Owls Head question

[Raymond]

I don't think anyone has yet pointed out that this higher-summit-to-the-north business began right here on Views From The Top when someone (sorry, I've forgotten who) posted his GPS tracing, which he had transferred onto a map when he got back home, and noticed that his turnaround point was south of the 4025 X on the map. He wanted to know if that meant that the summit we'd all been visiting over the years was, in fact, not the actual summit.

That set off the flurry of activity which led to more GPS tracks showing that the true summit was indeed to the north of the one where the sign had been posted.

As mentioned, the AMC still accepts either. The new AMC map shows the red path falling well short of the summit triangle, so it should be obvious that the summit is beyond the herd path, even if you don't read pages 209-210 of the White Mountain Guide.

GPS elevations seem pretty useless, to me. How many times did I set the known elevation at my home, and how many times since have I begun a walk at home with the elevation reading anywhere from 60 feet to 300-some feet? It should be 230 feet according to the paper map; 216 according to the CD-ROM map, and you'd think the GPS would realize that if I was starting at a known waypoint with a known elevation that the elevation should be set at that known elevation, and nothing else, but it doesn't.

 

[Kevin Rooney]

...and you'd think the GPS would realize that if I was starting at a known waypoint with a known elevation that the elevation should be set at that known elevation, and nothing else, but it doesn't.

Depending upon the make/model of the GPS, you can set the altimeter manually when you're at that known elevation.

 

[DougPaul]

GPS elevations seem pretty useless, to me. How many times did I set the known elevation at my home, and how many times since have I begun a walk at home with the elevation reading anywhere from 60 feet to 300-some feet? It should be 230 feet according to the paper map; 216 according to the CD-ROM map, and you'd think the GPS would realize that if I was starting at a known waypoint with a known elevation that the elevation should be set at that known elevation, and nothing else, but it doesn't.

Depending upon the make/model of the GPS, you can set the altimeter manually when you're at that known elevation.

This is true only if your GPS has a barometric altitude sensor.

The basic GPS gives not only position, but altitude. (It actually estimates the distance from the center of the earth and then subtracts it from a geoid model to give an altitude. Some of the early GPSes had inaccurate geoid models which would give local biases to the indicated altitude.) The altitude errors of a GPS are typically 2-3 times the position error, so expect errors on the order of 20-30 meters from a typical consumer GPS under good signal conditions. (Worse under bad signal conditions.)

Some GPS models also include a barometric sensor which can be operated in one of two modes: auto or manual calibration. If you use manual calibration, you can do as Kevin suggests. (You can also calibrate it from the air pressure given in aviation weather reports.)

The GPS altitude error varies on time scales of less than an hour but the long-term average is pretty accurate, the barometic altitude error varies on time scales of multiple hours but can be fairly accurate over short periods. Thus it is possible to calibrate the barometric altitude by slowly subtracting a fraction of the difference from the barometric calibration. Calibration by this method typically takes an hour or more to get the best accuracy. (Initial accuracy can be quite poor, but improves with time.) If I keep my GPS on (and autocalibrating) during the (2+ hr) drive to the trailhead and keep it on while hiking, it generally gives altitudes within 10 ft of the altitudes given by the summit benchmarks.

FWIW, Garmin gives the accuracy of their barometric sensors as within 10 ft when properly calibrated.

So the accuracy of the altitude readout of your GPS depends on how you use it.

Doug

 

[Raymond]

I set a waypoint in the driveway, right next to my car. Plus or minus 10 feet of accuracy, supposedly.

I know from the paper 7.5-minute U.S.G.S. topographic map that there is a 230-foot contour line touching the little black square that represents my house, so I crouch down so that the GPS (a Garmin GPSMAP 60CSx) is at ground level, and I manually set the altitude to 230 feet.

I can go off for a short walk around the neighborhood, and the altitude won’t be 230 feet when I return to stand next to my car. It won’t read 230 feet next time I set it to acquiring satellites while sitting in my car (I’ve just returned home from work), it hasn’t figured out that if it’s at or very near a waypoint with a supposedly set elevation, that that elevation doesn’t change. Short of a volcanic upheaval or sudden sinkhole, anyway.

I’m used to it by now, so I’m not really complaining about it, more just commenting on the assertion that the Owls Head map might be wrong because a GPS indicated that the elevation was eight feet higher than the map, that’s all. I’d trust the map over the GPS.

 

[DougPaul]

I can go off for a short walk around the neighborhood, and the altitude won’t be 230 feet when I return to stand next to my car. It won’t read 230 feet next time I set it to acquiring satellites while sitting in my car (I’ve just returned home from work), it hasn’t figured out that if it’s at or very near a waypoint with a supposedly set elevation, that that elevation doesn’t change. Short of a volcanic upheaval or sudden sinkhole, anyway.

Is your automatic altitude calibration turned on? If so, the system will change the calibration of the altimeter while you are walking. (In fact, the fastest adjustments occur shortly after you turn the GPS on.) Try turning the automatic calibration off (menu>setup>altimeter). Once the auto-calibration is off, the altimeter should act like any other purely barometric altimeter--you will have to calibrate it manually and the calibration with drift with the weather. (If you have a good mechanical altimeter, you might try using it side-by-side with the GPS altimeter. Ideally, they should act the same when auto-calibration is off.)

