Difference between revisions of "2009-07-16 57 16"

Latest revision as of 04:34, 8 August 2019

Participants

Gabriel
Jonatan
Ulf
Britt-Inger

Expedition

When staying at my (Jonatan's) parents' summer place, we're really happy for every day we get any opportunity to hash. A great deal of the graticule areas close to us are unsuitable for geohashing. Either because it's open sea or you have to exit the graticule in order to pass a bridge ~80 km south of the house and then go the same distance back again in order to reenter it. Today we were even more lucky than usual. The hash point was situated in an old quarry only a few (two or three, I think) kilometres from the house! If that weren't enough we had been in that quarry at the exact hash point only a few days ago. Strange but nice coincidence. :-)

Because of the extraordinary easy point today, we decided to walk and also wanted to make it somewhat more entertaining by trying to go for the tron achievement. In order to increase our chances of success, we hired two scouts (ehrm, my parents - Ulf and Britt-Inger). As a matter of fact, they turned out to prove very useful. Ulf was a very eager scout and always darted forward trying to fulfill that the tron route mustn't be crossed. Britt-Inger did her job by ponting out the silliness of geohashing, and how dumb it is to let fate decide where you go.

The journey to the hash point was actually slightly difficult. Ulf had an old map of the area, showing us a small path, right where we wanted to cross some terrain. The path was probably very suitable for our purpose about 20 years ago. Now it was hard to find, mostly grown over and filled with obstacles (shut closed gates, buckthorn...) on its way.

After slightly more than an hour we got out of the forest - bitten by insects, stung by buckthorn and, in the case of Britt-Inger, hit bloody because of an appointment with an evil sprig, which clearly aimed for (and also hit) her left eye. Even though we were a slightly tarnished group, now emerging from the the woods, we still celebrated that we made it through without spoiling the tron track so far!

Quarter of an hour later, we stood at the hash point in the quarry and looked at the traces we had left a few days ago when we last visited the place. After taking some photos of important things (mostly stones) we headed for the nearby water, and had a nice picnic there.

The return journey was very easy, because we were now able to use the ordinary stone road (instead of some improvised non-path) and returned safely at home after approx. 20 minutes.

Comments regarding the tron achievement and tracklog

If you look at the tracklog, you'll see that it actually crosses itself once. This was, however, not the case in the reality but was caused because by the gps flipping out and adding one point, which we did not pass, to the tracklog. This is also the explanation to the tracklog showing ~17 km/h as our max speed during the expedition, despite walking. There is, in fact, no reason we should continue to walk in a straight line for 15 meters after reaching the hash, despite walking through one body of shallow and very disgusting water only to reach nothing and then turn back and walk almost exactly the same way back to the point again only to turn in the right direction and continue walking that way... :-)

Comments regarding the probability for the déjà vu achievement

The déjà vu achievment seemed like such an improbable situation, given our current location, that we couldn't resist calculating the probability it would happen. The probability we wanted to calculate was that a close hash point fell on a spot that:
i) was within a somewhat close range from our current position (~less then two hours driving one-way with car).
ii) we had already visited.
iii) was actually documented (with photo or film) during our last visit.

First we made some guesses out of the blue. Gabriel guessed the probability of 0.0001%, and I (Jonatan) guessed 0.01%. Then we started thinking more seriously:

There are three different graticules that are actually reachable within the limit stated in i). Those are Västervik, Byxelkrok and Kalmar. When looking at our photos one can see that no photo was taken in the Kalmar graticule. That means that graticule has no impact on the probability and may be ignored. The estimated area in the different graticules that we've actually got on photo/film is approx:

Västervik: 47000 m²
Byxelkrok: 500 m²

The size of each graticule is then ~ 110 km * 60 km = 6600 km² = 6.6*10⁹ m²

Now we can calculate two different probabilities:

Västervik: V = (4.7*10⁴)/(9.35*10⁹) ~= 7.1*10^-6
Byxelkrok: B = (5*10²)/(9.35*10⁹) ~= 7.6*10^-8

We may then calculate the total probability by adding the probabilities for the two different scenarios:
B - The hash point falls within the documented area in Byxelkrok.
V - The hash point does not fall within the documented area in Byxelkrok, but within the documented area in Västervik.

This gives us the final probability:

B + (1 - B)*V = 7.6*10^-8 + (1 - 7.6*10^-8)*7.1*10^-6 ~= 7*10^-6

This means the probability that the hash fell at a place we already had documented and visited the day it did was 0.0007%! Of course when we leave, we will have been here for two weeks, so that tiny probability got a number of chances, but I'm still quite amazed of how improbable it is. And, I humbly have to admit, Gabriel guessed best...