Owl's Head Bushwhack

[bobandgeri]

As Spencer pointed out - I posted two shots of the GPS resting on top of the carin in front of the tree with the Owl's head sign.

Pic 1 shows the positon and altitude

Pic 2 shows the number and strength of the satallites as well as the EPE number.


We did not go over to the northern peak, so I would not draw any conclusion from my GPS readings. A true test would be to use teh same device to measure both spots.

 

[Papa Bear]

as well as the EPE number.

What's the EPE number? It was 14 FT but what does it mean?

 

[spencer]

EPE is estimated position error and refers to the "horizontal" position. I think it uses Garmin's proprietary formula. It uses a confidence interval to estimate the bounds of reason for your position.

It says nothing about vertical (elevation) error.


spencer

 

[bobandgeri]

From the Garmin Web Page:

Estimated Position Error (EPE)
A measurement of horizontal position error in feet or meters based upon a variety of factors including DOP and satellite signal quality.

 

[Papa Bear]

Before you move the summit - I checked the GPSR track of my last trip to Owls Head (2/9/2005), and looked at the properties of the location corresponding to the location of the little wooden sign on the summit. As you can see below, it shows the elevation at 4058'.

Fear not, I would never move anything anywhere. What I would like to do, is to get a group together (wanna come?) and find and measure the location and altitude of all three bumps that the map shows above 4000' with the same instruments and at the same timeframe (as suggested by Doug Paul above). The more instruments the better as that will hopefully rule out ideosyncracies.

The published specs I have seen on absolute altitude measurementrs with a consumer level GPS are not good (around 50': about 2x the accuracy of positional uncertainty which is speced at 3m at best. This is consistant with Bob's EPE of 14') but hopefully the relative accuracy will be better. So for example if we go to the sign and measure 4033' and then to the x4025 bump and measure 4049' and then back to the sign and measure 4031' and then back etc. etc. Thus we will get a notion of which one is higher (and let's not forget the 3rd or "northern bump" that the map shows above 4000'). Or perhaps they will be the same, within the measurement uncertainty. Incidently, according to what I have read, Bob's EPE of 14' last Sunday was very close to the maximum accuracy one can expect to get.

Then the AMC 4000 footer committee can take the numbers and do what they will. Eric Savage indicated he would like to join in such an effort. So much the better.

A question for Kevin: is the "40 ft" on your readout the positional accuracy for that day (equivalent to Bob's EPE)?

And a question for Spencer: do you have access to a "professional" GPS (whatever that means) and might be enticed to lug it up there if we can organize something?

For my part, I will hit all the bumps.

Edits: I fixed this up a bit in light of Doug's note which follows.

 

[DougPaul]

EPE is generally based solely upon the geometry of the satellites assuming ideal signal reception. Thus it is at best a lower bound on the expected error and at worst, complete fiction.

DOP = dilution of precision (based upon satellite geometry)
EPE = DOP * EPE_ideal_conditions
EPE_ideal_conditions considers errors from many sources (noise, uncertainties of signal propagation, etc)

EPE is a standard concept in GPS position measurement. (The formal definition is x% probability of being within the given distance of the true location. 90-95% probability is approximately 2 standard deviations.) There are 3D, horizontal (2D), and vertical error bounds. (EPE is the 2D horizontal.) Garmin may have a proprietary version, but the concept and a number of methods of computing it are quite public.

A rule of thumb is that the vertical estimated error is on the order of twice the EPE (subject to the same caveats above).

A typical EPE for a consumer GPS is 95% probability of being with ~30ft (thus vert ~60ft). Therefore both measurements (4033 and 4058 ft) are consistent (within a probability of 95%) with a true altitude of 4025 ft .

A typical consumer GPS will not allow us to establish a better altitude for Owl's Head than the USGS topo. That would require a survey grade GPS with a qualified operator at the helm. And possibly many hours of signal collection.

I outlined several methods (barometric and water/beer hose) for measuring small differences in altitude for 2 fairly close points which might give us amateurs a shot at finding a better "true summit", but these methods will not help us to measure the absolute altitude.

