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GPS Class ?

I am going on a GPS Class in a couple of days.

Anything I should ask ?

I often use GPS apps on my phone...trail tracking stuff....I find that some seem not to be as good as others at finding you...even in a clear, open area..I am currious how it locates you in that event.  And if there is something I should be looking for to ensure a good app/device.My last hile put me miles away from the trail head, though it did end up gettig the whole ACTUAL trail as well.


?#1 - I will ask about good Mobile GPS Apps

where are you taking the class? Love to take one to learn more.Any info would be appreciated callahan.Thanks much!!

Callahan said:

I am going on a GPS Class in a couple of days.

Anything I should ask ?

 Ask for directions!  (sorry I couldn't resist.)


Ask about geotagging your photos.

JimDoss said:

Ask about geotagging your photos.

 This is somewhat simple:

Then there is the alternative... Purchase a phone with GPS built in. Only problem is it will eat up batteries quicker. 

I find my Palm Pixi's GPS to be very accurate when outside.  The problem with it is you can't download a map for it to use when you are out of range.

There are several models of cameras that have geo-tagging built into them now. My understanding is that they chew batteries.

A bit of trivia about GPS:

GPS does not use triangulation.  It uses the intersection of spheres to find out where you are. Also just as time is important to navigation using a sextant it is important to GPS. Since GPS satellites are in orbit partially out of the Earth's gravity well they HAVE to use Einstein's theory of relativity in order for them to be accurate.

ocalacomputerguy repeats some folklore:

GPS does not use triangulation.  It uses the intersection of spheres to find out where you are. Also just as time is important to navigation using a sextant it is important to GPS. Since GPS satellites are in orbit partially out of the Earth's gravity well they HAVE to use Einstein's theory of relativity in order for them to be accurate.

Having spent more than a few years working as a systems analyst and designer for the GPS, some corrections and additions are in order (correct full name is Navstar Global Positioning System, and "GPS" refers to the WHOLE system, not to the handheld devices that so many call "a GPS" - correct term is "GPS receiver", "GPSR", or in the case of a cell phone, camera, etc, with some other primary function, "GPS-enabled device").

Devices that use the GPS for determination of location (usually meaning the 4-dimensional location in space time, 3 space dimensions and one time dimension) work by translateration (3 distances), which sloppy use is way too often called "triangulation" (3 angles). The position in 3-space is determined by the intersection point (ideally, since in real life, the intersection is a tri-pyramid in space, not a point), of four spheres, the radius of which is found from the flight time of the radio signal (approximately the free-space speed of light, modified by passage through the Earth's atmosphere, drawn about the computed position of the satellites (using the ephemeris contained in each satellite's radio transmission plus the time hack and estimated clock error, also in the transmitted signal). Why 4 satellites? The calculation is dependent on having a correct and accurate time at each of the satellites and the receiver. But since clocks (especially the one in the receiver, but even the highly accurate atomic clocks in the satellites) drift and are always a bit off, the clock corrections must be calculated and the positions repeatedly recomputed (an iterative process) many times to converge on a solution. An error of 1 nanosecond (0.000000001 second) means an error of about 1 foot in position.

The satellites are not "partially out of the Earth's gravity well". They are simply at a different position in the Earth's gravity field. In the calculations, account must be taken of 3 primary effects due to relativity - relative motion (the satellites and receiver are all in motion relative to one another, hence a Special Relativity effect), in different parts of the gravity field of the whole universe (the Earth's field being the major component, hence one of the General Relativity effects in the group of acceleration effects), and the Sagnac effect (basically another acceleration effect due to the satellites moving in a rotating reference frame). Without taking these 3 effects of living in a relativistic universe instead of a Newtonian one, the errors can add up to a kilometer or more.

But, ya know, the end user really does not need to be concerned with all this. Even the least expensive GPSR does all the computations for you. It will even give you your position in whatever obscure datum your maps have - NAD27 (the system on almost all USGS topo maps), WGS84, NAD83, Old Egyptian, Old Hawaiian, Potsdam, ... It will also give you distances in miles or kilometers, altitudes in feet or meters, whatever system is most familiar to you and matches the maps you have. You can work in Latitude-Longitude, UTM, MGRS, or whatever coordinate system you have on your maps (or geocaching list). You can even just feed in a street address and it will give you driving directions from where you are to where you want to go.

You do have to think a bit, though - the widget will not warn you that its map is 10 years out of date, or that the straight line you want to take leads over a cliff.

"Very accurate!"??? Oh, really? The error budget for your consumer GPSR at present is a circle of 7 meters (20 feet), with the possibility of 3 meters (10 feet) if you are in range of the Wide Area Augmentation System (WAAS, for most of North America, and soon with EGNOS, a European augmentation system). A little caveat - there is a little effect called "Dilution of Precision" that comes from the positions in the sky of the satellites your GPSR is using. And there are such things as the canyon and canopy effects that come from heavy vegetation ("canopy") and buildings/mountains/being in a deep ravine ("canyons") that block the view of the satellites. Plus the radio signals that the GPSR depends on for making its 4D determination are reflected by multiple surfaces, giving an effect called "multipath" (used to be I could say "that's what causes the 'ghosting' on your TV set", except that has largely gone away with digital TV). Hey, it's better than the 100 meters back when "Selective Availability" was active (deliberate smearing of the positions for defense purposes).

