julianh

Interestingly, OziExplorer for Android http://www.oziexplorer3.com/android/oziexplorer_android.html is in Beta testing now; version 1.05 and earlier versions did not correct for the Geoid / Ellipsoid error, but this correction is now applied in version 1.06, so my Android now reports true elevation much better in OziExplorer.

GPS altitude accuracy has improved tremendously in the last few years. Even after SA was turned off, for a long time altitude accuracy was horrendous and I always had one of my baro altimeters handy when out in the mountains and never used GPS altitude. I'd been away from playing with my GPS units for a few years until I recently bought the 62st and I've been astounded by the improvements in GPS altitude accuracy. Newer (and more) satellites; wide adoption of WAAS; improved receivers, antennas, and more powerful processing chips in handheld units; and likely improvements in s/w have all been contributors. GPS altitude has actually become usable! Dave There is still an issue on many units, such as car sat-navs and mobile phones, where the manufacturer / programmers have "decided" that accurate elevation information is not important - the difference between the "ellipsoid" and the "geoid". The GPS processor unit calculates your elevation relative to the standard ellipsoid; however, local Mean Sea Level (as defined by the geoid) can deviate by a considerable height above or below the standard ellipsoid - see for example: http://www.esri.com/news/arcuser/0703/geoid1of3.html http://www.esri.com/news/arcuser/0703/graphics/geoid3_lg.jpg Some units (e.g. most if not all Garmin outdoor recreational hand-helds) correct for the deviation between the ellipsoid and the geoid; however, many units do not. Where I live (Brisbane Australia), Mean Sea Level is about 30 metres ABOVE the ellipsoid. My Summit HC corrects for this, so when I go to the beach, it reads around 0 m to 5 m. However, my car sat-nav and my Android phone don't correct, so they tell me I am at about 20 - 30 m elevation when I am standing at the water's edge. Plots of elevation loss / gain are fine, but your absolute elevation may be in significant bur relatively consistent error. (This issue may not affect all brands, but it is certainly an issue with my no-name sat-nav and my HTC Wildfire phone.)

Hmmmm - I've never seen that - on the open (sea) water, my Summit HC always reads around 0 m - 3 m or so, which is about right for the tidal range around here. What sort of boat do you have - a power boat? As I noted above, air speed affects local pressure (do a Google for "Bernoulli"); if you are moving quickly, you can get an error of several metres, depending on whether your unit is in an area of high or low local pressure zone - this will depend on the layout of the boat. Also - what unit do you have, and does it correct for the deviation between the ellipsoid and the geoid - see my following post for more information.

I used this graph as source - close enough for a rough approximation. A "rough approximation" maybe - you probably shouldn't quote your "rough approximations" to 8 or 9 digits of precision! (If you look at the graph, you will see that the relative rate of change of pressure with elevation - given by the slope of the curve - is significantly greater near sea level than it is at higher elevations.)

Seems to be a lot of confusion about air pressure units here. For the record - 1 atmosphere at sea level is approximately equal to: 101,300 Pa / 101.3 kPa / 1,013 hPa 14.7 psi 29.9 inches Hg 408 inches H2O (34 feet) Each metre (say 3 feet) of elevation gain (near sea level) reduces your ambient pressure by approximately: 0.0001 atmospheres 11.8 Pa / 0.012 kPa / 0.12 hPa 0.0017 psi 0.0035 inches Hg 0.047 inches H2O Does anyone want any other pressure unit conversion factors? (Bushels per acre? Gallons of water per square millimetre? Elephants per square mile? ) Hope this helps! EDIT: @dfx - I think my conversion factors are more accurate than your factors for elevation changes near sea level - the rate of change of pressure with elevation is decidedly non-linear over the altitude range you interpolated.

As others have said, you are generally better off calibrating using known elevation rather than barometric pressure, if possible. However, if all you know is the barometric pressure, you are better off entering the pressure in metric units (hPa) because the Garmin units allow you to enter to an accuracy of 0.1 hPa (an error of 1 part in 10,000), which is a LOT more precise than 0.1 inches of Hg (i.e. an error of 1 part in 300). Don't stress about it too much, though - as long as you have auto-calibrate turned on, the unit will correct the initial error itself within 15 - 30 minutes typically, once it has a reasonable GPS fix. Yes, it would be great to be able to enter the second decimal place if using in Hg - this is a long-standing problem with Garmin units - it has been reported numerous times to Garmin over the last 5 - 10 years, but they have never done anything about it.

