Re: Challenge Terrestrial Caches below sea level

Hello,

i plan a challenge, where a cacher has to find a number of caches below sea level, and the challenge requirement is the sum of the negative altitude of the caches. The number of caches found is not important, only the altitude.

I found out that there are already simple checker for this, but i would like to make a more precise challenge.

There are caches with 5.000 meters below sea level (mostly mysteries) and so on, which should not count for this challenge, because the cacher should have been at the terrestrial spot in reality.

The deepest terrestrial point is around 420 m below Sea Level at the dead sea, so i guess the checker could only check for caches between -420 meter and -1 meter below sea level?

Is this possible?

Capsai

It would certainly be possible however I would urge caution.
Firstly is is probably worth you reading this FAQ on how elevation is arrived at: http://project-gc.com/Home/FAQ#2384662308

Basically the elevation of the cache is a complete guess, it assumes that the cache is at ground level, and then guesses the height based on a few data points that could be up to 90m away, any features like ditches/holes/craters may well not be included in these values.

For positive heights these constraints aren't too much of a problem and people can generally live with it (or not normally notice the difference). For negative heights this is more problematic, a cache that was found underground will be reported as being a positive height of the surface even though the cacher may know they were hundreds of m below ground level (and therefore potentially also below sea level). Similarly in areas that are mostly flat and at sea level it may not register that a specific ditch/feature was actually a few m below sea level. The final potential issue is that sea-level itself is a relatively imprecise measurement - different countries maps may use a different sea level base height than that of the GPS system (I'm not sure what project gc deals with). Again generally this isn't that important, but when cachers are looking for negative heights then they well be in the error bounds and disappointed that project-gc gets different results than their GPS or map displayed.

Let us know what you decide

Thank you for the information.

I don't have problems with discrepancies between the real height and the PGC height, because i can catch that off by setting the challenge not so hard. Yeah, i just could say: Find terrestrial caches with a summerized negative height of 500 m, but i think this is pointles and difficult to reach.

I found some caches in Germany, that are placed in a now recultivated, former 360 meters deep surface mine (example: GC3JT3K is listed with -78 meters), so i think a realistic challenge could be summarized -100 meters.

The only thing i want to avoid is that someone comes up with an mystery placed in the oceans with a listed depth of -5000 meters or so.

The other possibility would be to exclude multis and mysteries since they're (generally) listed at a different location to where the cache itself is.

Yes, that could be an idea.

But i think, i forget the idee for this challenge.

I asked the owner of GC3JT3K if this cache is really -78 meters and he answered me that its wrong. The surface mine was filled during the recultivation, so this cache is likely in +50 meters height, because of incorrect data.

There is two FAQ items about elevation
http://project-gc.com/Home/FAQ#2384662308
http://project-gc.com/Home/FAQ#4179902386

My guess is that the information is from SRTM3 https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission
The data was collecter during https://en.wikipedia.org/wiki/STS-99 in 2000
So if it is a deep open-pit mine then you will get those elevations
edit: If you use historical images on google earth you will see the pit. 200 the eastern part was partially refilld but where the cache is is still a deep pit



Edited 1 time(s). Last edit at 08/19/2016 02:11PM by Target.. (view changes)

There is no feedback from the cacher who requested a checker for more than 1.5 months therefore this thread will be cancelled. The request can always be resurrected by placing a new request when this becomes a necessety.