A non-GPS method for determining the absolute altitude would be to compare the barometric pressure to that at a known nearby location (such as an airport). Big airports transmit the effective sea-level pressure on ATIS (automatic terminal information service) radio so that aircraft can set their barometric altimeters. (Whitefield seems to post the data hourly on the web, but I don't see a listing for an ATIS radio frequency.) This would require temperature and humidity compensation for maximum accuracy. I don't know what the accuracy of this method would be.

bobandgeri:
The summit pictures of your gps are consistent with a reading to the accuracies described above. However the satellite display and EPE can fairly reliably tell you when you have a bad fix, but cannot reliably tell you when you have a good fix. There is an effort (RAIM--receiver autonomous integrity monitoring) by the FAA to develop methods for guaranteeing the accuracy of a GPS reading. WAAS (wide area augmentation service) is part of this system.

Doug

 

[bobandgeri]

bobandgeri:
The summit pictures of your gps are consistent with a reading to the accuracies described above. However the satellite display and EPE can fairly reliably tell you when you have a bad fix, but cannot reliably tell you when you have a good fix. There is an effort (RAIM--receiver autonomous integrity monitoring) by the FAA to develop methods for guaranteeing the accuracy of a GPS reading. WAAS (wide area augmentation service) is part of this system.Doug

Agree completely - consumer GPS units get you in the area with more or less accuracy depending on the signal strength and number of satallites. I have found our unit to be fairly accurate when compared to know positions and altitudes, but would by no means think a consumer unit could be used to officially indicate which of two close bumps was the highest - although Papa Bears proposal is an interested experiment. It would show the differances between GPS models.

Ditto with altimiters - they are only as good as the last accurate calibration.

Papa Bear - if you do the hike in late Fall we'd be interested in joining in - or even better - how about a Winter trek?

 

[Papa Bear]

Papa Bear - if you do the hike in late Fall we'd be interested in joining in - or even better - how about a Winter trek?

Sounds interesting.

But I may want to combine a mapping trip with another trip down the Lincoln Slide (and maybe a bushwhack of the Owls Head northwest ridge), and that may depend on when Greenleaf Hut closes. It makes a great deal of difference (to me ) starting at Greenleaf at 6:00 AM vs. starting at Lafayette Place at 4:00 AM.

Let's talk after I knock off a bunch of stuff in a month or so.

 

[DougPaul]

The published specs I have seen on absolute altitude measurementrs with a consumer level GPS are not good (around 50': about 2x the accuracy of positional uncertainty which is speced at 3m at best.

The 3m EPE number is with WAAS and assumes everything else is perfect. Not gonna happen in the trees. (And even if it does happen, you won't be able to tell when.) And, as stated in an earlier post, WAAS can also damage accuracy.

And a question for Spencer: do you have access to a "professional" GPS (whatever that means) and might be enticed to lug it up there if we can organize something?

FWIW, survey-grade GPSes start in the tens of thousands of dollars... I have read knowledgable comments to the effect that they can be rented. And, of course, you have to know how to use them. (This is not a reflection on Spencer in any way. Just some general background info.)

It might be possible to record raw satellite signal arrival times (possible using undocumented interfaces to some consumer GPSes) with a computer and post-process the information in conjunction with correction data available on the network to obtain fixes with accuracies on the order of a meter or so. (I just know it can be done, don't know the details.)

BTW, it is very interesting to look at bobandjeri's tracks as they approach on the valley trails--a number of small deviations are evident. Don't know how they were carrying the GPS and whether they were using the internal or external antenna. One would generally expect better signals on a summit than down in the valleys. But in this case, both have trees.

Doug

 

[bobandgeri]

BTW, it is very interesting to look at bobandjeri's tracks as they approach on the valley trails--a number of small deviations are evident. Don't know how they were carrying the GPS and whether they were using the internal or external antenna. One would generally expect better signals on a summit than down in the valleys. But in this case, both have trees.
Doug

We use a Garmin ll+ unit that has an external attena and carried it at the top of the pack. We lost the signal 4 times. We had the unit to record the track points every 20 seconds. It was set to wrap so we lost the points from the start to @ 2/3 of a mile before the turn around point at teh suspension bridge on the way out - I hand drew the start up to that point in, so ignore anything there.

After that point - one deviation is where we crossed the river by mistake and then came back again. The trail has been changed in several places which would help explain why the track does not match the map completely.

 

[DougPaul]

We use a Garmin ll+ unit that has an external attena and carried it at the top of the pack. We lost the signal 4 times. We had the unit to record the track points every 20 seconds. It was set to wrap so we lost the points from the start to @ 2/3 of a mile before the turn around point at teh suspension bridge on the way out - I hand drew the start up to that point in, so ignore anything there.

Thanks--I've recorded both better and worse tracks.

After that point - one deviation is where we crossed the river by mistake and then came back again. The trail has been changed in several places which would help explain why the track does not match the map completely.