BTW1, the error on that 230ft contour is 90% chance of being within a half contour interval. (A contour at 230 ft suggests 10 ft contours, if so the error should be 5 ft or less.)

BTW2, you can always read out the GPS altitude from a menu off the satellite page. (The 60CSx and AFAIK all other Garmin GPSes with barometric sensors use the barometric altitude everywhere except on this menu. The GPSes without barometric sensors use the GPS altitude everywhere.)

BTW3, Garmin GPSes with barometric sensors can be manually calibrated from a known altitude, known pressure, or from the GPS altitude.

BTW4, for general use in the woods (particularly if I am leaving the GPS on for a long period), I prefer to use auto-calibration. That way I don't have to worry about manual calibration and since I know how long the GPS has been on, I have a good idea how trustworthy the altitude number is.

BTW5, autocalibration will calibrate the GPS to the correct altitude plus the geoid model error. Thus it may tend to calibrate the GPS to some offset of the correct altitude.

I’m used to it by now, so I’m not really complaining about it, more just commenting on the assertion that the Owls Head map might be wrong because a GPS indicated that the elevation was eight feet higher than the map, that’s all. I’d trust the map over the GPS.

The accuracy of a consumer GPS (with or without a barometric altimeter) is inadequate to make such a statement with any reliability. USGS Topo map elevation accuracy standards are 90% of all points are required to be correct within half of the contour interval. (http://erg.usgs.gov/isb/pubs/factsheets/fs17199.html). Since the 24K scale map has a contour interval of 40 ft, this would be an expected error of less than 20 ft. FWIW, the DEM in my NG TOPO! gives 2008 ft as the highest point. (The stated accuracy of the DEM is 1 meter.) I don't know how the altitude (2025 ft) of the "X" marking the summit was determined, so I don't know the accuracy.

Probably the only way to really resolve the question is to have a surveyor head up and make high accuracy measurements--there are survey GPSes that can make measurements to centimeter accuracies. In the meantime, I think that Dr D's barometric evidence that the "new" summit is higher than the "old" summit is fairly strong, even if we do not know the actual height to, say, a 1 foot accuracy.

Doug

 

[DougPaul]

I can go off for a short walk around the neighborhood, and the altitude won’t be 230 feet when I return to stand next to my car.

I was taking a rehab walk today, so I decided to perform a similar experiment:

I turned the altimeter auto-calibration off on my 60CSx GPS, manually calibrated it to 144 ft (the altitude of my living room window sill (from a 24K topo + ground-to-sill offset), drove ~3mi, walked ~2mi over ( and back over) a 100 ft hill, drove home, and checked the altitude at the same spot as the calibration: 144 +- 1 or 2 feet. The total elapsed time was about 1.25 hrs and the GPS was on the entire time.

The pressure according to www.wunderground.com was:
2:56pm 30.08 in
3:56pm 30.07 in
4:56pm 30.07 in
The calibration was around 3:25pm and the end-of-trip time was ~3:40pm. A .01 inch change in pressure would give ~8 foot change in indicated altitude.

I also just tried calibrating the barometric altimeter by setting the barometric pressure (30.04 at ~5pm) from the ATIS (Airport Terminal Information Service) broadcast from a very nearby airport. My 60CSx read 179 feet and my 60CS read 150 feet for my window sill height.

My walk happens to take me by a benchmark at the high point of the hill. It is labeled 312 ft--add 4 ft to give 316 ft (the GPS was in my pack). The GPS track gives high points of 308 ft (3:52pm) on the first pass and 310 ft (4:08pm) on my return pass.


I also took a look my data records from the GPS-bakeoff #2: http://www.vftt.org/forums/showthread.php?t=15025 , recorded with the auto-calibration on. The data records were 12-13 hrs long (recorded every 30 sec) from a stationary 60CSx mounted near my ceiling* (topo map altitude of 150 ft). The average altitude of the data records (recorded on different days) ranged from 158 to 172 ft and the standard deviations of each record are about 10 ft. (2 standard deviations (20 feet here) is about a 95% confidence interval.)

* The skyview was reasonable, but certainly not perfect--much of the southern sky is through a tree. And, of course, the signals must go through the ceiling and roof (wood frame building, no significant metal above).

Doug

 

[RoySwkr]

I don't think anyone has yet pointed out that this higher-summit-to-the-north business began right here on Views From The Top when someone (sorry, I've forgotten who) posted his GPS tracing,

Their screen name was "Capt Caper", maybe if they had used "Expert Geographer" people would have taken them more seriously :
http://vftt.org/forums/showthread.php?t=3232

A well-known AMC leader placed a geocache at the then-summit and geocachers (presumably experienced GPS users) found it at coordinates that were well S of 4025 (need free geocaching account to read coordinates):
http://www.geocaching.com/seek/cache_details.aspx?guid=8c342351-5b04-48ac-854d-59c76b5de465

Even after every track log actually produced showed the path ended before 4025, the hiking community wouldn't believe it, partly because the map in the AMC WMG showed the path continuing to 4025. (I am told that the AMC did not use their advanced GPS technology to survey this path because it wasn't an official trail, but just drew it on the map.) But after several years and the efforts of various people, I believe there is now general agreement that where the path ended at what was thought to be the summit for many years was not 4025 but maybe .2 miles S and that is how the present WMG map shows it.