The upshot of this is that you cannot drive down the freeway using only your GPSR for navigation - you still have to look outside to check which lane you are in, plus the maps are not all that accurate that are used in your GPSR.

Geotagging - Yes, there are several cameras with GPS chips built in. Plus Nikon and several companies working with Nikon (not Canon, though) have the capability of using an external accessory that will write the camera's position, compass pointing direction, lens tilt, and time directly into the image's EXIF file. Of the devices available, the one I use most is by a company named Solmeta (located in China - yes, I know, but almost all the geotaggers are made there these days). There are other devices that attach to the hotshoe of the camera (the flash attachment slot) and record the location and time for later download and with software, attaching to the EXIF file.

Here is an example:

And here is a shortened part of the EXIF file that is attached to the original photo:

Artist:      BILL STRAKA                        

Copyright:              2011                                                 

Camera Info

Device:   Nikon D300S

Lens:       VR 18-200mm F/3.5-5.6G

Focal Length:        18mm

Focus Mode:          AF-C

AF-Area Mode:       Dynamic, 51 points

VR:          ON

AF Fine Tune:        ON(0)


Aperture: F/16

Shutter Speed:       1/640s

Exposure Mode:    Programmed Auto*

Exposure Comp.:  0EV

Exposure Tuning:

Metering:                Matrix

ISO Sensitivity:      ISO 400



Latitude:  S 9°25.277' (9°25'16.6")

Longitude:              W 77°27.392' (77°27'23.5")

Altitude:  5693.00m

Altitude Reference:                Sea level

Heading:                32.30 (M)

UTC:       7/10/2011 13:27:9.00

All digital images have an EXIF file attached, though not all parameters are available.

To see where this photo was taken, enter the GPS-derived latitude and longitude into Google Earth. Remember it is SOUTH latitude and WEST longitude.

THE most important question for you to ask is "How can I use a map and compass to navigate?" I am glad that everyone is using a product in which I had a hand. But, really, folks, an electronic widget, as wonderful and miraculous as it is, is no substitute for learning the basic skills of navigation. And the most important tool for that is that mass of protoplasm inside that boney container sitting on your shoulders - your brain.

Check out your local REI

whomeworry said:

Callahan said:

I am going on a GPS Class in a couple of days.

Anything I should ask ?

 Ask for directions!  (sorry I couldn't resist.)


 Hah so funny....

i was waitin in the store the other week with me GF and she wz off shoe shppn or shoud I say trying every pair on along with everthing else.

I went to broadcast a msg ovr the in store PA that her party her lost party was found in the GPS section.  Damn she found me.

Ok, BillS...that was interesting. But from reading that, I still cannot understand why, when I was at Red Rock, Calico 1 Trail-head and started my app, I get a message that says: Satellite does not have a strong position, do you want to continue? to which I respond YES. At the end of the hike, I ht the FINISH button and it has my starting point 14 miles away from where I started. OH and I am not in any canyons, wide open sky above a flat area.

Bill S said:

..the most important tool for that is that mass of protoplasm inside that boney container sitting on your shoulders - your brain.

 Arrgh!  I'm am glad you clarified that, me mates thought you was referin' to me parrot!  Arrrrrrgh!  (Don't ask what's under me bucking hat.)



giftogab said:

At the end of the hike, I ht the FINISH button and it has my starting point 14 miles away from where I started. OH and I am not in any canyons, wide open sky above a flat area.

 It those pesky thechno kobolds; they infest every handheld electronic device and play tricks when you most want the thing to work. 


giftogab said:

 But from reading that, I still cannot understand why, when I was at Red Rock, Calico 1 Trail-head and started my app, I get a message that says: Satellite does not have a strong position, do you want to continue? to which I respond YES. At the end of the hike, I ht the FINISH button and it has my starting point 14 miles away from where I started. OH and I am not in any canyons, wide open sky above a flat area.

 Yeah, so. Your problem is?

The clue is that message "Sat not strong". When you turn the widget on, you have to allow some time for it to acquire a sufficient number of satellites, read the messages, do the computations, and do a few iterations to refine the position. You can't just turn it on and start running down (or up) the trail. The widget (or app) thinks it is at the location where it was when previously on. So it assumes the satellites are at the same position in the sky. There is some computation using the internal clock (which drifts typically even more than your wrist watch) and the old ephemeris (the table of data describing the orbit of each satellite) so it knows which satellites to look for and where they are in the sky. As it locks on to each visible satellite, it reads the updated information and calculates an approximate position. If you start tracking when you first turn on the widget, it may show the start point as the point the widget was turned off last usage. If you give it a few seconds, it may log one of the intermediate guesses in the iteration process.

