How accurate is elevation data on OpenStreetMap etc?

[KneesUp]

I thought I'd come to work up a hill I normally avoid today - because it's a hot day - why wouldn't I want to cycle up an unnecessary hill before getting to work ...

Anyway, it felt like harder work than I'd expected it to (I had panniers on, I'd not drunk enough, I didn't want to get too hot before work ... and I'm not that fit) but having plotted it, it comes out as depressingly benign looking, which just goes to show how much fitness I don't have

However, it also shows sections as being downhill, which I don't recall, and flat bits, which I also don't recall. The plotting program uses data from OpenCycleMap, so I just wondered - is it me that's just less fit than I though (they were heavy panniers ...) or is this data not that accurate?

 

[I like Skol]

When holidaying in Cyprus many years ago we went on a 4x4 tour to a remote part of the island. At one point the driver stopped the vehicle on a long, straight remote road and then asked us all to have a good look around and tell him which way was uphill. We all told him it was obviously that way (can't remember if it was in front or behind us) and he then released the hand brake and to our surprise the vehicle began to pick up speed pretty quickly and roll UP HILL!

This anecdote is not entirely relevant to your question but does show that you can't necessarily trust your own judgement of what is up, down, flat or steep with any great certainty.

 

[Slick]

When holidaying in Cyprus many years ago we went on a 4x4 tour to a remote part of the island. At one point the driver stopped the vehicle on a long, straight remote road and then asked us all to have a good look around and tell him which way was uphill. We all told him it was obviously that way (can't remember if it was in front or behind us) and he then released the hand brake and to our surprise the vehicle began to pick up speed pretty quickly and roll UP HILL!

This anecdote is not entirely relevant to your question but does show that you can't necessarily trust your own judgement of what is up, down, flat or steep with any great certainty.

You don't gave to go quite so far afield to get the same affect.

https://en.m.wikipedia.org/wiki/Electric_Brae

 

[ColinJ]

I found an uphill road in Wales which was incredibly deceptive. I was going up it so fast that I thought I must have a massive tailwind, but when I stopped to take a photograph I realised that there was no wind and I had actually been descending!

There is a drainage channel on the right in the photo below. Water flows down it ... in the direction that the camera is pointing! Every time I walk or cycle along that bridleway, it catches me out. It really looks like it is going uphill!

 

[Ajax Bay]

However, it also shows sections as being downhill, which I don't recall, and flat bits, which I also don't recall. The plotting program uses data from OpenCycleMap, so I just wondered - is it me that's just less fit than I though (they were heavy panniers ...) or is this data not that accurate?

I think the data are likely to be accurate but the algorithm used (for example averaging over a greater distance than a rider would 'naturally') may generate these phantom downhill/flat bits.
@Dogtrousers has suggested how to check that sort of thing, if so desired. Proper basic (eye to screen/paper) mapwork as opposed to electronic interpretation.

 

[mjr]

The plotting program uses data from OpenCycleMap, so I just wondered - is it me that's just less fit than I though (they were heavy panniers ...) or is this data not that accurate?

"OpenStreetMap does not try to be a general elevation database ... Another source of elevation data are gridded elevation data sets. Such are often used in outdoor maps to render relief, like OpenCycleMap and Reit- und Wanderkarte and elevation values can be determined from displaying these maps as a background layer in editors. Beware however that such data sets contain a significant amount of noise and errors and absolute elevation values can easily be off by 50m." https://wiki.openstreetmap.org/wiki/Key:ele

 

[mjr]

"Height Data is from NASA’s SRTM" http://www.opencyclemap.org/docs/

"Height data - I know the contours are often wrong. That’s a story for a different day. Unfortunately there’s no easy way for you to fix them, since it’s a deeply technical thing. If anyone is reading this and knows of better DEMs than SRTM, ideally on a global scale but country by country ones would be OK, then let me know. They need to be either in the Public Domain or available with an OSM-compatible license." http://www.opencyclemap.org/donate/