Nov 222010

Freakonomics is an interesting book. It brought back a lot of memories of projects I’ve worked on and papers and presentations I’ve seen. Some of the special bits for me are the surprising statistical results. Like: Car seats haven’t done much to save kids’ lives, but they’ve made some

Filtering Water

companies a lot of money. I’d love to have the time to sit and look for this sort of stuff, it sounds cool. And I was just vaguely citing something like this a couple of weeks ago when camping: I didn’t bother to bring my water filter. I thought about it and figured that:

  1. Parks organizations feel they need to warn people about giardia (‘Beaver Fever’) so that they don’t get sued
  2. Water filter manufacturers warn people about it so that they can make money.

The kids and I drank unfiltered water from the Ghost River all weekend with no ill effects.

But mining statistics is a funny one. Working in digital radiography and developing image processing algorithms made me think a bit differently. Radiologists are looking for unusual things in images, not necessarily the standard things; hey, look two lungs! Patients come in with all sorts of deformities, like missing fingers, legs, you name it. So an image segmentation algorithm that looks for a hand in an image by trying to identify the 5 digits may be a bit unreliable; you don’t always get to see 5 digits on an ER radiograph of a hand! And when you do – they might not present themselves in a sensible set of straight lines. Crushing trauma does a lot of crazy stuff to the phalanges. Then you have X-ray images with metal plates and other implants – but these aren’t necessarily the norm, so you need to make sure that your algorithms can deal with stuff that isn’t normal, and most of the stuff you see isn’t normal. After all, that’s why these people are getting X-rays taken!  What about the few dead pixels in a CCD that you might encounter.   Suppose it’s 5 out of 16 million pixels that are bad.  Statistically insignificant.  But important because they distract the reader from the real pathology – a human eye will pick out the flaw immediately.

Similarly, if I had been the one who got sick with beaver fever after drinking water from the Ghost River, I probably wouldn’t care so much about how unlikely it was.

 Posted by at 10:27 pm
May 032010

Ah, at long last.  We got both the kids onto cross-country skis and headed out into the woods this winter.  It’s been an 8 year hiatus, really, and while we haven’t been back-country skiing, the world has moved past us.

The simple cross-country skiing was fun.  Vanja figured out how to move quite quickly on the flats, struggled a bit on the uphills, and did ok on the downhills as long as they weren’t too steep.  But this was all tracked skiing.  You don’t have to think too much about the direction your skis are pointing, as long as you don’t lift them too high!

We decided on a bit of adventure – back country skiing, with an overnight camp in a snow cave, or more precisely a quinzhee.  We went with Dan and Magda and Paul.

Despite the overall lack of snow at lower elevations, the trail up Mosquito Creek seemed to be well covered.   We got the kids light-weight gear, and since we already had boots that we’ve used for back country skiing for ever – we rented back country gear.

Back country gear appears to have changed.  You can see that Monika’s skis are about as wide as she is.  Turns out that works great if you have plastic boots.  Our leather boots are comfy and soft and just can’t dig the edges of these wide skis into hard packed trail.  On the way in – no problem.  On the way out – agony; snowplow to slow down?  not a chance.  Dig in an edge to turn?   No way – the skis stay flat on the snow and you simply go the direction of steepest descent.

The way in was fun.  It’s slightly uphill, which is where Vanja excels, Ronia packed it in and rode (and slept) in the pulk.

The pulks are fun too, basically a covered sled that’s strapped to your waist with two long bars.  The first time you go down a hill with one, you realize what the bars are for.  Side hills are exciting as Dan and Magda demonstrated, starting with and overturned pulk and ending up with both almost over their heads.  I couldn’t get the camera out in time to catch them at their most spectacular!

Dan and Magda dig out.

Once in camp, the quinzhees can actually be quite exhausting to build.  You have to understand that we’re basically standing on one to two meters of powder and you can’t just dig a snow cave.  You have to build it.

You start with stamping down an area of snow that’s roughly the size you want the cave to be inside plus a meter on either side.  Then you start piling snow on top of that.  You don’t have to pack too much.  In about an hour, the snow will have refrozen into a fairly rigid mass, by morning, you’ll have to work hard to move it.

So once you’ve refreshed yourself with some soup, and let the snow set up, you start digging.

Dan digging in for the night.

Dan digging in for the night.

Dan got stuck doing all the digging himself which explains the lack of photographs from his camera.  I on the other hand had a team that managed to stay fresh enough to do the digging for me.  I just needed to move the snow away from the door as fast as I could!

Vanja digging in

Vanja digging in

Anyway, Vanja says we can do it again next year.  The skiing wasn’t so good he said, but making the snow cave was fantastic!


 Posted by at 7:42 pm