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Ralsina.Me — Roberto Alsina's website

Endomondo Lied To Me

A week ago I re-s­tart­ed my di­et and ex­er­cise plan. Since I am gross­ly over­weight, the ex­er­cise plan is ba­si­cal­ly "walk around 5km ev­ery day, fat guy". Since I am a nerd, I want­ed da­ta so I could stop ly­ing to my­self about how much I was walk­ing.

I had seen En­domon­do men­tioned in my tweet­er time­line a bunch of times and the fea­ture­set looked pret­ty much ex­act­ly as I need­ed:

  • Track my walk­ing

  • Keep his­­to­ry

  • Show it in google maps (be­­cause it's nice)

It even did things like track­ing calo­ries burnt and so on.

The on­ly prob­lem was... it re­al­ly sucks at fig­ur­ing out how much you walked. It con­sis­tent­ly over­es­ti­mates by around 50% the dis­tances, and since it cal­cu­lates the av­er­age speed based on time and dis­tance (and the time mea­sure­ment is cor­rec­t) it over­es­ti­mates speed by 50%, which then means it over­es­ti­mates calo­ries burnt by (I am guess­ing) 125%.

How did I ver­i­fy that En­domon­do is wrong, and avoid the ob­vi­ous ex­pla­na­tion of "y­our GPS is bro­ken"?

  1. I tracked my­­self us­ing En­­domon­­do and Google Trails at the same time.

  2. I coun­t­ed steps ro­­man-mile style (count ev­ery "left­­-right", mul­ti­­ply by 1.6)

  3. I mea­­sured the path I walked in Google Maps and Bing Maps

All those mea­sure­ments tell me a walk of 1100m +/- 150m is mea­sured by En­domon­do as 1.68 km

Ver mapa más grande

Why does this hap­pen? I could as­sume En­domon­do is just crap, and prob­a­bly be right, but try­ing to come up with a "in­ter­est­ing" ex­pla­na­tion, I am lean­ing to­wards noisy mea­sure­ments. For ex­am­ple, if En­domon­do saw my po­si­tion shift­ing ran­dom­ly 10 or 15 me­ters left or right it would prob­a­bly add enough noise to make the path 50% longer (for a much more fun ex­am­ple of this, read this pa­per (by no oth­er than Benoit Man­del­brot!) but this does­n't ex­plain why Google Trails works so much bet­ter (un­less Trails does some­thing smart with an­tialias­ing and in­ter­po­la­tion).

If you use En­domon­do, care to share your ex­pe­ri­ence? I am re­luc­tant to 1-s­tar it in Google Play with­out in­de­pen­dent con­fir­ma­tion.


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