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Alameda County Highpoint

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Alameda County Highpoint

Postby Bob Burd » Tue Apr 23, 2013 2:22 am

At one time it was thought Rose Peak was the highpoint of Alameda County at 3,817ft, but in 1991 Dinesh Desai was studying maps and found a point to the east with an extra contour, making for an elevation of 3,840ft+, which he named "Discovery Peak" and has since become official with the BGN as of 2011. It turns out there is a second point, 1.6 miles SE of Discovery Peak, that has a spot elevation of 3,841ft. Trees intervene to make it exceedingly difficult to get a level between the two points, and since the spot elevation is a mere 1-foot over the closed contour, it is easy to dismiss it as "unlikely". I had actually visited this second point four years ago but didn't have a GPS or other measuring intrument and didn't give it any further thought.

I was back in the area today to visit another nearby peak and went over both points a second time, now equipped with a Garmin 550t. Though the absolute elevation reading isn't too accurate, I've found the relative measurement between two close points to be fairly good in other situations, within about 5ft of previously reported elevations. Today's reading showed Discovery Peak to be 18ft lower than the spot elevation point, which caught me by surprise.

Does anyone know of technical reasons why this measurement technique is not to be trusted?

(I know there are many who could care less about county highpointing, and I apologize for wasting your time here. Thank you for not using this post to point out how silly this all is - we highpointers are well aware of this, but can't help ourselves.)

View of Peak 3,841ft from Discovery Peak:
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Summit block of Peak 3,841ft (about 8ft in height):
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Re: Alameda County Highpoint

Postby mrchad9 » Tue Apr 23, 2013 4:31 am

I'm glad I did both! Was worth the quick run over to the boulder anyway.

It seems the contour lines could be not more accurate than a GPS? At least using relative measurements as you did. I'm curious if anyone has knowledge of topo map accuracies.
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Re: Alameda County Highpoint

Postby boyblue » Tue Apr 23, 2013 7:00 am

This last Saturday morning Nora and I dropped our son off at the Ranchos Los Mochos BSA Camp and we decided to take a hike up from Del Valle. We thought about how cool it would be to hike all the way to the county highpoint, but we're pretty out of shape and we got a late start, so we settled for Rocky Ridge. A far more realistic objective for us. :)

RockyRidge.jpg
Nora on Rocky Ridge 4/20/13
RockyRidge.jpg (1017.73 KiB) Viewed 663 times


I have to smile when I think about how if we had actually made it (and I have no doubt that we could have back in the nineties), I would now be saying to her, "Oh, hon... Guess what..." :oops:

Edit: (Trying to stay on topic...)

Bob Burd wrote:I was back in the area today to visit another nearby peak and went over both points a second time, now equipped with a Garmin 550t. Though the absolute elevation reading isn't too accurate, I've found the relative measurement between two close points to be fairly good in other situations, within about 5ft of previously reported elevations. Today's reading showed Discovery Peak to be 18ft lower than the spot elevation point, which caught me by surprise.

Does anyone know of technical reasons why this measurement technique is not to be trusted?


It might be a good idea to simultaneously use a well calibrated aneroid based altimeter along with the GPS. I used one BITD that had 100' increments and it was extremely reliable and accurate to about 50'- less than a contour interval on a 15' topo. Unfortunately, I realize that this is not be enough to truly define the difference between these two summits, but I think that some of the high end instruments sold at REI used to have an accuracy of +/- 20' which may be enough to confirm or deny an 18' difference. Maybe someone has one they could lend or maybe they'd like to go on an important mission. :)
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Re: Alameda County Highpoint

Postby Teresa Gergen » Tue Apr 23, 2013 3:00 pm

There are many documented topo map contour errors in Colorado. Some quads are notoriously inaccurate.

