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Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: GPS compass on 05/17/2012 13:52:06 MDT Print View

"If you're not going in the direction of the waypoint, adjust."

Jerry, that's pretty much it, isn't it?

The real compass can become handy if your GPS receiver has a dead battery or total failure. Also, if you operate in places where the GPS signal can't reach, like in a deep ravine or a cave, or underwater.

--B.G.--

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: GPS compass on 05/17/2012 14:02:35 MDT Print View

Yeah, you should have a manual compass in case the GPS dies.

I have a little cheap-o plastic compass.

And the wind speed vane on my ADC Wind. One of the blades is red. That one points North.

Neither is very accurate but good enough to keep you going generally in the same direction.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: GPS Compass... on 05/17/2012 14:02:45 MDT Print View

"But if I have been moving and have stopped (which is what I think we are really discussing)..."

I don't believe that this distinction was ever made.

To do the test. Stand motionless in one spot and turn on your GPS receiver. Give it however long to lock up a position fix. Note the directions on the Satellite Information screen. Now, without moving off your spot, rotate your body around until you are facing some other arbitrary direction, say 90 to 180 degrees around. Now note the directions on the Satellite Information screen. How does the first screen view compare to the second screen view?

They are the same, since the GPS receiver still can't tell which way it was facing. It knows what might be in view in the sky and where those satellites are by direction, but it doesn't know which direction it is facing.

All of this changes when you add in the digital compass feature that some models have.

--B.G.--

Mike W
(skopeo) - F - M

Locale: British Columbia
GPS Compass... on 05/17/2012 17:27:10 MDT Print View

>> "But if I have been moving and have stopped (which is what I think we are really discussing)..."

I don't believe that this distinction was ever made. <<


By the simple fact that this is a backpacking forum, I think we can assume that we will be moving. I don't know about you Bob, but when I'm sitting in front of my computer, I don't need to turn on my GPS to find out which way is North.

I'm not looking for an argument, I just thought I'd mention a feature that many GPS users overlook. If you don't want to use this feature then don't.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: GPS Compass... on 05/17/2012 23:43:58 MDT Print View

What some new GPS users will discover after a period of lots of use is that different GPS manufacturers have tweaked many of the small details of operation. The basic position fixing operation is cut and dried, but lots of the small details are not. An example is how the receiver treats a fixed position. If you are in a fixed position, how does the receiver really know that it is in a fixed position? If the velocity gets extremely close to 0.0 mph, then that is one indicator. However, even a fixed position will give a "snail crawl" track display due to the small errors of the system. How many feet is it allowed to seemingly crawl before it determines that it is not in a fixed position? Is it one foot, ten feet, or fifty feet? For years I used a receiver that had a Position Averaging Mode. Run it for four hours in one spot, and then it knew precisely where it was (not just about where it was).

Further, few manufacturers specify these kinds of small details for user operation. Those are considered to be trade secrets. Lots of these small techy details fall into the category of "heuristics." That means guesswork or maybe common sense. It would really help some users if the heuristics were spelled out. That way, they could know what the GPS receiver is thinking and guessing, and that could help the user to know when a GPS blunder is happening, and then what to do about it.

If the receiver is a so-called mapping receiver, should the display be forced to be North Up, or should it be Destination Up? What if there is no destination? There are just all kinds of techy details hidden in there, and it becomes great fun to try to unravel the secrets.

--B.G.--

Mike W
(skopeo) - F - M

Locale: British Columbia
GPS Features... on 05/18/2012 01:24:27 MDT Print View

Well I certainly agree with you on that point Bob.

I'll sound a bit like Roger on this one because I have to say that many of the features are driven by marketing spin with a focus on creating sales rather than better GPS units.

The one that bugs me the most is related to the point we have been discussing (stationary location). This one really frustrates me because you may see sat nav systems in cars that show the satellite page and they include the location of the sun and moon on the page. So why does this bug me... because I have never seen this feature on a handheld unit. Why?... because that would allow me to align my GPS with the sun or moon and accurately use the GPS as a compass while stationary. Why don't the manufacturers offer this feature? Because they would lose sales of GPS's with electronic compasses, so I suspect you will never see the sun and moon position on a handheld unit even though it's easy to program in (in fact I suspect it's already programmed in but they don't allow you to access it).