Yeah, strong wind blowing onto or across a door can certainly create quite large forces that you have to push or pull against when opening or closing a door. However, the differential pressures due to normal HVAC systems are a LOT smaller than strong wind pressures - otherwise, people would be getting their arms ripped off EVERY time they went to open a door!

25 to 35 Pa, not kPa. There are 101.6 kPa in one standard atmosphere, so 25 to 35 kPa is 25% to 35% of an atmoshere - or 4 to 5 pounds force for every square INCH of surface area in American units. That is a LOT more than hurricane force winds, and it would sure make your ears pop! As per my previous post - 25 to 35 Pa would only generate 2 or 3 metres or so elevation error.

... and don't let 'em talk you out of this! The effect is even more pronounced in many office buildings. Geez! How high do you guys pressurise your buildings in the States?! I would have thought the maximum pressure differential between inside and outside of a mechanically ventilated building is probably only 5 mm water gauge (50 Pa) or so? (Think about it - how hard do you have to push to open the door to get inside or outside? 5 mm water gauge pressure differential would create a total pressure load on a standard 2.1 m x 0.9 m door panel of about 10 kg force - do you have to push / pull with a force of more than 10 kg just to overcome the air pressure differential and open the door?) Each mm of water gauge pressure differential is roughly equal to one metre of elevation difference, so I wouldn't have thought the HVAC pressure effect would lead to apparent elevation errors of more than about 5 metres or so.

Yes, but that doesn't change the air pressure, so this won't register on your barometric altimeter.

Definitely use auto-calibrate - in my experience, with auto-calibrate on, it should keep an accurate elevation record (+/- 5 m say) all day, adjusting for changes in barometric pressure. Things that can upset the barometric altimeter: 1) Your speed - air speed changes air pressure. If you are in a moving car for example you may find your elevation reads wrong by 10 m or more at highway speeds. The actual error depends on a whole lot of factors - whether windows are open or shut, etc. (Also - don't expect it to work in a pressurised aeroplane, where air pressure is artificially maintained to approximately 2,000 m effective elevation.) 2) Wind speed - if you are in very gusty conditions, you can see apparent elevation rise and fall with the wind gusts. If you are standing in strong steady winds, try turning your back to the wind, then face the wind - you will probably see your apparent elevation change by a couple of metres. 3) Gripping the unit too tightly, or covering the vent hole - the pressure sensor is inside the casing, and needs to be freely vented to atmosphere to register the air pressure. If you squeeze your unit tightly, the air pressure inside the unit will rise briefly, showing an apparent drop in elevation. This will correct itself in a couple of seconds when you relax your grip. 4) Faulty unit? Being aware of the above factors, and proper calibration / usage, I have found my Summit HC to be accurate to about +/- 5 m all day every day for normal walking / hiking use. If you are consistent;ly getting bigger errors than this, it suggests something may be faulty. Hope this helps!

janandsteve, I don't have current access to a marine GPSr or marine charts (BlueChart or similar), nor a current set of official tide tables for Australia, so I am working somewhat off memory. However, in the not-too-distant past, I have programmed computer applications to generate tide tables from the official published data using Excel, Mathcad, C_++, etc, so I have some knowledge of the sorts of algorithms that are used. In Australia, the "official" tide charts are the Australian National Tide Tables (ANTT): http://www.hydro.gov.au/prodserv/antt.htm This does not necessarily mean that ANTT is the most sophisticated mathematical tide model around; just that it is the national standard. In the ANTT, as well as the annual day-by-day tabulations of high and low tide predictions, you are given a set of parameters for each port so that you can program your own tide predictions at any time of any day. Some of these parameters are permanent (unchanged from edition to edition), while others are valid for only one year, and are updated in the ANTT each year. I assume this takes into account factors such as additional historical data, more sophisticated analysis of the historical data, morphological changes (tidal effects of recent dredging, seawall construction, etc), and so on. The ANTT provides a couple of different algorithms, which allow you to use more or fewer terms in your predictions. Properly programmed, you will get very similar (but not necessarily identical) predictions as the published tables, depending on which algorithm you use, and how many terms you carry in your calculations. In general, you need to update some of the terms every year, to keep your algorithm totally aligned with the latest copy of the ANTT. If you don't update these parameters every year, you will still get "reasonable" tide predictions, but they will gradually "drift" from the current published ANTT tables. I would suspect that if you buy new BlueChart maps, and compare with older revisions, you will find very similar but not identical tide predictions. I think that ALL of the tide parameters are embedded in the BlueChart data, and probably none are "hard-wired" into the GPSr. You should get identical predictions for any Garmin device running the same version of BlueChart, but you could get different results if you use different versions of BlueChart. For example, you can load the BlueCharts in Garmin MapSource on your PC and get your tide predictions that way - most PCs have no GPS unit or sunrise / sunset function. With respect to why the 62s gets different results - it could be just that a different version of BlueChart is loaded, or it could be a different algorithm (with more or fewer terms, using the same or different tide data model), or it could just be a programming bug - I don't know which! However, 30 minute deviation for the time of high / low tide strongly suggests tide parameters which are badly out-of-date, or else a significant programming bug. Hope this helps!