Solution - as soon as you get to the parking place, turn on the widget (app or handheld receiver) and place it in the open while you gather your pack, water bottle, etc. Look at the EPE (Estimated Position Error, a number that usually appears on the satellite page). This is a wildly optimistic magic number that is computed by a "company proprietary" method that accounts for the arrangement of the satellites being used (if they are in the same small part of the sky, the EPE is very large, if they are spread over the sky, the EPE is small, and as the iterative process proceeds, the EPE gets smaller). In short, give your unit time to converge to a reasonably accurate position.

Way back in the early days of the GPS (I mean the whole system, which is what the acronym really stands for - satellites, ground control system, and the user widgets), the DOP (Dilution of Precision) number was displayed. As I mentioned earlier, this is the error factor that accounts for the actual distribution over the sky of the satellites. The position error estimate is the error budget (7 meters) multiplied by the DOP. But the manufacturers want their units to look good, so they display a "position error" that is somehow related, though they won't say exactly how they do their calculation (unless you get a survey-grade instrument). Typically on consumer units, you want the displayed error estimate to be less than 20 feet or so. When you travel more than a couple hundred miles from last use, you may not see the number displayed until it shows 100 feet. Wait a few minutes and it will get down to a much smaller number. Garmin, Delorme, and Magellan will get down to a claimed number of 2 or 3 feet after a few minutes with a clear horizon and sky. When you go geocaching or benchmarking (2 of the GPS-related games), it is common to get a "ground zero" indication ("0 ft to go") while 10 to 20 feet from the cache or benchmark monument. After you use your widget for a while, you kind of get used to the vagaries.

Oh, while most consumer handheld GPSRs display a "position error", most vehicle units do not. The apps I have used or seen on people's phones, iPads, etc also do not display the estimated position error. So the only advice I can give there is  - give the widget a few minutes to settle in on the location.

I think that must be it...the time..... because I have been gearing up THEN launching the app. Thanks, Bill.....must seem pretty elementary to you . :)

Thanks Bill S.

Why can't someone explain that in a book?

It seems when I read books the author spends too much time explaining things that are not vital to what I wish to understand.

I want to understand how things work, so that I can figure things out for myself.

Maybe I'm not good at picking books, haha.

I do have "Be expert with map & compass" by Kjellstrom and I like it the best of the several I have.

Something like that appears in some of the geocaching books and most of the "how to use your GPSR" and "navigation, including GPS" books. Larry Latham's "GPS Made Easy" is an example (I passed that along to him after seeing his first edition).

If you aren't in too big a hurry, I will probably be doing another wilderness navigation course at the Sierra Club's Clair Tappaan Lodge at Donner Pass in June. The first day (a Saturday) is "staying found" with no  aids, then map and compass, including computerized maps, with Sunday devoted to GPS receivers. Very much hands-on, in the field in the beautiful Sierra ("The Range of Light"). The lodge is a great place to stay - excellent meals, great atmosphere (t was built in the late 1930s by Club volunteers out of HUGE lodgepole pine logs - great programs year around, or just lounge around the huge "living room" or the well-equipped library). Sierra Club members get a discount on room and board.

Hey Bill - how will we know if you're doing that class and how do we sign up and how much does it usually cost (roughly)?

Sounds great - I'd like to sign up myself and my dad

There should be no need for taking a gps class after reading this thread. billS explained it all! That was great info.

langcow said:

Hey Bill - how will we know if you're doing that class and how do we sign up and how much does it usually cost (roughly)?

Sounds great - I'd like to sign up myself and my dad

 I will be at Clair Tappaan Lodge for Thanksgiving weekend. One of the topics I will be discussing with Peter (CTL Manager) is scheduling the Land Navigation class, along with when Barb and I will be leading our Snowshoe Ecology day hikes. When the dates get settled, they will be listed on the CTL Calendar. Two ways to get to the calendar - go to, then under "Outings" click on "Lodges and Huts". CTL is the first and most prominent one on that page. Click on the CTL link, then on "Activities". Or just click on the word "Activities" above.

CTL with a little bit of snow.

CTL with a little more snow. In another month or so, the snow will be piled up to the roofline.

I think the cost of lodging and 3 meals is about $55 or $60 for Sierra Club members and $5-10 more for non-members. And that's 3 really good meals, too. When you participate in programs and outings based at the lodge, there is a small administrative fee added, which I think is about $30 for a 2-day weekend program like the Land Navigation course. The Lodge sets the fees when they are given by volunteers like me. If it is an outside group (such as the Wilderness First Aid courses), the outside group sets their own fees.

