Method of certifying a route
The difficulty of evaluating a Trail-Running race
The search for a sensible measure
A route's distance and elevations are the two main data to define a trail-running race and it is particularly important that they be as reliable as possible. Unlike a normal flat road, trails are by nature irregular. It is thus necessary to keep in mind that there is no exact measure of the distance and altitudes of a route.
Let’s take, for example, a rocky trail. To go from point A to point B, a small animal will have to climb over each rock. Thus, he will travel a much greater distance and elevations than a human that will simply walk over each rock. Likewise, during a trail-race, an agile and/or a rested runner will jump over a small ditch, unlike another who will have to climb down into the ditch then back out making the distance and elevations travelled by the second runner much greater. Another example is that of downward rocky and turning path where a nimble runner will jump from stone to stone while a less agile runner will be more wary and choose a longer route.
The measure of a route on paths, be it in distance or elevations, is limited to the most reasonable assessment possible always obtained by using the same measurement method so that it is possible to compare routes.
The relevant tool?
A second problem arises: that of the method used to obtain this evaluation. Should we, for example, measure the distance by unrolling a string over the whole length of the route? At first glance, this would seem to be the most accurate method but it is actually very impractical – if not impossible – and gives a random outcome. Should a wheel be used, as for road races? What about very irregular paths (i.e.: a scree)? Anyways, these methods offer no solution for the measurement of altitudes.
Paper maps has been, until recent years, the most commonly used tool but giving a very broad approximate. The measure of distance with an opisometer is, by nature, imprecise and the calculation of altitudes requires a very delicate, fastidious and difficult reading of the altitude curves to spot the high and low areas.
Nowadays, it is easier to use cartography software. Nevertheless, drawing a route on computer can be flawed with numerous mistakes due to the limited precision of the used software (the drawn path never truly corresponds with the actual path) and the user’s imprecision; on a very steep terrain, how can we guarantee that the drawn point is exactly on the path and not 5 meters above or below? Thus, the altitude can easily vary by several meters.
In recent years, the use of GPS tools has become common. Henceforth, the best way to measure a route is to walk the full route with a GPS tool in hand.
A GPS' limitations
But, beware; the gross data given by this apparatus is fundamentally approximate and false!
The importance of the GPS' sensitivity
Indeed, your GPS, like any measurement tool, has only a limited accuracy. This accuracy depends first of all of the number of satellites it can connect to (the more satellites, the better the accuracy). But, in difficult areas (under a cliff, in a forest…) the GPS does not pick up the signal as well and the accuracy diminishes. Thus, we advise you to use a GPS with a good antenna, thus more sensitive to the signals.
The frequency of the tracepoints
Secondly, the question of the frequency of the GPS points is brought up. A GPS does not draw a straight line but picks up points at a given frequency. Thus, the obtained line is made of different segments and the measure of the route is the result of the sum of all of these segments. If the frequency of these point is too small (for example once a minute or once every 100m), the track will be too perfunctory and as result wrong (e.g.: some turns have been cut out).
But, if the frequency is too great (e.g.: one point every 2m or every second), the distance between each point is under the accuracy threshold of the GPS tool resulting in a crisscrossed line longer than the actual route.
Lastly, if the GPS tool is set up to pick up a point every x seconds, when one stops in one place, the GPS records a great number of points at this same area which, due to the GPS’ limited accuracy, do not all have the same latitude, longitude or altitude. As a result, your pause becomes a fictitious route artificially lengthening the distance and elevations of your race. For all of these reasons, we advise you to set up your GPS tool to pick up a point every 10 meters.
After having travelled the route of your race, your task is not yet completely finished. The satellite signals received by your GPS can sometimes be disturbed, notably in rocky areas. This can result in rogue points (distant of tens, if not hundreds of meters from the actual position). The only way to fix this is to upload your track to cartography software and examine the track to detect and erase these fallacious points.
Apprehending distance and elevations
It is now time to measure the distance and elevations. Once again, things are more complicated that they seem.
Simply summing up the altitudes between two points all along the route gives a largely overestimated approximate of the total elevation increase and decrease. Indeed, the GPS’ measurement of altitudes is flawed with multiple accuracy errors. Added up along the whole length of the route, these errors can give an overestimation of over 30% of the total elevation gain and loss!
The importance of these errors depends of the GPS tool used.
If these errors are solely calculated by the GPS, they can be quite important. Indeed, the altitude is estimated by triangulation, similarly to the latitude and the longitude, which only satellites on the horizon can calculate. As a result, the measured altitude is up to 3 times less precise than that of the latitude and longitude.
In order to better this accuracy, some GPS tools have a pressure altimeter partnered to the GPS that can smoothen the measure. Indeed, when one does not move, the GPS’ altitude can differ by a few meters while the atmospheric pressure is constant (at least during a short period of time). This increases the accuracy, though it remains imperfect: your GPS tool will continue to give you altitudes within a 1 to 2 meters margin though you remain still.
This is why it is necessary to smoothen the altitude curve by calculating a moving average that does not take into account elevation differences between points that are under a determined threshold. The value of this threshold is fixed and depends of the GPS you use. The values chosen by ITRA are the result of many years of experimentation and comparison of data given by different instruments for a given route and of study of the different methods used in the world of outdoor sports. The required threshold is of 3 for GPS tools with a pressure altimeter and of 10 for those without a pressure altimeter.
Two methods exist. The first is in relations with the official definition of distance; the measured distance between 2 points of the Earth’s crust following its curvature. If this ‘scholar’ calculation is necessary to measure great distances, it is of limited use for distances of several dozens of meters or less because, on a sloped path, it does not take into account the altitude of the points and tends to underestimate the actual distance.
Our preferred method is to also take into account the altitude of the points. This results in the use of the Pythagorean theorem:
d = flat distance (the ‘scholar’ calculation according to the latitude and longitude of the points)
e = altitude gained/lost