janandsteve, I am pretty sure that Garmin marine units do NOT use the algorithm for sunrise / sunset to determine tide times. I would be 99% sure that it uses a completely separate algorithm for tide times, based on "Harmonic Analysis". (e.g. See http://en.wikipedia.org/wiki/Theory_of_tides , and especially, scroll down to the section on "Harmonic Analysis".) The algorithm parameters for various tide stations are included in the BlueChart marine charts that you load onto your GPSr. If you search the various on-line tide table data, you will find a bit of variation on time and height for many tide stations, depending on which source you use. A key determining factor is the number of higher order harmonic terms ("constituents") used, but don't lose track of the fact that it is only an approximation, and other factors such as weather patterns (high and low pressure, prevailing wind direction, etc) can significantly alter the ACTUAL tide height and time of high and low tide at any tide station.

Which might matter - if there was ANY rational basis in believing the Fish and Hunt Calculator in the first place! I mean - does anyone REALLY think that Alaskan Grizzlies, Indian Tigers, Australian Barramundi, and Scottish Pheasants all follow the same rules in deciding when to come out to feed?! (Or maybe they all carry Garmins in their fur / feathers / fins, and consult the tables to find the best time to forage? ) http://www.arkansasstripers.com/best_times_to_fish.htm janandsteve, I am fully aware that there is a so-called "Solunar Theory", and this is programmed into Garmins. My point is - does anyone REALLY belive that ALL different types of (edible) animal, bird and fish use the same "rules" to decide when they should go out and feed? (And by corollary, that's when we should go out hunting and fishing.)

Which might matter - if there was ANY rational basis in believing the Fish and Hunt Calculator in the first place! I mean - does anyone REALLY think that Alaskan Grizzlies, Indian Tigers, Australian Barramundi, and Scottish Pheasants all follow the same rules in deciding when to come out to feed?! (Or maybe they all carry Garmins in their fur / feathers / fins, and consult the tables to find the best time to forage? )

My main suggestion would be to get any current model Garmin mapping unit that is in your price range, and then download and install the free ShonkyMaps from http://shonkylogic.net/shonkymaps/ (don't let the name put you off - it's 100% legitimate!), which will give you 1:250,000 topographic maps for all of Australia. You can use this as a starting point, and see whether you think you need to pay for proprietary Garmin maps, such as street maps for car navigation, higher resolution topo maps, etc. Colour screen is HIGHLY recommended for a mapping unit - trying to use full-colour maps on a grey-scale screen can be pretty tricky. (Almost all recent mapping units would have colour screens, but there are still older grey-scale units in-store at plenty of places I have been to recently, like Anaconda.) External antenna? Forget it - it might have been advisable once upon a time, but just make sure you get a recent model with high-sensitivity chipset - these will hold a lock under VERY heavy tree cover. Again, nearly all recent units are high-sensitivity, but Anaconda etc still stock some older low-sensitivity models. (e.g. I use a Garmin Summit HC which I have owned for 3 or 4 years now; it works fine even in dense tropical rainforest.) Paperless means you probably want to look at the Oregon / Dakota range - these models should tick all of your boxes. Just be warned - most Garmin map-sets in Australia are licenced for only a single unit, so you would need to buy two licences to install on both a hand-held and a car sat-nav - and you can't put Garmin maps onto a TomTom sat-nav, etc. Note also that most hand-helds make pretty poor substitutes for dedicated in-car navigation (small screen, feeble voice prompts, etc), and most car units are not ideal for hiking (fragile, limited battery life, not fully water-proof, etc). While it is possible to get a single unit which can act as both a hiking unit and an in-car unit, it will probabaly not be ideal for both tasks. Hope this helps!