Then there arose a story that Miriam Underhill had been to 4025 on her winter trip but that during a period of bad deadfalls ~1990 the path was not reopened all the way. We'll never know for sure, but Miriam's photo seems to better match the "old" summit and a guy with initials HE who has climbed Owls Head ~50 times said there was always that higher bump beyond the "old" summit.

Lastly, there seem to still be people who feel that the "old" summit may be higher than 4025. While this is theoretically possible from the map, I have heard plenty of reports that much higher readings were obtained on 4025 and at best that both were about the same - nobody seems to find the "old" summit notably higher.

You don't need to climb 4025 (yet) for the AMC patch, and I don't know if the Trailwrights have rendered an opinion. Since the AMC insists for historical reasons that you must climb S Hancock which the USGS says lacks a 200' col, perhaps it is only fair that for historical reasons you don't have to go to 4025. But like it or not, the 4025 summit is almost certainly higher so if visiting the true highpoint is important to _you_ a visit is indicated.

 

[kaseri]

Lets assume the Forest Service has removed the sign & cairn at the new summit. Would there be any other way to identiify the new summit? Are there any natural landmarks that I should look out for?

 

[jniehof]

Would there be any other way to identiify the new summit? Are there any natural landmarks that I should look out for?

It's somewhat more open than the old summit--the old summit is a clearing surrounded by thick woods, the new summit is a small...not sure what you'd call it, almost a room...with thinner woods around, you can peer out a bit. The cynical could look for boltholes where the sign used to be a little above 6'.

 

[bikehikeskifish]

Photographic evidence indicates a pretty clear view to the slide on Lincoln from the so-called new summit. This view is absent from the so-called old summit, but may be visible prior to being at the actual so-called new summit.

To my earlier point... how do you ever know, if you cannot see in 360 degrees (you know you're on top of the highest point on Adams) and there is no official marker? You don't

Tim

 

[Kevin Rooney]

...To my earlier point... how do you ever know, if you cannot see in 360 degrees (you know you're on top of the highest point on Adams) and there is no official marker? You don't

Tim

Tim - I'm confused. Did you really mean to say Adams? The summit of Adams is all busted up and bouldery, but ... it's pretty easy to determine which rock is the highest, even in poor visibility.

 

[bikehikeskifish]

Tim - I'm confused. Did you really mean to say Adams? The summit of Adams is all busted up and bouldery, but ... it's pretty easy to determine which rock is the highest, even in poor visibility.

My point exactly -- The highest rock is obvious, even without the pin sticking out.

Tim

 

[kaseri]

I understand the USFS point of view but it has to be better environmentally for them to just allow a blazed/cut a trail to the summit. Having hikers roaming/bushwacking about the summit area looking for the "true" summit just doesn't make sense to me from an environmental standpoint.

 

[kaseri]

So... lets beat a dead horse here...

I have another Owls Head hike planned for this Wed and would like to get clarification once & for all if the location in the following photos is or is not the "true" summit.

http://picasaweb.google.com/kaseri/OwlsHead61408?authkey=rHyQmNzrTc8#5212153788848329602

 

[Tom Rankin]

So... lets beat a dead horse here...

I have another Owls Head hike planned for this Wed and would like to get clarification once & for all if the location in the following photos is or is not the "true" summit.

http://picasaweb.google.com/kaseri/OwlsHead61408?authkey=rHyQmNzrTc8#5212153788848329602

Please provide time, date, phase of moon, direction you are facing, wind speed, cloud cover, height of camera, number of rangers hiding in the bushes, shoe size and birthday. Then, *MAYBE* we can help you!

 

[kaseri]

Best I can do is the date... 6-14-2008



What say you......?

 

[Dr. Dasypodidae]

Best I can do is the date... 6-14-2008



What say you......?

I am quite certain that you are sitting on the bench at the "old summit," based on the scars on the tree behind you. But, I have not been there in a while, so my guess is that the bench is long gone?

 

[kaseri]

Recent trip reports do not mention this bench. A ranger was recently spotted on Owls Head so I'm reasonably sure the ranger dismantled the cairn and bench. I wont know for sure until I go back on Wed.

 

[MichaelJ]

I believe a sign on the summit and cairns down low were dismantled two Saturdays ago.

 

[bobandgeri]

Big Earl and a couple of others were there on Sunday - in his trip report he has a link to photos for both summits that he took. Another thread by the person who carried a flag to the summit for Flags on the 48 last week, reported that the ranger left a small carin at the new summit.

Here is the link to Earl's Trip Report:

http://www.vftt.org/forums/showthread.php?t=25336