Salonanarchist | Leunstoelactivist

Cycling: Garmin altimeter compared to elevation databases

During a very rainy ride in Scotland, my Garmin altimeter appeared to be off: on some of the steepest climbs it failed to register any gradient. Afterwards, I tried the «elevation correction» feature on the Garmin website, which generously added over 750m to the total ascent the device had measured. This was certainly more satisfying, but it left me wondering. Can the weather affect the Garmin altimeter? And how accurate is the recalculated ascent?

Garmin’s recalculation service works basically by looking up the gps locations of your ride in an elevation database. Strava offers a similar service. Below, I analyse the Garmin and Strava recalculations for a number of rides. Note that this is only an exploratory analysis and that no firm conclusions can be drawn on the basis of this rather small set of observations. That said, here are some preliminary conclusions:

  • If you want to boost your ego, let Garmin recalculate your ascent: chances are it will add (quite) a few metres. Strava’s recalculations tend to stay closer to the original measurement. When it does make changes, it frequently lowers the number of metres you’re supposed to have climbed, especially on relatively flat rides.
  • In theory, you’d expect weather changes to affect the ascent measured by the device, because the altimeter is basically a barometer. In practice, weather changes don’t seem to have much effect on the altimeter.
  • It appears plausible that heavy rain does in fact mess with the altimeter.

In the graphs below, the colour of the dots represents the region of the ride. Red dots represent the Ronde Hoep, a flat ride to the south of Amsterdam. Blue ones represent the Kopje van Bloemendaal (north, south), the closest thing to a climb near Amsterdam (it’s not high but quite steep). Green dots represent the central area of the country and include the Utrechtse Heuvelrug, Veluwezoom, Rijk van Nijmegen and Kreis Kleve (the latter in Germany).


By default, the graph above shows how much the Garmin recalculation differs from the ascent measured by the device (graphs may not show in older versions of Internet Explorer). The closer a dot is to the dashed line, the the closer the recalculated ascent is to the original measurement.

For rides shown on the left part of the graph, where the device measured less than 500m ascent, Garmin’s recalculation often adds about 50 to 100% or more. With higher ascents, the recalculated ascent is closer to the original measurement, although it still tends to add about 30 to 50%. The highest dot to the far right of the graph is the rainy ride in Scotland; here Garmin’s recalculation added over 35%.

With the selector above the graph, you can select the Strava recalculation. You’ll notice the scale on the y axis changes (and the dashed line moves up). Also, a few red dots enter the graph. These are rides along the Ronde Hoep, which is a flat ride. For these rides, Garmin’s recalculation added up to 750% to the ascent measured by the device; therefore these dots were initially outside the graph area.

The Strava recalculations are similar to the Garmin ones in that the correction is larger for relatively flat rides. Unlike Garmin, Strava lowers the ascent in these cases, often by 15 to 50%. For rides where the device measured a total ascent of over 500m, the Strava recalculation tends to be pretty close to the original measurement.

Weather changes

It has been suggested that changes in the weather may affect elevation measurements. This makes sense, since the Garmin altimeter is in fact a barometer. Wikipedia says that pressure decreases by about 1.2 kPa for every 100 metres in ascent. In other words, if net atmospheric pressure would rise by 6 mBar, this would cause the device to underestimate total ascent by about 50 metres, so the theoretical effect wouldn’t seem to be huge.

The graph above shows how much recalculations differed from the original measurement, with change in pressure on the x axis. Note that the effect of recalculations is here in metres, not percent. I tried different combinations of pressure measures and recalculations and in only one case - the Garmin recalculation shown above - the correlation was statistically significant (and the regression line much steeper than the Wikipedia data would suggest), so this is not exactly firm evidence for an effect of weather change on elevation measurement.

Heavy rain

It has been suggested that heavy rain may block the sensor hole and thus affect elevation measurement. This may sound a bit weird, but I have seen the device stop registering any ascent during very heavy rain. Among the rides considered here, there are two that saw really heavy rainfall (the Scottish ride and a ride in Utrechtse Heuvelrug on 27 July). These do show some of the largest corrections, especially in the Strava recalculation. So it does seem plausible that rain does in fact affect elevation measurement.

In the spirit of true pseudoscientific enquiry, I tried to replicate the effect of heavy rain by squirting water from my bidon onto the device during a ride in Utrechtse Heuvelrug. This didn’t yield straightforward results. At first, the device registered implausibly steep gradients and it turned out it had interpreted the hump between Maarn and Doorn as 115m high, more than twice its real height. About halfway, unpredicted rain started to fall, mocking my experiment. Strava recalculation didn’t change much to the total ascent but it did correct the height of the bit between Maarn and Doorn, so it must have added some 50+ metres elsewhere. Be it as it may, the «experiment» does seem to confirm that water can do things to the altimeter.