As you can see, the cost of meals and lodging is really cheap compared to what you would pay in Truckee or at Lake Tahoe (very nearby). The lodge is run like a hostel - most people stay in the bunkrooms or "family" rooms, bring your own sleeping bag (the bunks have mattresses), though there are a small number of "cubbies" (2-person tiny cubicles). Like hostels, meals are served "family-style", and you are obligated to do a "hostel duty" each day you are there (helping prepare or serve meals, sweep the dining room or living room, and similar).

rob5073 said:

There should be no need for taking a gps class after reading this thread. billS explained it all! That was great info.

 So, send money to cover the expense and wear and tear on my fingertips for keying it all in ;)

Bill S,

I would love to come stay at the Lodge and take your class, I really would, however the economy has beaten me up pretty good the past couple of years. I am really trying to do something about that so I can do more things I want to do and can afford to travel some.

I would love to spend some time out west.

The class was good, not great but good.  I learnt a few things, refined a few more that I thought I knew and discarded a few that were teetering on the edge.

So in the end I was glad I went and am now torn between two models that don't either do all what I want but each differently almost all of what I need.

Callahan said: now torn between two models that don't either do all what I want but each differently almost all of what I need.

 So, what do you want in a new widget? Keep in mind that there will never be a single unit that will do everything you want. Sometimes you have to get two devices to do everything. Plus, remember the old rule about electronics:

You narrow your choices, go to the store, make your final selection, take the sealed box to the cashier, pay for your purchase, walk out to the parking lot, and as you get in your vehicle, two new, improved versions have already been released.

2011 version:

You do a thorough search on the Web, including on-line reviews, make your selection, enter the order, pay with Paypal, receive the confirmation of purchase, receive the tracking number, and on the morning of scheduled delivery, receive 5 emails announcing 5 new versions, all improved with a dozen new functions.

Just like, as I key this in, I see a popup on my screen for "new updates" for a piece of software I use frequently (that sentence is NOT a joke).

Hey W S,

Here to keep this in point form as best as possible and thanks for any additional input you can confuse me with.

I own two GPS units and have access to a 3rd, listed respectively

Garmin GPSmap76

Garmin GPSMap640  (newly purchased and still returnable)

Garmin GPSmap76cx

I am looking for a unit that is and in no particular order,

- highly water resistant or waterproof (floats ?)

- Trail/Route map-able for a trail of routes with up to 2000 waypoints total

- able to have additional map/s loaded e.g. TOPO or CNNT

- Touchscreen is plausible

- able to be hardwired for power

aditional nicety's,

- automobile audio routing

- marine charting

So where does this leave me ?

Most of the consumer GPSRs on the market currently are waterproof to IPX7, which means accidental immersion to 1 meter depth for 30 minutes. There are a few units that float (go to your local West Marine and ask them). There are also several companies that make plastic cases and pouches that allow you operate the buttons (not touch screen, though) and will float.

Most consumer "pocket" GPSRs can be loaded with maps and with routes of "GPS trails" that you draw on one of the computerized map programs, such as NatGeo's Topo! or Delorme's Topo North America, or in Garmin's case, their proprietary MapSource. NatGeo's Topo! is entirely scanned USGS maps, and is updated fairly frequently (free for life, as far as I can determine - I have had various State maps in Topo! for over 20 years, and they are still updating them when I request on the Web). Delorme, Magellan, and Garmin use vectorized maps, which are less accurate, mainly the contour lines and linear features (roads) which are stored as a series of points that are connected. All three do offer scanned maps as well at extra cost. Delorme and Garmin also offer maps of foreign countries, with Delorme's of significantly higher quality than Garmin's (my comparison is based on Peru most recently). All 3 allow routes of more than 2000 points, depending on the model.

My Magellan 610 is touch screen. That's nice sometimes (larger display), but touch screens are not really usable in winter with your insulated gloves. I have tried out several "touchscreen gloves", and they work ok, though they are just liner gloves. They all have problems in the corners of the screens. Plus touch screens get dirty quickly, as anyone who has an iPad, iPod, or Android touchscreen phone knows. I just find buttons to be more dependable and easier to work with gloves.

Almost all consumer GPSRs can be connected to outside power, either through a proprietary connector (Delorme) or micro-USB (Garmin, Magellan). On trips, I usually run one plugged into the cigar lighter (my TomTom is far better for road trips, though, especially trying to navigate around ancient cities noted for their arts and antiquities - it got me to the doorstep of my hotel in the center of Firenze, for example).

Voice directions for road trips? Although my Delorme PN-60w does this, the TomTom and other dedicated automobile GPSRs are far superior for voice quality. That's why I said, sometimes having 2 devices works better - the car units don't have enough battery life for hiking, and those few handhelds that have voice have really poor quality.

Marine - Magellan has the best marine charting (that's their background, after all), but Delorme has a good set of marine symbols plotted on their charts. Garmin is ok.