For that situation, I'll project a waypoint from my current position and navigate to it. Much easier and less error prone. Having an accurately calibrated compass has the advantage that your GPS can effectively project a course line between your start point and your projected end point, and it will tell you how far off the course line you are - left or right - as you progress along the line. E.g. suppose you are traversing some very heavily wooded country; you come to a clearing (or climb a tree), calibrate your compass, and project a course line to the next visible waypoint (e.g. a hilltop, shelter, or whatever), which you estimate might be about 2 km away (but it could easily be 1.5 km, or 2.5 km or ...). With a calibrated compass and using the course line feature, you can navigate across country, staying as close as practical to your projected course line, moving back on line after you have had to deviate to negotiate the terrain, and have a chance of coming up to your target. If your compass is not calibrated properly you could be several degrees off track, and end up missing your target by several hundred metres, which may make it impossible to find in heavy scrub. All of the above may have zero relevance for navigation where you have full topo maps installed, or where you have accurate coordinates for all your waypoints (e.g. geocaching), but it can be vital in back-country hiking with limited topo mapping detail. Whatever works for your needs is fine.

Did you check against a good magnetic compass to verify that the direction it is pointing to doesn't change? I have the compass calibrated to true north and, for geocaching purposes, east is still east and south is still south, nothing has changed between battery replacements. luvvinbird, If you don't calibrate your compass, you may well find that it points approximately to magnetic north. (It's probably unlikely to be out by more than say 10 to 15 degrees or so.) If all you want to know is which way is north(ish), its probably fine. However, if you need to set a navigation course to (say) 27 degrees east of true north and walk that course for 5 km, you had better calibrate before heading off - or you could miss your destination by a couple of km!

Some smart-phones go beyond just cell-phone tower triangulation to get the A (Assisted) in AGPS. My HTC Wildfire (Android) also uses Wi-Fi, so if you have Wi-Fi turned on, and there is a Wi-Fi network within range which has been mapped by Google's StreetView cars, it can use the very limited reception range of Wi-Fi to get an approximate location within 50 metres or so, even with no GPS reception at all. This is significantly better accuracy than can be achieved using cell phone tower triangulation. This system works really well in urban areas with good StreetView coverage, not much value in rural areas with no Wi-Fi connectivity. You don't have to be connected to the Wi-Fi network, just have it within range. For example, sitting in my office building, with zero GPS Sat's visible, Google Maps will still find my location to the correct corner of my building! I suspect many current generation smartphones have similar capability.

Measure the length of your GPS; climb the tree, marking off GPS lengths as you go; count how many GPS's tall the tree is ... Voilà!!!! or ... Find a forest ranger or lumberjack who knows how tall the tree is; tell him you will give him your GPS if he will tell you how tall the tree is. Hope this helps!

OziExplorer CE for Android is currently in Alpha test for a select few. I am pleased to report that while the Alpha release only has basic functionality so far, it works fine - and the maps are entirely legible on the HTC Wildfire's "low-res" QVGA screen. Sure, a super hi-res screen like the HTC Desire HD would be great, but it is nice to know that it is entirely possible to design sophisticated Android Apps that work fine across the whole range of supported screen resolutions. (Kudos to Des for the job so far - I can't wait for the full functionality!) I think this is one of the great things about the Android platform: the whole concept envisions from the start that Apps to be developed to run on a wide range of platforms - from micro-screen devices (think latest generation iPod Nano sized devices with 2" square screens) right up to giant TV screens - and the free development tools support this right out of the box. Sure, it takes a tiny bit of thought on the part of the software designer to think about the how the App will look and perform on a range of screen resolutions, but honestly - it's not that hard when it's all built into the system and the developer tools. At the risk of inciting an Apple vs Android war - does the Apple "iThingy" App development platform seamlessly support _EVERY_ Apple iDevice from the smallest iPod Nano right up to the latest Retina-screen iPhone and iPads? How about the _NEXT_ generation of "iThings" - do you think current iPod / iPhone / iPad Apps will be supported on a 40" iTV or 400" iCinema?

