GPS trackers and altitude – la croix de gattigues

All hikes start from the iron cross of Gattigues

Any attempt to get a good fix on cumulative climbing using GPS devices (handhelds, or smartphones) is fraught with error. Anyone who has used these GPS tracking devices knows that, while longitude and latitude are quite accurate, the altitude measurements at any given point vary from accurate to completely off the mark. GPS chip quality, embedded software and various external factors affect these readings. Walking at the base of a steep limestone cliff will cut out half of the sky and half of the satellite readings. Walking through a wet forest under wet leafy trees degrades the signals too. And there might be times when military services move the geopositioned satellites to serve a military purpose thereby degrading consumer level accuracy. So in this article, I compare a couple different apps to the Garmin GPSMap 64s to see if there are any conclusions.

I used the Garmin GPSMap 64S to record a track of (theoretical) distance 10.1 km. On the same hike, I turned on the trackers for 2 smartphone applications described below.

First is the Android app – Ultra GPX logger (cost about 5 €). This app’s primary focus is logging GPS tracks. The app can create GPX, KML, NMEA or CSV track logs. It is able to stop logging to save battery when the device is stationary (the Garmin GPSMap 64s does not know how to do this and consequently adds error to the track record). The app also uses a Kalman filter – a mathematical technique to filter out noise in the altitude recordings and (theoretically) provide better accuracy. This might be used on the Garmin too, but I did not see any mention in the docs. There are bunch of other settings and I am far from sure if I set it up properly. The app does a very good job on logging longitude and latitudes too. The app lets you export track logs and view them on a base map. For altitudes, we’ll look at some charts below.

Second is the Android app – Orux Maps (free). This app has more functionality than Ultra GPS Logger (e.g. logging heart rate if you wear an HRM belt) but still does a good basic job of logging the track, displaying it on a base map, providing speed and altitude charts, etc. The app is a bit hard to get used to as it makes use of multiple icons for navigation. Several of the smartphone apps I tried have this problem which a result of the limited space on the smartphone screen.

So here are the charts.

Click for Garmin vs theoretical

Garmin altimetric profile vs theoretical

This chart shows that the Garmin altitude measurements are fairly good. They track fairly close to the theroetical chart, but there is anoticeable distance shift. In fact the Garmin device is overestimating distances compared to the theoretical chart and introducing a lag in the chart. Garmin says this track was 10.6 km in length. Theoretically the track is 10.1 km long. I suspect this difference is due to Garmin continueing to record short distances between readings so introducing errors while the device is stationary. (We stopped a lot on this walk).

Click for Orux vs theoretical

Orux Maps altimetric profile vs theoretical

Here, it’s interesting to note that Orux tracks much closer to theoretical than the Garmin. Remember this log is obtained from the smartphone so not the same chip and software. Orux says the track is only 10.4 km long – also closer to the theoretical value. The Orux track only starts to show longer distances after about 7 km. Overall this altitude chart is as good as, or better than Garmin. But there’s a big catch. There is significant point to point variation in the altitude readings visible as up-and-down spikes on the chart. If you try to calculaten cumulative climbing on this (unsmoothed) data, you’ll get a substantial over estimate.

Click for Ultra GPX Logger vs theoretical

Some conclusions

The Garmin device is better than the smartphone apps overall. While it overstated distances it has a much smoother reading on altitudes. The smartphone altitude profiles are very jaggy – indicating lack of stability of the GPS meansurement. Perhaps this is just the smartphone chip quality.

The distance readouts are better with the smartphone apps. This is likely due to the ability to stop recording when the device is stationary.

The experience during this trial leads to some other unrelated observations though. Viewing a smartphone screen in bright sunlight is a non-starter. Carrying a smartphone by hanging in on your backpack or your belt is ill-advised. And battery life is badly affected by continuous use of the smartphone over a 3-4 hour walk. Smartphone apps often rely on network connections to obtain basemap tiles. The Garmin with large screen fonts, interchangeable batteries, and rugged build has none of these issues.

Overall Garmin wins hands down on useability, but is closely pursued by (at least) one smartphone app in regard to track data quality.

Hiking in south central France

This site is about walking, mostly in south central France, specifically the Cévennes “mountain” range, the Grands Causses area with it’s plateaus and canyons, the departments of Gard with the garrigue landscapes, the Vaucluse, the Herault, sometimes the Ardèche, the Avéyron and the Drôme. There are descriptions of interesting walks, but sometimes also the non-interesting ones. There are occasional articles about the hardware and software you might use while hiking and in particular handheld GPS devices and smartphones. There are also references to cartography both in the field and on the computer screen back home. The progress in the area of web mapping in the past several years, with the arrival of services like Google Maps, Nokia Here (now “Here we go”) and the OSM Open source map service, has been spectacular.

However, hiking in this region is more than just walking with eyes focused forward or concentration on the GPS screen. It’s also about observing and understanding the landscapes, villages, farms, ancient ruins and various geological curiosities. Often the subject of interest is not the “what”, but the “why”. Why did people settle here hundreds of years ago? Why did the ancient trade routes pass this spot?

In this region, traces of human activity are everywhere – in the still occupied villages and farms, but also in the undergrowth, on the riverbanks and and on the abandoned hillsides. This site aims to explore these subjects, traveling on foot, at about 3.5 km per hour.

This site is also for people who like maps. There are maps in nearly every article, often with interactive features. Try them out.

For every walk mentioned in an article, the site provides a downloadable gpx file suitable for use on a handheld gps device. The site provides additional access to a database of some 700 circuits also complete with downloadable gpx. It is worth noting however, that a gpx track is a perishable commodity. Over time waymakings change, land owners block walking routes, obstacles appear. For this reason, when available, dates of the most recent passage on the circuits are also provided. Use with care.

Everywhere you go on these hiking routes you are likely to see a field cross. They can be in iron, stone, or wood and can be simple or ornate. These field crosses are supposed to have protected travellers from bandits – as supposedly no bandit would dare attack near a symbol of the all powerful religion.

So the french word “croix” translates to cross and holds the connotation of the cross as a religious symbol. However, in the context of this site, cross means crossroad or intersection. “La Croix de Gattigues” refers to a crossroad in a hamlet in the Gard department at which, no surprise, there is also a fine an iron cross.

Click for Garmin vs theoretical

Click for Orux vs theoretical

Click for Ultra GPX Logger vs theoretical

Leave a Reply Cancel reply