It's also why they won't put replaceable batteries in their wrist watch units. That would interfere with the hiking GPS sales. They have to make sure we buy one for each of our hobbies... a GPS for Golf, Hiking and running... Oh, and for your car!

Edited by skopeo on 05/18/2012 01:25:06 MDT.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: GPS Features... on 05/18/2012 01:35:53 MDT Print View

"I have to say, I'll sound a bit like Roger on this one because I have to say that many of the features are driven by marketing spin with a focus on creating sales rather than better GPS units."

Oh, no argument on that.

Even dinky little sales and marketing departments know how to spin the features on GPS products. I know. I spent years there.

Often features are in there and advertised, but they have virtually zero practical function. They are in there just so that they can be advertised, and the competition doesn't have that feature. In other words, it looks good on paper.

The primary function of these features is to extract the maximum amount of cash from the user's wallet.

Part of the user problem is that they really don't understand exactly how GPS works (or doesn't work). They don't understand the practical limitations. They don't understand why they should not buy a 14-ounce solar charger to charge up their 8-ounce GPS receiver on the trail. A really good marketing department moves the user past the decisionmaking [should I buy this or not?] and moves them into the concept area [wouldn't this be cool?].

--B.G.--

Stuart R
(Scunnered) - F

Locale: Scotland
Re: Re: GPS Compass... on 05/18/2012 06:49:32 MDT Print View

However, even a fixed position will give a "snail crawl" track display due to the small errors of the system

The track that you get when the receiver is stationary is more like what you would expect from an inebriated spider. It really annoyed me that there was no option for me to specify "do not record track points if less than 0.x MPH", so I wrote an application to post-process an gpx file to remove such points.

I also discovered that the latest firmware update for the GPSr had increased the precision of the lat/lon degree coordinates in the gpx file to TEN decimal places! You may appreciate how ridiculous that is if I tell you that that 0.0000000001 degrees latitude is equivalent to 0.01 millimeters!

Nathan Hays
(oroambulant)

Locale: San Francisco
No elev on iPhone on 05/18/2012 10:41:29 MDT Print View

I noticed tracks on my iPhone 4S don't have elevation unless the app has built in topos. Tracks without elev suddenly get them when I upload to a server - presumably they combine lon-lat track data with a topo.

If this is true - I haven't found independent confirmation - then one should be careful about any elev data off the iPhone. The reason is that the x-y points are strongly affected by whether you are on a steep slope or not. The signal tends to get drawn in and up the hillside by the multipath reflections and ground plane conduction. If the x-y is off by even ten meters in such a case, it could easily place your elev way off since it derives the elev from a topo. Note also that the topos are not super-super accurate either.

A great example of this is when I cross the Golden Gate bridge - 250 feet over the water, but my GPX track from the phone has me at sea level.

When I look closely at iPhone tracks on Google Earth, I see they do not exactly follow the clearly visible trail I walked. I know GE is often off, but it is off by a fixed vector over a small enough area. I find that north and south slope tracks "suck up the hill" rather than being both off in the same direction. The hidden effect of this is again in elevation.

I use elevation info (along with fall line heading) when orienteering from a topo and I'm not sure if the distant peaks are what I think they are. I have found that my baro altimeter is more accurate than the iPhone elevation once I take into consideration weather variance (recal at known spots).

On a side note, my electronic altimeter, which is spec'd to 10%, reports exactly 10% shorter elevation change than was actually travelled. So if I recal at Road's End (5035) and hike up Copper Creek to the benchmark at Lower Tent (7825), the unit shows 7550 = 10% short of my known location. Just the opposite is true going down - it reports a net 2500 feet, ending up at the correct recal elevation. I sure wish the unit would take a correction factor input, but at least I can keep myself occupied with the math.

My GPS is an ancient Garmin eMap, so I wouldn't use it for any comparison with what may be available today. Has anyone else compared the elevations in the GPX files between smartphone and a dedicated GPS?

Nathan Hays
(oroambulant)

Locale: San Francisco
Benchmarks on 05/18/2012 10:52:41 MDT Print View

BTW, since long before I had ever heard of geocaching, I have been 'collecting' benchmarks. Somewhat informally in the dark years before GPS, and now electronically. What I do is find a BMxxxx entry on the topos, then hike to the spot and try to find the brass benchmark itself. Then I record a new waypoint in the GPS unit at that spot and name it according to its elevation.