I have gone back and forth with a simple Garmin eTrex Legend GPS between two potential high points on numerous summits, where one point will read as much as maybe 50 ft higher than another, and then on the way back, that point will read much closer to the other, even all in the space of 15-20 minutes. I have not found even relative GPS elevation readings to be meaningful. Of course, we have weather in CO, year round. :)

You can also mouse over two potential summit areas in Google Earth and read the ground elevations it reports. Does anyone know how accurate this method might be for relative (not absolute) heights?
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Re: Alameda County Highpoint

Postby mrchad9 » Tue Apr 23, 2013 3:09 pm

How do they figure out where to put the contour lines in the first place? Obviously no one goes out there and walks in circles around these mountains.

I would imagine there is an accepted confidence level/error range for topo contours... it isn't zero, but +/- something with a 95% confidence level. Anyone know?
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Re: Alameda County Highpoint

Postby DougHemken » Tue Apr 23, 2013 4:08 pm

Here is a simple experiment you can do.

Turn your receiver on and leave it in one place until the battery runs out. Put in a fresh battery and download the last track. Check the minimum and maximum elevations.

I did this with my GPS on the kitchen table overnight, 7.74 hours in one location. The estimated elevations had a range of 58.37 feet.

In another experiment with averaging track data, I found some individual tracks are off by over 100 feet from the "average" track. Interestingly, I could be in the middle of a run on a fairly accurate track, see it drift off by 100 feet for 20-30 minutes, and then drift back to a fairly accurate estimate.

Consumer grade receivers are usually reasonably accurate - enough to realistically speculate about the county highpoint - but you can't tell when you have that accuracy and when you don't without some other reference.

There are lots of technical reasons that consumer grade receivers are not accurate enough for the measurement you are hoping to make. My suggestion would be to take (or collect from others) as many readings as you have patience for, spread over many hours or days, and then average them.
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Re: Alameda County Highpoint

Postby boyblue » Tue Apr 23, 2013 4:19 pm

mrchad9 wrote:I would imagine there is an accepted confidence level/error range for topo contours... it isn't zero, but +/- something with a 95% confidence level. Anyone know?


http://egsc.usgs.gov/isb/pubs/factsheets/fs17199.html

Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error by more than one-half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale.
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Re: Alameda County Highpoint

Postby Bob Burd » Tue Apr 23, 2013 4:58 pm

The model I use has a built-in altimeter based on barometric pressure to provide more accuracy on elevation than one can get on a GPS signal alone (my older GPS was wildly varying because of this). In the spec sheet, Garmin gives the accuracy as +/-10ft. It took me 35 minutes to go from one summit to the other. There was a slight breeze most of the day with steady weather conditions, so I doubt the barometric pressure changed all that much during that short interval. Still, it's impossible to be certain I suppose without more data...

As far as trusting contour intervals - they are not all that accurate on the crags. One example that comes to mind is a 13,000-foot summit just north of Ski Mountaineers Peak near South Lake. The topo shows three points with a closed 13,000-foot contour. In reality, there are two local summits and the southern of these is physically located between two of the three closed contours.
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Re: Alameda County Highpoint

Postby sierraman » Tue Apr 23, 2013 5:23 pm

Before I use my GPS for any land survey work I check the internet (usually the California Survey and Drafting Supply site) to see what the constellation of satellites will be for the day. With that information I can predict when my GPS accuracy will be good (good enough for my survey objectives anyway) and when it will be not so good. I plan my survey schedule accordingly. I would not be surprised to see some vertical drift over a 35 minute period based on satellite position. Keep in mind also, GPS was designed to be predominantly a latitude and longitude measurement tool. With a correction signal you can get centimeter accuracy for the x and y coordinates. Commercial users and the military rarely need such accuracy for the z coordinate.
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Re: Alameda County Highpoint

Postby DougHemken » Tue Apr 23, 2013 6:30 pm

As a null hyothesis, suppose both peaks are the same height, the average of your two spot heights. Then both of the deviations are < 10 ft. And +/- 10 is just a typical error, not an absolute limit.

Your measurements are pretty suggestive, but I'd like to see a few replications. Even five replications would go a long way. I think it's worth pursuing, it wouldn't take that much collective work to figure this out.
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Re: Alameda County Highpoint

Postby youngclimer123 » Sun Apr 28, 2013 6:01 pm

huh i will have to go there now
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