Mountain Pass Elevations Discussion

[+rakeck]

Doing a lot of travel throughout Colorado last few weeks and I began thinking about something ... the mountain passes were posted with their elevations many, many decades ago, long before satellites and GPS technology. I somewhat understand the methods engineers might have used at that time to determine a pass's elevation but honestly, how much accuracy can be in these numbers? As an aside, it blows my mind how accurate they in fact were given what they had to work with.

 

Example, If you look at Loveland Pass, CO. The sign at the tops says 11,990, Google Earth says 12,008, MY WAAS Garmin 62s with a roof mounted GPS puck and altimeter not used for altitude correction said 12,019. I'm just asking questions to see if any here may have any relevant knowledge on the subject or opinions they may care to share. If satellite technology were to see where corrections may be called for, I wonder if states would undo a half-century worth of history and replace the data and the signs for say 40 feet of "error". Does it make a difference to anybody? Furthermore, I suppose it could be said that sea level is relative to what over centuries?

Edited July 13, 2013 by rakeck

[WildHog]

It depends on which vertical datum the information came from. Many signs are still posted using a vertical datum from 1929 even though there is a more accurate one from 1988. It's the same with lat/lon. Many of the "you are here" markers use the datum from 1923 which is different than the default GPS datum. So if you take your GPS there the readings won't match the sign.

[+ngrrfan]

States are not going to replace all the signs simply because it is too cost prohibitive. A survey team would have to take the readings, the data sent to the sign shop where the sign is made, and then an installation crew has to go out and replace the sign. And those signs in Colorado are pretty big so it isn't like just painting new numbers on them.

[+rakeck]

I wonder how much discrepancy there is from the 1929 datum, the 1988 datum and what today's datum may be?

[+Team CowboyPapa]

.... how much accuracy can be in these numbers? As an aside, it blows my mind how accurate they in fact were given what they had to work with.

 

....

Apparently, plenty much accuracy in those numbers. Wikipedia quotes the elevation of Mt. Everest as determined in 1856 to be 29,000 ft:

"It was first measured in 1856 at 29,000 feet (8,839 m), but declared to be 29,002 feet (8,840 m) high. The arbitrary addition of 2 feet (0.6 m) was to avoid the impression that an exact height of 29,000 feet was nothing more than a rounded estimate."

 

More recently:

"On May 22, 2005, the People's Republic of China's Everest Expedition Team ascended to the top of the mountain. After several months' complicated measurement and calculation, on October 9, 2005, the PRC's State Bureau of Surveying and Mapping officially announced the height of Everest is 8844.43 m (29,017.07 ft)."

[xyzee]

From Corpscon:

Loveland Pass, Vertical 29 Datum: 11,990 US Survey Feet.

Vertical 88 Datum: 11,996.086 US survey Feet.

88 is most recent listed.

[+rakeck]

In comparing numbers found on many recent printed maps and Wiki, and actual signs at the CO summits, I'm finding there is a discrepancy of more than 20 feet one way or the other on about 60% of them. I've actually seen what has got to be actual sign creation 'typos on some because there are 300 foot and 900 discrepancies on some when compared to Wiki and and Goggle Earth.

 

Anyway, I just find it interesting for now as I'm building a pass rating system and guide for truckers

[WildHog]

I wonder how much discrepancy there is from the 1929 datum, the 1988 datum and what today's datum may be?

 

There can be a very large discrepancy sometimes depending on where you are. Parts of Texas have actually lowered due to pumping oil from the ground.

 

Also keep in mind when you look up surveys that a US survey foot is different from the foot you would measure using a regular ruler.

[+GeoTrekker26]

 

Also keep in mind when you look up surveys that a US survey foot is different from the foot you would measure using a regular ruler.

 

Technically true but it has no bearing on the current discussion. The difference on the above cited Loveland pass is about 1/4 inch which is well within the expected error of commercial GPS units.

[+Team CowboyPapa]

... how much accuracy can be in these numbers? As an aside, it blows my mind how accurate they in fact were given what they had to work with...

Comparing the two estimated Mt. Everest elevations that are 150 years apart, I see a difference of 0.06%. Not much change IMHO.

Edited July 13, 2013 by Team CowboyPapa

[Colorado Bear]

Rakeck;

 

Reading your posts you seem overly concerned with the accuracy of the elevation.

 

In your example you drove over Loveland pass and think the GPS unit you are using is the gold standard for being correct.

 

Did you stop and find the benchmark on top of the pass, set your GPS on it and let it average the point over several minutes? If not the elevation you were at just may be closer than you think, The antenna was on top of your vehicle, how far away for the benchmark were you? Although WAAS was turned on, were you receiving it?

 

You are concerned over a difference of 29 feet, This is well within the accuracy of the Garmin 62s.

 

There are GPSr that you can rent, buy that accurate enough to use for land survey, however I suspect you do not want to pay the price.

 

So after reading your posts here and the one on your need to change the elevation profile on trout creek, Looking at your profile you have not found any geocaches. I am curious what is it you are trying to accomplish?

 

Colorado Bear