At this point, I use the Magellan for local hikes, taking photos (reasonable camera in it), and caching. For "serious" use, such as the Cordillera Blanca Environmental Expedition, where we had to mark the locations of our samples accurately and dependably, the PN-60 Delorme is better. In addition, the PN-60W can also communicate to home with either the SPOT Communicator (uses the GlobalStar satphone system) or 2-way texting with the new inReach system (just released to market - Alicia and I got a chance to play with the prototypes at last summer's OR Show and wrote a bit about it in the blogs). The inReach uses the Iridium satphone system to send text messages both ways. Both the SPOT and inReach accessories will send tracks that your family and friends can follow if you are doing, say, the John Muir Trail (a friend and his family did this with their SPOT last August).

Cheers Bill,

That all sounds good.

I think I have choosen a good one though I am very close to changing the 640 to the Montanna.  BUT to add the auto voice commands, Naut. Charts and auto and Marine/MC mounts will make it grossly more expensive than the 640.  Yes, as you say, compromise here and there and have two.

TAT here I come.  (next year)

Even though I have various Garmins, I far prefer Delorme and Magellan. In part this is because of the very aggressive marketing that Garmin has done over the years - exclusive agreements and all that. Besides, in many ways, Magellan and Delorme have better technology, although Garmin has more bells and whistles, many of which are not all that useful. An example - in many Garmin units, the barometric altitude is tho envy one you can have in the screen displays, because many people believe the legend that the baro altitude is more accurate. Yes, you can see the GPS-derived altitude by going to a calibration screen. But not in a normal page. People forget about the atmospheric pressure continually changing and the real lapse rate rarely matching the standard atmosphere. I often run into the situation where I set the altitude at a known point, climb a peak a couple thousand higher, and find a difference in the displayed altitude of 200-300 difference. The GPS-derived altitude is usually within 20 feetof the surveyed altitude Delorme and Magellan let you set the GPS-derived altitude as the displayed one. Also, Garmin wants a lot for their proprietary maps.

This is very interesting.

Bill S. how accurate do you feel (or know) the GPSR's barometric altitude reading compares to that of an analog altimeter like a Thommen Classic or TX? Or put another way, what do you think the loss of accuracy would be in the example you give above (setting & climbing 2000') between the GPSR's  barometric pressure reading and that of an analog altimeter? Is the analog Thommen more accurate than the GPSR's barometric reading?

I know my query may be an an oversimplification of the factors involved such as human error and changing pressure, but I was wondering about the accuracy capability of the two devices themselves based on how they are designed / made.

I do understand the functional difference between the Satellite derived altitude and the barometric pressure derived altitude on the GPSR, but I'm sure there is more I can learn.

Also any clarification if I am using incorrect terminology would help too.

I agree exactly about the Barometric Altitude, toooooo in accurate if important. For me some accurate altitude may help a lot with navigation.  I glad that the Garmin GPSmap640 does not altitude with pressure.  You might say it is not under presure to disclose how high it is,,,,,,,, yeeeaaah maaaan pass it around again.

OK here's an idea.

Assuming that barometric sensor was accurate and you have a unit with both barometric and GPS based altitude you could sync them at the start of your trip, then use the difference to measure barometric pressure for weather prediction.

John, the Delorme PN-60 allows you to display both the GPS-derived altitude and the barometric-derived altitude on the data page at the same time. However, this doesn't really help with forecasting. The problem is that the barometric altitude is derived from the absolute barometric pressure (also called the ambient pressure), adjusted to sea level equivalent, then converted to altitude using the ICAO Standard Atmosphere table for temperate latitudes. Unfortunately, the real atmosphere is never exactly the same as the Standard Atmosphere. That is the major reason for the discrepancy between the barometric altitude you set in the morning while standing at a lake whose surface altitude has been well-surveyed and marked on your map and the altitude you get on the summit a couple thousand feet higher (or conversely, set the altitude on the peak while standing on the USGS benchmark, and find a discrepancy when you get back to the lake). To do weather prediction, you need to track the pressure at a fixed location or altitude (say, on a boat on the ocean/sea level).


Short (over)simplified answer - The barometric altitude given by the Thommens mechanical, calibrated aircraft barometric altimeters, and modern GPSRs will match to the precision you can read each if you (1) set the altitude at the same base to be the same and (2) allow them to settle for a few minutes when you have climbed or descended, although the response times may differ by a fair amount. The barometric altitude of wrist altimeters (found in watches and heart rate monitors) will generally be within 5 meters for the better units. In inexpensive watches with built-in altimeters and in some inexpensive dedicated electronic altimeters can vary by 10-20 meters from the better quality altimeters. This is based on my personal experiences and watching the devices people bring to my land navigation courses.

A further, still (over)simplified explanation:

First, the Thommens analog aneroid altimeters are very high quality. Plus, they are mechanical, hence have no batteries to run down.