Makes me want to return to some spots I have been but passed up or never found the benchmark.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: No elev on iPhone on 05/18/2012 14:10:20 MDT Print View

Nathan, if you hike to the very top of the Copper Creek Trail, you are at Granite Pass. On the east side of the trail there is a metal benchmark. Try using that to calibrate.

Due to plate tectonics and uplift and erosion, elevations like that are not perfectly constant. I believe there is a web site that lists the benchmarks, the last date of survey, and what the corrected elevation was at that time.

On second thought, hiking up and down the Copper Creek Trail is way too much work.

I've studied lots of 3D topo maps, and I haven't found any that are particularly accurate for elevation.

--B.G.--

Nathan Hays
(oroambulant)

Locale: San Francisco
Re Calibration on 05/18/2012 14:45:58 MDT Print View

Yup, there are benchmarks all along the trail. Been over Granite Pass 5 times now. Last time we followed the string of lakes NE of Goat Pass and Glacier Lakes. No evidence anyone had ever been there. Left it that way.
Nobody for miles

R K
(oiboyroi) - M

Locale: South West US
Re: Re: No elev on iPhone on 05/18/2012 15:24:52 MDT Print View

I've studied lots of 3D topo maps, and I haven't found any that are particularly accurate for elevation.

Bob, perhaps the differences you've found could be explained by the differences between the geoid and the ellipsoid? Just a thought.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: No elev on iPhone on 05/18/2012 15:32:03 MDT Print View

Uplift is a fraction of an inch a year

If it was 1 inch, in 100 years it would rise 8 feet - in our lifetimes this is not significant

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: No elev on iPhone on 05/18/2012 17:07:21 MDT Print View

"Bob, perhaps the differences you've found could be explained by the differences between the geoid and the ellipsoid? Just a thought."

Yes, the different models each yield different elevation numbers for peaks and other well-surveyed points. Part of the problem is that when your GPS receiver shows you some piece of map, you don't really know which model it came from. Or, if you access some model's maps on the net, you don't really know there, either.

Back in the old days, consumer GPS receivers first showed up in the marine market (yachts, fishing boats, etc.). There, they weren't too worried about elevation since they figured that the ocean was at sea level, and fresh water lakes and rivers didn't matter. So, the early NMEA protocols didn't really address high elevation details so well. Now, we pay the price with elevation confusion.

Tectonic uplift is significant in many areas. I think it is very pronounced in the Himalayas. The summit elevation on Mount Everest keeps getting higher, so you better get over there and climb it before it gets even worse. Some of the other mountain ranges are actually sinking due to their own weight.

--B.G.--

Paul Hatfield
(clear_blue_skies) - F
Averaging on 05/18/2012 17:49:36 MDT Print View

> The basic position fixing operation is cut and dried, but lots of the small details are not.

The normal consumer GPS receivers with the integrated displays are doing some sort of averaging when they display GPS coordinates. It's probably an exponential moving average, but as you say, the manufacturers tend not to disclose what they do, which is unfortunate.

BlueTooth GPS receivers (example: GlobalSat BT-359) tend not to do such averaging, and will give you the raw coordinate data, non-averaged.

I don't believe that consumer GPS receivers have improved much in the last 5 years, but I haven't been watching the field closely.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Averaging on 05/18/2012 19:34:38 MDT Print View

"The normal consumer GPS receivers with the integrated displays are doing some sort of averaging when they display GPS coordinates. It's probably an exponential moving average, but as you say, the manufacturers tend not to disclose what they do, which is unfortunate."

Kalman filtering gets used a lot in the normal front end to mid-parts of the receiver. By the time the position data is streaming along, some manufacturers seem to have different ideas about the best way to do averaging.

A good experiment is to put a GPS receiver in your car, and drive down a flat highway at a fixed speed with the cruise control set on. Let's say it is 50 mph. Then watch the speed displayed on the receiver. It will likely be floating around from 49.5 to about 50.5. I don't think that is a sign of an imperfect cruise control. I think it is something about the sampling rate and round-off error mixed with a display rate issue. Constant data display updating is not a top priority.

Military GPS receivers, like in a jet flying at Mach 2, use a much faster sampling rate, so they get different kinds of errors. They also use a lot more power.

--B.G.--