I took total ascent data measured by my Garmin Edge 800 and obtained a recalculation from the Garmin Connect and Strava websites. Subsequently, I looked up weather data from Weather Underground (as an armchair activist I do appreciate their slightly subversive name). Weather Underground offers historical weather data by location, with numerous observations per day. I wrote a Python script that looks up the data for the day and location of the ride and then selects the observations that roughly overlap with the duration of the ride. There turned out to be two limitations to the data. First, it appears that only data at the national level are available (the Scottish ride yielded data for London and all Dutch ones data for Amsterdam). Second, for the day / location combinations I tried there was no time-specific data for precipitation available, only for the entire day.

Because of these limitations, I also took an alternative approach, looking up data from the Royal Netherlands Meteorological Institute KNMI. This did yield more fine-grained data, although obviously limited to the Netherlands. In the end it turned out that it didn’t make much difference for the analysis whether KNMI or Weather Underground data is used. Code from the scripts I used for looking up weather data is here.

I tested quite a few correlations so a couple of ‘false positives’ may be expected. I didn’t statistically correct for this. Instead, I took a rather pragmatic approach: I’m cautious when there’s simply a significant correlation between two phenomena but I’m more confident when there’s a pattern to the correlations (e.g., Garmin and Strava recalculations are correlated in a similar way to another variable).

The spread of the fast food strikes in the USA

[Updated 6 December 2013] - On 29 November last year, 200 workers in fast food restaurants in New York went on strike to demand decent wages. What seemed exceptional at the time, has only grown since, culminating in a national day of fast food strikes in over 100 cities last week.

Their demands are justified, the NYT noted: “we’re talking about big, profitable companies, which are big and profitable in part because they rely on underpaid labour”. You can support these workers by telling fast food chains like McDonald’s and Burger King that low pay is not ok.

Embed code for the map (may not display in older versions of internet explorer):

<iframe src="" frameborder=0 width=510 height=380 scrolling='no'></iframe>


Data on strikes was collected from various sources and may be incomplete. I used d3.js to draw the map and setTimeout to time the transitions. For some reason I couldn’t get this to work with a for-loop without the latest transition terminating the previous ones or all transitions using the last value of i, so I hard coded each step of the iteration.


American norms require more space for car than for bedroom. How about Amsterdam?

«Odds are, your bedroom is smaller than your car’s: your city nearly requires it to be», this infographic explains (via Herbert Tiemens). Perhaps so in the US or Canada - but how about Amsterdam?

  • According to national regulations (see p.152), a residence area - which may coincide with or comprise a bedroom - must be at least 5 m2. This is not to say bedrooms are usually that size; it’s a minimum. For example, many bedrooms in the new Oostpoort project in Amsterdam Oost are 9.5 m2, 11 m2 or larger (these examples regard houses that are not in the ‘affordable’ category).
  • For new houses, the Amsterdam West district requires 0.6 parking spaces per house if it’s affordable housing and 0.8 per house in other cases (where, as the Zuid district puts it, «it may be expected that [households] have a median or high income and own a car»). Parking norms in some other districts are higher, e.g. 0.6/1.1 for the aforementioned Oostpoort project; 0.7/1.0 in Zuid and 1.0/1.3 in Nieuw-West.
  • For guidelines regarding the size of parking spaces, the Amsterdam municipality refers to expertise platform CROW. CROW advises 2.5 x 5 metres or 2.0 x 6 to 7 metres depending on the type of parking space; that amounts to a surface area of between 12 and 14 m2.

This implies that the required space for car parking may vary from 7.2 to 18.2 m2 per new house. Or 1.4 to 3.6 times the minimum size of a bedroom. Of course, most bedrooms will be larger than the minimum 5 m2. That said, your bedroom may very well be smaller than the required car space.


Cycling: are women catching up with men

Photo by DAVID ILIFF. License: CC-BY-SA 3.0 / via Wikimedia

In a petition already signed by 88,000 people, riders including Emma Pooley and Marianne Vos ask for women to be allowed to participate in the Tour de France and other cycling events:

We seek not to race against the men, but to have our own professional field running in conjunction with the men’s event, at the same time, over the same distances, on the same days, with modifications in start/finish times so neither gender’s race interferes with the other.