That happens. And when the GPS is getting good signals, it can easily be more accurate than the original map. (Trails may not show on survey photos. Washburn found many inaccuracies when he GPS mapped the trails around Mt Washington.)

It is particularly interesting to record tracks both inbound and outbound--that way one can check accuracies without relying on the map.

Doug

 

[bobandgeri]

I think (and I very well could be wrong), that leg length is how far you went to the next track point. Most units can be configured to record tracks either every so many seconds/minutes, or automatically based on course changes.

So I read that to mean that you traveled 40ft in 12 seconds at an average speed of 2 mph in a direction of 281 degrees true.

You might want to configure your GPS to read magnetic so it matches your maps.

 

[DougPaul]

I think (and I very well could be wrong), that leg length is how far you went to the next track point. Most units can be configured to record tracks either every so many seconds/minutes, or automatically based on course changes.

The more recent Garmins can record track points by time, distance from the last track point, or auto. Auto hopefully places points at a rate that depends on the shape of the track.

You might want to configure your GPS to read magnetic so it matches your maps.

My maps are oriented to true north... Its my compass that has the obsession with magnetic north. (Actually, it has a settable declination, so I do all my navigation in true.)

The other thing to worry about is the datum (should be given on the map legend). USGS topos are NAD27 and GPSes use WGS-84 internally. A GPS can translate between WSG-84 and another datum on input and output to enable the user to see the datum of his choice. Mapping software can also be set to read in/write out in a user chosen datum. NEMA (specs for digital I/O to/from a GPS) specify WSG-84, but not all software and GPS manufacturers follow the spec. Propriatary GPS I/O specs can obviously do whatever they want.

Doug

 

[Papa Bear]

... USGS topos are NAD27 and GPSes use WGS-84 internally. ...

Relatively recent USGS maps use WGS84/NAD83 and the Topozone version of the USGS maps use WGS84/NAD83 by default. There is a pull down menu to change this. Since many of us use Topozone (or other computer generated) maps exclusively, I would just leave everything in that Datum and forget about NAD27.

Here is the info on the South Twin Quad (7.5', 1995) which includes Owls head:

Map center is UTM 19 291647E 4891168N (WGS84/NAD83)
South Twin Mountain quadrangle
Projection is UTM Zone 19 NAD83 Datum

 

[DougPaul]

I would just leave everything in that Datum and forget about NAD27.

I prefer WGS84 myself. The key issue is that everyone should state the datum so one can make sure that everyone is using the same one.

Doug

 

[bill bowden]

Owl's head Summit?

I've done the fairly easy bushwhack from the "south summit" of Owl's Head to the "possible north summit" a few times (or at least twice), there used to be a very rough herd path. I was trying to be absolutely sure I got the SOB. finding the high spot of the possible N summit will be a trick because it reallly is flat and wooded (not terribly thick but wooded) I appear to recall some very interrupted viewsfrom where I thought was the "north summit" My efforts were pre-GPS and my altimiter at that time was plus or minus 50 feet so I can't contribute to your discussion except to say that I had no sense of being any higher.

Love to hear how this comes out.

 

[bobandgeri]

We use WGS84

 

[DougPaul]

We use WGS84

Suggestion/request of anyone giving a set of cordinates--Please, please, please include the datum. Saves confusion due to the risk of different assumptions.

I'm not singling bobandgeri here--confusion is possible in any open community and any community which does not have a prior standard. Many people (myself included until I started researching GPSes) are not even aware of datums. In the outdoors community both NAD27 and WGS84 are commonly used contributing to the possibility of confusion. The difference in the Whites is on the order of 230m.

Doug

 

[NH_Mtn_Hiker]

I visited Owl's Head this past Sunday and after GPS'ing the location of the summit sign, I ventured North on the herd path for a little ways. There is a spot about 80-100 feet north of the summit sign and several feet West of the herd path that is clearly at a higher elevation than the sign by about two feet. This spot is covered however with moss and brush, but I doubt there's a couple of feet of it.

Summit sign vs 4025

Note:WGS84 used. I've also noticed the "Terrain Elevation" on World Wind isn't very accurate.

Looking forward to solving the summit issue.

 

[spencer]

Sometime this fall when I can find time I'll come down with a Trimble unit and we can fix this right.

I'm thinking it won't be until the end of October. Who would want to join me?

send me a note and we can form a party to work on this. I know several folks have already expressed interest in going.

spencer