--like a bicycle wheel but with a solid rubber tread on a steel rim--makes a very stable circumference) which we find does a better job of averaging out small trail irregularities than a smaller wheel. Curiously, Rolatape claims accuracy for their measuring wheels without special calibration of the same 0.08% as the RRTC targets for measurement repeatability!

How much time can trail measurement take?

DD: Well, as with any careful measurement, you can't rush the process, and you have to be prepared to back up and start over if something goes wrong or measurements don't repeat to sufficient accuracy. It took us the better part of two days to complete the certification measurements for the Bizz Johnson Marathon. We had to establish our own custom calibration course on a section of the Bizz Johnson Trail surface, measuring it with a 100' steel tape and correcting for temperature. Then the measuring bicycle got a flat about eight miles out, essentially requiring that we start over. After that we chose to do segments of no more than about seven or eight miles at a time complete with repeats and calibration checks before doing the next section, so that in the event of another "instrument failure," we would not lose too much data. But we never had problems with repeatability, typically about 0.02% even on the relatively loose dirt surface of the Bizz Johnson Trail.

What about marking the trails on race day?

DD: We try not to make trail running an exercise in navigation! Many runners are accustomed to road events where they are pretty much forced to follow the race route, and they often tend to "zone out" while running. On a trail run, they do have to pay at least a little attention to where they are going, but our routes are well-marked, complete with mile markers, and we double-check for vandalism immediately before start time. The mile markers are the main reason that we have to measure the trail one more time for race day--we can't leave permanent markers in place. In addition to mile markers, we typically post direction signs at all trail crossings and turns, generous wrong way signs along incorrect route choices, and orange ribbons along the correct route away from any trail junction or intersection. We also may station a "traffic cop" at any difficult intersections where runners may be taking multiple routes, either for different races or different legs of the same race. The marking process isn't fast either! It can easily take six hours or so to fully mark a half-marathon route even if one runs between marking locations. And, of course, there is unmarking to be done after the race as well!

What mileage do you use for race results?

DD: I presume you mean, "what distance do we use to calculate "pace" for a race?" We use the actual measured distance, of course (including the 1.001 "safety factor" if the race has a certified distance). Trail races are often advertised at "standard" distances (marathon, half marathon, 10K, 5K). Where possible, we try to adjust the start or finish location, or the length of an out-and-back dogleg to make race distances "right," but sometimes that just isn't practical, and the real distance may be a little longer. Our courses are never short, because we can always add a little extra distance somewhere if the natural trail length is short. If a race is long, we try to let runners know, both before and after the race, exactly how far they did go (though if they ignored all the wrong way signs, they're on their own!).

At Redwood Trails events you always have results available right away. I understand that you using your own timing software. Aren't there existing commercial software packages out there?

DD: Sure, there are existing commercial packages to manage race results. But as one of the most active race management organizations around, we tend to develop our own ideas about how things should work, and it can be problematic to impossible to customize someone else's package. I've been programming for some 35 years now, and had plenty of support from Eric Gould (who has award winning software design credentials himself), my wife (another MIT PhD), and my son Ethan who is now a freshman at Stanford. We wanted a robust, easy-to-use package that would allow us to manage our complete events from pre-registration through immediately available results at the event and same-day posting of results with an easy-to-use interface on the web site. It might seem that there's not much to the task of recording start and finish times for a couple of hundred runners, but it turned out to be a good deal more challenging than we had expected.

Thanks for the interview, David. Looking forward to the Palo Alto Vista Run on March 5!

Cheers,

SD