The way an aneroid barometer works is that at their heart, they have an aneroid. This is a metal canister that is sealed with a vacuum inside. Air pressure is working to crush the canister, but the metal is strong enough to simply flex. This flexing is transmitted through a system of levers and gears to the needle, rotating it to point to the calibrated scale. The Thommens altimeters are made to very high precision with jeweled bearings, just like the finest traditional Swiss watches (no surprise, since Thommens is a Swiss company). The scale is calibrated to the ICAO Standard Atmosphere for temperate latitudes. The lever and gear chain is made of materials combined to compensate for temperature (remember that metals expand when heated and contract when cooled, but different metals expansion and contraction differ - which allows combining them in the transmission chain to compensate for each other).

Digital electronic altimeters use a strain gauge of the solid state variety. They measure the strain (= deformation) and convert that to an electronic signal. They require a battery, as do all electronic widgets. Temperature affects batteries, plus it affects the electronic properties of the circuit of the digital barometer/altimeter (an altimeter is, after all, just a barometer with a different scale). Digital altimeters have software designed to compensate for temperature and battery fluctuations, but still use the ICAO Standard Atmosphere table.

The digital altimeters in most current GPSRs are quite good and will agree very well with the pressure you would measure with a Torricellian barometer (that's the liquid mercury barometer, and ultimately is the most accurate way to get pressure, and hence altitude - but no one these days is going to carry a mercury barometer with them, even though historically that's the way it was done in the early days).

Keep in mind that a barometric altimeter is by its very nature dependent on the ambient pressure of the atmosphere. If you are in an airplane in the pressurized cabin, you see the altitude corresponding to the air pressure in the cabin - typically 6000-8000 feet for most long flights (during the last hour of our flight from Lima to Cuzco, the cabin altitude went from 6000 to 11,200 ft, the altitude of the Cuzco airport). There is a diurnal variation in the atmospheric pressure at a given location of perhaps 0.1 to 0.2 in.Hg (equivalent to 100-200 feet). A low pressure system may drop the pressure by a half-inch Hg (equivalent to 500 ft), while a hurricane eye may be as much as a full inch or even 2 (equivalent to 1000-2000 ft - roughly 1 inch pressure difference is equivalent to 1000 ft altitude, or more accurately, the air pressure at 18,000 ft is half that at sea level). The record drop in a hurricane was 4.23 inches.

Thank you Bill S.

That was a big help!

One thing I forgot to note - the Thommens altimeter graduations are 10 ft intervals, though you can estimate the in-between values. No real sense in doing that, though. The Suunto "wrist-top computer" is calibrated in 1 meter steps. The Delorme PN-60w can be calibrated to in 1 foot intervals using the local sea-level equivalent pressure, your known altitude, or synching to the GPS-derived altitude.

Suunto, set to the "official" altitude of my back patio (8 ft, according to the city's surveyor - that's the "100-year flood level', so the floor of the house has to be at least 1 inch higher).

Thommens, set to the 10-foot marking.

The PN-60w, with the barometric altimeter turned on, calibrated to the GPS-derived altitude. I turned it on, let it get a lock on the minimum 4 satellites, did the calibration (just select "use GPS altitude" in the "calibrate" menu, not the "automatic update" option). I did not leave it on long enough to settle or average, but the reading holding it in my hand was 5 ft until I set it on the patio to take the photo.

The altitude from the USGS Palo Alto quad is 7.5 ft, since it is halfway between the 5 foot and 10 foot contour lines. The nearest benchmarks are both at 5 feet, one at 0.53 miles, the other at 1.37 miles from the back yard. The official accuracy of USGS maps for surveyed points is one half the contour interval. Sea level pressure is 1013 hPa standard, compared to the 1024 hPa ambient pressure measured on the PN-60w, 11 hPa higher, which translates to about 320 feet below sea level. Actually, we have a high pressure system over us right now, with the Palo Alto Airport reporting 30.28 inches Hg, compared to the standard 29.92 inches, a difference of 0.36 inches, which, using the rough rule of thumb of 1 inch Hg per 1000 ft, would say 360 ft below sea level. That's why you have to calibrate your barometric altimeter frequently!

Thanks again for the explanations, I enjoy learning to use these instruments and understanding how they work.

I need to purchase a newer GPSR (better receiver) and I still don't have a Thommens.

After that (next year?) I hope to get a dissolved oxygen and turbidity meter for stream sampling, which is a hobby of mine.

The list seems to keep growing.

Mike G.

My family is looking at getting a newbies like us, this is like tap dancing in a mine field (and we can't use a GPS to get out) --[ok, couldn't resist] 

But really, what is a good one--would like compass, but esp altimeter and barometer.  

What's the best bang for the buck for the stuff that we need?  I read all of this. But some DIRECTION would be good to get started. (pun mostly not intended)

Like any other outdoor gear, it depends on what you are going to do with it. First, I seriously doubt that you want a GPS. That means Global Positioning System. A GPS primarily consists of a constellation of satellites, a ground control system, and a community of users who operate user equipment, including among other things, GPSRs (which are GPS receivers). I suspect you really want a GPSR, since a GPS would be priced far out of anyone's price range other than a government. There are 2 currently operational global positioning systems - the US Navstar Global Positioning System and the Russian GLONASS. There are 4 more under development - the European Galileo, plus Japanese, Chinese, and Indian systems.

