How consistent is elevation?

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I placed a multi-cache, and for the second clue the cacher derives the coordinates from the number of a benchmark that is found at the top of a hill. I wanted to know if I could use the elevation there as the number in the formula for the latitude, rather than using the benchmark number both times. How consistant would the elevation reading be from individual gps reciever to reciever? Maybe if I stated that the numbers would add up to 25 or something...

Deer laugh when they hear my name!

http://www.geocities.com/ihunts

Remeber, the accuracy of a GPSr is usually +/- 16-30' depending on your reception and other factors. You might have 20 people head to that location and get 20 different elevations.

I've read that elevation error is usually 3 times worse than 2D position error. Probably not a good idea.

"Just because I don't care doesn't mean I don't understand."

Consistent, wouldn't even come close. The error can be at least twice (and up towards 3 times) the horizontal error.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

What's your gps? If you have a eTrex Vista or Summit, the units have an barometric altimeter which is very accurate if you caliberate it at known elevations and at trailhead. I've had my unit get to within 5 feet of the actual elevation. Other units can't do this...they use sats for elevation data and I wouldn't rely on those. Instead buy an altimeter. You will still need to caliberate though. All altimeters need to be caliberated before each use. Hope this helps.

What you might be able to do is round the altitude to the next highest 100' and use the first 2 digits (assuming you're over 1000'). My house is at about 4050'. Rounding up would give you 4100'. Then you would use 41 for your calculations. If the altitude is close to XX50' then you shouldn't run into too many problems. The other problem is that some GPSr's may not have an altimeter. You should be able to get an approximation from a topo map though.

I have found the MeriPlat GPS gained altitude becomes very close when I allow it to average over a 20 minute or longer span of time at the same location. It will still bounce by as much as 20 feet though when it has to switch to another satellite when it loses track of one going behind the horizon mask.

The trouble with barometric altitude gauges is you also have to compensate for the temperature of the day every X hundred feet. I forget the exact formula but it was enough of a high math I figured, if I can get within 50 feet of the actual altitude, I'm doing good.

An alternative idea is to use an old fashioned compass and map and triangulate your cache location with known landmarks. For the general cache hunter, this could be too much effort. For someone who wants to bone up their mapping skills or those whom want to learn to triangulate, this could be just the challenge.

If you choose to triangulate, make sure you indicate which brand map you used. I've noted different brands will vary based on which database was used to create the map. You may also want to indicate how many degrees declination you used on your compass.

Cheers!

TL

That benchmark has an elevation assigned to it somewhere even if it's not stamped on it. That elevation won't change...

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Wherever you go there you are.

quote:

That benchmark has an elevation assigned to it somewhere even if it's not stamped on it. That elevation won't change...

=====================

Wherever you go there you are.

Right up until that one earthquake moves the mountain and closes all bets.

quote:
Originally posted by TotemLake:

quote:

That benchmark has an elevation assigned to it somewhere even if it's not stamped on it. That elevation won't change...

=====================

Wherever you go there you are.

Right up until that one earthquake moves the mountain and closes all bets.

True. But the assigned elevation won't change and so even if the benchamrk was FUBAR due to natural catastrphy you could still use it's elevation for a number in the LL of the cache as Huntinglady was thinking.

At least until they fixed. But since this situation won't come up all that often I'm sure HL could handle it!

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Wherever you go there you are.

I am curious as to why the altitude would be off more cooridinates.

quote:
Originally posted by SpongeFamily:

I am curious as to why the altitude would be off more cooridinates.

Here is a good explaination.

"Just because I don't care doesn't mean I don't understand."

With that article being over 2 years old now, and some of the advancements in GPS technology, would that still hold true?

Brian

Team A.I.

quote:
Originally posted by Brian - Team A.I.:

With that article being over 2 years old now, and some of the advancements in GPS technology, would that still hold true?

Hold true, Yes as the underlying principles haven't changed and neither has GPS technology as in the "actual" GPS side of things. Even with SA off the relative difference still exists.

Actually there's really been no new advancements in GPS technology (proper) for quite some time. However things will catch up in a rush over the next few (or more) years.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

Using the elevation assigned to the bench mark would allow 'lazy' cachers to get your clue without ever going to the bench mark.

Make'm find the bench mark and offset the clue to the next stage by some value on the BM.

don

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