Among other things, they want to «debunk the myths of physical ‘limitations’ placed upon female athletes». So how about those limitations? In an opinion article in NRC Handelsblad, Sanne van Oosten of WOMEN Inc. argues that the world hour record for men (49.7 km) is only slightly higher than for women (46.1 km). And Guardian cycling columnist William Fotheringham observes:

Over the years there has been a convergence between the distances men and women race, as men’s professional races are becoming progressively shorter, and women’s gradually longer.

He doesn’t specify which races this applies to. The distances of the UCI world championships haven’t changed much, at least not since 2004. Below are the distances of the Olympic individual road race since 1984, the first year women were included. I collected the data from different sources - surprisingly there doesn’t appear to be a single source that has consistent records of distances and times over that period (not even Wikipedia!). Of course, to better understand the data one should also consider how much climbing was involved.

Distances Olympic individual road race, 1984-2012

The absolute difference hasn’t changed much: men ride about 110 km more than women. The relative difference has decreased substantially: until 1992, the distance for men was 2.4 times the distance for women; by 2012 that factor had shrunk to 1.8. So this confirms that distances are converging. Of course, the distance women race is still shorter than most Tour stages.

In a slightly cryptic article, it has been argued that the distances for women must be shorter than those for men: otherwise women’s speed would drop and they wouldn’t be able to display their technical skills («corner, change direction, or maintain their trajectory while looking at their opponents») optimally. So is women’s speed dropping as Olympic races become longer?

Average speed of winner Olympic individual road race, 1984-2012

The graph above shows the average speed of the winners of the Olympic road races since 1984. While the difference between men and women has somewhat increased, it’s not the case that women’s speed has dropped. On the contrary, the winner of 2012 was 8% faster than the winner of 1984 (over a distance that was two-thirds longer).

In short, I can see no particular reason why women riders shouldn’t get the chance to prove themselves in the 2014 Tour de France.

Graphs may not display in older versions of Internet Explorer.


OV-fiets gaat live informatie bieden over beschikbaarheid huurfietsen

Alle data & grafieken-geeks retweeten momenteel een kaart die laat zien op welke locaties er nog goedkope huurfietsen beschikbaar zijn bij populaire projecten als Vélib’ in Parijs, Bicing in Barcelona en Citi Bike in New York. De maker van de kaart, Ramnath Vaidyanathan, heeft voor honderd bike sharing-projecten de beschikbaarheid van huurfietsen in kaart gebracht. Dat betekent in de eerste plaats dat er dus al honderd van dit soort projecten zijn en in de tweede plaats dat al die projecten actuele gegevens aanbieden over de beschikbaarheid van huurfietsen (via een API).

Nederland komt in het lijstje niet voor. Ondanks het feit dat het Amsterdamse witte fietsenplan vaak als inspiratie wordt genoemd voor dit soort projecten, hebben wij zelf geen echt bike sharing-project (lees hier waarom). Wel hebben we de OV-fiets, maar die biedt weer geen actuele informatie over de beschikbaarheid van huurfietsen. Althans, nog niet. Een woordvoerder van de NS laat desgevraagd weten dat er momenteel aanpassingen worden gedaan aan de ICT waardoor «in de nabije toekomst» wel actuele informatie over de beschikbaarheid van OV-fietsen kan worden geboden. Dat is goed nieuws.

Overigens is het mij nog niet vaak overkomen dat de OV-fietsen op waren. De Fietsersbond heeft een aantal keer onderzoek gedaan naar de OV-fiets. In 2011 was de beschikbaarheid nog het belangrijkste probleem volgens respondenten; in 2013 was dat niet langer het geval. De NS zegt constant in de gaten te houden of er voldoende fietsen beschikbaar zijn en zonodig bij te sturen.


Ramnath Vaidyanathan has mapped the availability of bikes in bike sharing programmes across the world. Although the Dutch ‘White Bicycle’ plan is often cited as inspiration for such initiatives, there’s no ‘real’ bike sharing programme in the Netherlands (read why). Dutch Railways does offer OV-fiets rental bikes (note that the OV-fiets may not be easily available to tourists), but doesn’t have an API that provides realtime data on the availability of bikes. That is, not yet: when I asked Dutch Railways about their plans, a spokesperson indicated that this data will be made available ‘in the near future’.



I’m not the first to mistake this for a tank speed limit sign - nor the first to be slightly disappointed to find out it’s in fact a weight limit for tracked and non-tracked vehicles crossing a bridge (with lower limits for two-way traffic). Oh well, I did learn a cool German word: ‘Gleiskettenfahrzeug’.

More Verboden toegang.