Further, you probably want a consumer GPSR, and since you are inquiring on Trailspace, I suspect you want a hiking-oriented GPSR, and not a marine, aviation, or automotive GPSR. Then again, you might want a geocaching-oriented GPSR. I will also assume you are not interested right now in geotagging your photographs. I will stick with hiking-oriented variety for now.

A number of the "features" are a matter of personal taste. Of the GPSRs currently on the market, my top recommendation is the Delorme PN-60. It has all the features you want and more. If you get the PN-60w version, you can add either a SPOT Communicator or the new inReach (currently being delivered, according to the ads). With these, you can send messages to your "team", and with the inReach you have 2-way text messaging anywhere on Earth. Some people really really want touch screens, not only on their phones and pads, but also on their GPSRs. Magellan and Garmin make touchscreen versions, plus most automotive GPSRs these days have touch screens. While I like my Magellan 640 for many things (especially geocaching) and make a lot of use of my TomTom automotive GPSR in new locations, getting around road construction, and in foreign countries, I find the touchscreen to be problematic in hiking/backpacking/ski-touring use, especially when I have to wear gloves. Touchscreens basically do not work well with gloves on, plus they always get greasy fingermarks and become less responsive and require frequent cleaning to be fully legible. For backcountry use, buttons just work better than touchscreens.

You say you want a compass. The compass in any electronic widget, including all GPSRs is a flux-gate compass. This means it requires fairly frequent recalibration (including every time you change the batteries). Flux-gate compasses shorten the battery life dramatically, from the 15-20 hour range for GPSRs without a compass or keeping the compass turned off down to 10 or less hours. Regardless of whether you have a compass in your GPSR, you really really really must have a "real" compass with you in the backcountry. As discussed many times here on Trailspace, an inexpensive baseplate compass ($10 or so) from Brunton, Suunto, or Silva (Johnson Worldwide Associates) will do everything you need in the backcountry, plus it won't die when the batteries run down or are cold.

You say you want an altimeter. All but the very cheapest current handheld GPSRs display the altitude. This is automatically part of the 4-dimensional positioning determination of a GPS receiver. The GPS-derived altitude is more accurate overall than a barometric altimeter, since it is not affected by changes in air pressure. But since you mention a barometer, I suspect you think you want a barometric altimeter in your GPSR. The mid-range and up GPSRs currently on the market all have barometric altimeters. When the barometric altimeter is turned on, it cuts battery life almost as much as the flux-gate compass, so much so that the manufacturers are recommending (ins some cases, stating is as "require") using only Ultimate Lithium batteries in their GPSRs with both compasses and barometric altimeters. Garmin, unfortunately, does not allow you to turn off the barometric altimeter, and in some of their models displays the GPS-derived altitude only when calibrating the barometric altimeter (although they do have an autocalibrate mode). There are separate barometric altimeters, both handheld and included in "wrist-top computers" such as heart-rate training monitors by Suunto and Polar. These have very long-lived batteries (2-5 years). But again, electronic altimeters, whether in a GPSR or elsewhere, are useless when the battery runs down.

As for the barometer itself, since a barometric altimeter does depend on air pressure changes to show the altitude, and the barometer depends on air pressure changes to be usable for weather tracking, you can use the barometer in its weather mode only when at a fixed location, or by recording the air pressure at locations of known altitude. Which means you should take an introductory meteorology course.

One feature appearing in more and more GPSRs, as well as your phone and pad is a camera. My Magellan 640 has a fairly nice camera, but by no means as nice as my little shirt pocket camera in terms of the photos it produces, and of course far poorer than my DSLR with its interchangeable lenses.

Ok, so after all that, what do I recommend? You can go to EMS, REI, Fry's. Best Buy, etc etc. and look at all sorts of GPSRs. You will find that Garmin dominates the market. The reason is not that they are any better or that the "user interface" is any easier to use (the easiest use interface is the one you first learn and get used to, besides which "the Garmin interface" changes between their button and touchscreen models and as new "features" are introduced). Rather, Garmin has had a very aggressive marketing campaign for years. Add to that Magellan's financial problems which has put them through a series of owners (currently a Chinese company), Lowrance's low profile in the outdoor market (they are big in marine and aircraft), and Delorme's newness to the market. Delorme has been big in the map market for decades, and the computerized map market for more than 10 years (their Street Atlas is the most complete and up to date computerized street map software - Rand McNally is a subsidiary, as is Thomas Bros.). Delorme has had a tiny module to use with your laptop in the car with their electronic map software for years, with the PN series (20, 40, and now 60 - I have all 3) being relative recent.