Wat kost zo’n basisinkomen (en is dat wel de juiste vraag)

Sargasso is een brede discussie gestart over het onvoorwaardelijk basisinkomen - een soort AOW voor alle leeftijden - en ook bij GroenLinks staat het op de agenda. Aan een basisinkomen zijn allerlei voordelen verbonden, zoals: meer vrijheid; minder bemoeizucht; gezondere verhoudingen op de arbeidsmarkt (werknemers zijn iets minder afhankelijk van hun werkgever); geen straf voor werklozen die weer aan het werk gaan en meer draagvlak voor het sociale stelsel. Tegenstanders hebben moralistische argumenten («je kan mensen niet zomaar geld geven») en ze voeren aan dat het onbetaalbaar is.

Wat dat laatste betreft: de kosten van een basisinkomen hangen natuurlijk af van de hoogte ervan. De huidige bijstand bijvoorbeeld komt globaal neer op 395 euro per volwassene plus 530 euro per huishouden (uitgaande van 1320 euro voor een echtpaar en 660 plus maximaal 265 per persoon voor een alleenstaande). Als je ervoor zou kiezen om jongeren geen eigen inkomen te geven dan komen de totale kosten bij dat soort bedragen op ruim 110 miljard euro. Maar je kan natuurlijk ook andere keuzes maken - bijvoorbeeld rekening houden met het feit dat mensen bij een basisinkomen in principe de mogelijkheid hebben om wat bij te verdienen.

Vul hieronder de bedragen in om een variant door te rekenen. Je kan kiezen voor een bedrag per persoon (eventueel uitgesplitst naar leeftijd), een bedrag per huishouden of een combinatie:

Bereken de kosten van het basisinkomen

Maandbedrag per persoon tot 18 jaar:

Maandbedrag per persoon 18 tot 65 jaar:

Maandbedrag per persoon 65 jaar en ouder:

Woonkostentoeslag per huishouden per maand:

Overigens zegt zo’n bedrag niet zoveel over de uitvoerbaarheid. Om een basisinkomen te financieren zal je waarschijnlijk de belasting moeten verhogen, maar tegelijk geef je dat geld ook weer terug aan de burgers. De vraag is daarom niet zozeer wat kost het maar wie gaat erop voor- of achteruit en zeker ook: wat voor effect heeft het op hoe mensen zich gedragen. Critici voorspellen dat mensen massaal zullen stoppen met werken (lijkt me sterk); optimisten verwachten dat de economie juist dynamischer wordt.


Voor de berekeningen heb ik cijfers voor 2012 gebruikt. De uitvoeringskosten heb ik geschat op krap 0,4%, dat komt overeen met de uitvoeringskosten van de AOW (dit soort regelingen zijn veel efficiënter dan meer bemoeizuchtige regelingen zoals de bijstand). Het aantal inwoners per leeftijdsgroep (hier) en het aantal huishoudens (hier) zijn afkomstig van het CBS. De huidige uitgaven aan sociale zaken en arbeidsmarktbeleid zijn ontleend aan de Miljoenennota.


Gezeur over wel of geen zadeltasje

Er is een boek uit waarin uiteenlopende fietsers - van Laurens ten Dam tot Dries van Agt - vertellen over hun favoriete trainingsrondje. De meeste routes zeggen me niet zoveel, omdat ik de omgeving niet ken. Ik geloof eerlijk gezegd ook niet dat ik ze uit ga proberen.

Wat ik leuk vind aan het boek zijn de gewoontes van de fietsers. Neem je de korte route, of fiets je nog een stukje door. Wat voor eten neem je mee (sultana’s? dadels?) en hoeveel (veel fietsers blijken te vertrouwen op tankstations om extra eten en drinken in te slaan). Op de fiets flarden tekst bedenken voor een schrijfklus waar je mee bezig bent. Dankjewel zeggen als andere fietsers opzij gaan. Het gezeur over wel of geen bel op je fiets, wel of geen zadeltasje.

Wat dat laatste betreft kunnen de puristen het doen met het commentaar van Marianne Vos:

Het deert mij niks om met spatborden en een zadeltasje te fietsen. […] Wel grappig dat trimmers altijd denken dat ze er professioneel moeten uitzien terwijl professionals er als trimmers bij fietsen.

Het boek is een tikkeltje pretentieus. Heuvelachtig terrein heet ‘geaccidenteerd’. Het boek is gezet in een speciaal ontworpen lettertype ‘gebaseerd op de luchtband’ met twee puntjes in elke hoofdletter O. Maar dat geeft niet.