All of the big 3 have maps that will display on the screen of their handhelds, with capability of getting worldwide topographic maps. The problem is the tiny screens, of course - CARRY A PAPER MAP!!! You can get scanned USGS maps to go on most of the better models from each company. And you can get many foreign country street and topographic maps - extra cost, of course.

As mentioned above, the Delorme PN-60 is my current top choice (wait 6 months, and I will find something "newer, better, faster, ..."). My main reason is dependability and ease of interfacing with computerized maps. I have used the PN-60w in Antarctica, up to the summits of peaks in the Andes, wandering the deserts in Arizona, and other places in the year I have had it. I have used it with a SPOT to send tracks and messages back home to Barb and to my son in Wisconsin. At the OR Show last summer, I (and Alicia) got a chance to try the prototype of the inReach, which looks really good (particularly the free-form texting feature). I do like the Magellan touchscreen models a lot. I used the 640 in Antarctica, where I had a problem with taking my gloves off in the -30 to -40 weather, but have little problem with the touchscreen (except greasy fingerprints) here in California from Pacific shore to Donner Pass. Yeah, I do like my Garmin 60CSx, though the short battery life when the compass is on is annoying (I leave the compass off almost all the time). I haven't tried the newer 62CSx yet.

Great Thread!!  Hell of a Job OGBO!! 

I have always been a map and compass type of guy, some times just the map to really push it and have a little fun.  Recently though I have been thinking about getting a device to "Help" in trail locating and the like. 

Although these are not GPSR devices, I would like to know your opinion on the wristwatch models that display; Time, altimeter, barometric pressure, compass heading (How dose that work?), and other watch like features, stop watch, alarm, etc.

The model I have been looking at is the Casio Men's PAW1100-1V Pathfinder Atomic Solar Watch

Did I mention it's solar!  :)  It's not cheep, around $180 to $200, I don't really know what GPSR's cost, but for me that is a big investment.  I am actually trying to get it as a Christmas present, we'll see. 

Thanks for any info Sir Bill.


The majority of the "wrist-top computers" out there, like the Casio ones (they have several models) have pretty poor altimeters and compasses in them. I had a Casio similar to the one you mention for a while. However, it was poor enough that when the battery ran out, I didn't bother to replace the battery. You pretty much have to get into the price class of the better Suunto, Timex, and Polar instruments, or go to the ones with GPS chipsets in them, like Suunto, Polar, and Garmin make. Another alternative is the Kestrel pocket weather instruments, particularly the 4000 and 4500 series. Unfortunately, these are all a bit higher priced than your Casio. But that's what you pay for better quality and dependability.

The compass feature uses a miniature fluxgate compass. In the less expensive ones like Casio, the compass gets de-calibrated very easily. Even the better Timex and Suunto models get de-calibrated easily. When we had a Ford Explorer, the truck's electronic computer system had enough RF leakage to decalibrate electronic compasses.

I did a gear review of some Timex watches that Timex provided for a Trailspace Gear Review Corps test, with the WS-4 being the more sophisticated one. At the time the list price was $200. The one I had was their "carabiner" model. It worked very well, though there were some features I thought could be changed slightly. Unfortunately, my test of the WS-4 ended when on a day hike, the "carabiner clip" somehow unclipped without my noticing it. Despite scouring the roughly 1 mile of trail, asking other hikers, and having the rangers post a "lost" note, it never re-appeared. Still, I liked the WS-4 well enough that I have considered actually buying one for myself.

I find that I personally use my Suunto (lacks an HRM) and my Polar (has an HRM) frequently - almost every day hike and orienteering competition, as well as on my expeditions, like Antarctica, Cordillera Blanca, Kilimanjaro, etc. They have proven very dependable and well worth the extra cost.

The date for my next Land Navigation course at the Sierra Club's Clair Tappaan Lodge at Donner Pass has been set for June 16 and 17, 2012. There will be a get-together on Friday night June 15, at the lodge. The link gets you to the Activities page for CTL, though the schedule there does not extend through June at present. I do not know what the cost of lodging and meals plus the course is at this time, since that is set by the Sierra Club.This being the Sierra Club, we will also have a bit of Leave No Trace discussion and other conservation issues, very much non-commercial. Part of keeping the costs down is that you get to do a "hostel duty" each day you stay at the lodge, like helping prepare or serve the meals or help sweep the floors. And like most hostels, you bring your own sleeping bag.

The lodge is in the High Sierra, a short distance off Interstate 80, not far from Truckee. It is most definitely not a 5-star hotel, being a bit on the rustic side. But that's part of the fun.

Thanks a lot for the info Bill, and the review was very helpful.


So now all you have to do is get a new GPS and attend. 

Callahan said:

So now all you have to do is get a new GPS and attend. 

 Actually, no. I have a set of loaner GPSRs. By using the loaners, I can pre-load several scenarios. If you bring your own, you need to bring the connector cable so I can load several waypoints and a route.

January 20, 2021
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