OziExplorer CE will be coming out in an Android version in the not too distant future (Beta for Android expected out late this year, with iPhone and Windows 7 to follow some time later) - check out http://www.oziexplorer.com/ The Windows desktop and Windows CE version has been around for several years, and is well regarded. You can scan your own maps on your PC (or download scanned maps), convert to OziCE format, and load them to your Android for use even when there is no phone coverage. Hope this helps!

Ummm ... Surely, when your phone is in "Airplane" mode, ALL transceivers are turned off, so you won't get data from the cell-phone network OR from WiFi OR from Bluetooth, so the HTC QuickGPS app (and similar apps) won't work anyway. (Or am I missing something?) The GPS receiver, however is only a receiver, not a transmitter, so should be able to continue to work even when in Airplane mode. (It certainly works this way on my HTC Wildfire Android.)

A better screen makes a difference for photo browsing, videos, icon definition, application executuion, map images . . . I was talking in context of caching but even so, the screen is ok for all of those, given the detail of online maps I don't feel like I'm missing anything ... Do you not use topo maps on your phone? When I am out in the back country I load up oziExplorerCE with my topo maps. A larger screen would be nice (I have a HTC Vogue which has a smallish screen). Also, GCzII can display google maps, satallite, and terrain. I think GeoBeagle does too. I just got an HTC Wildfire, and am thrilled with it. Yes, I was fully aware of the higher screen resolution on other models* - at a price. I looked at several Android models in the store, and yes, you can see the difference, but for my needs, the QVGA is plenty good enough so far. I am looking forward to trialling the upcoming Beta of Ozi CE for Android, to see how it runs on a QVGA Android. I figure that if / when I get high-res-envy, I can always upgrade later. Prices will only go down, and features will go up, so for me, the Wildfire is an unbelievable value entry-level Android smartphone. For anyone who is thinking of dabbling with Android smartphones, but is a bit nervous about the costs for a high-end model, check out the Wildfire in-store if you can. * What I REALLY wanted was the HTC Desire HD, but the cost was significantly higher for me. My phone supplier offered me a free Wildfire if I renewed my current budget plan - which I wasn't actually thinking of leaving anyway - so this was a nil-cost option for me. To go to one of the high-end options like the Desire, I would have needed a significant cash outlay and / or upgrade to a more expensive contract plan. I figured the option of a free Wildfire was a great way to experience the whole Android thing - and I am very impressed so far.

I recently upgraded from a Windows Mobile 6.5 smartphone to an Android 2.1 (HTC Wildfire). I'm not an expert on the precise location technology used, but what I can confirm is that it is remarkably fast and remarkably accurate. As I understand it, on Androids which are equipped with WiFi and GPS (which seems to be most of them), the hierarchy of getting a location fix is something like this: 1. If there is a current mobile phone connection, use mobile cell tower triangulation to get a rough fix - this is basically instantaneous, if you are in mobile range. 2. If there is a WiFi connection in range, use this to get a more accurate fix, if the location of the WiFi transmitter is known. As far as I can tell, you don't need to be connected to the WiFi network, it just has to have the WiFi ID. 3. If GPS satellites are detectable, use GPS for a high precision fix. I'm not sure how all of this works - I just know that it DOES work, and it is all pretty transparent as far as the user is concerned (but you can turn off WiFi and / or GPS and / or aGPS if you want to save battery power, for example.) It automatically switches to the highest accuracy location available, as soon as a better fix is made. For example, if you drive into a tunnel or basement with no GPS reception and no WiFi, but still have a phone signal, it will give you a coarse estimate of your location, presumably based on cell phone triangulation. Step outside, and the GPS precision instantly kicks back in. For example, if I activate Google Maps on my phone while sitting in my office, within about 2 seconds, it pin-points my location to about 20 metres actual precision or better (although it draws an "error circle" with about 40 metres radius), even though I am indoors, and it can't get a GPS fix, and I am not connected to any WiFi networks (although I do have WiFi turned on, and my phone can "see" them). This is better precision than I would have thought possible using cell phone triangulation alone. I assume this is because Google has mapped all the WiFi network signal strengths by driving along the street in their WiFi-slurping Google Street View cars, and it must use the relative signal strengths to work out which side of the building I am on. (Or something like that... ) As soon as I step outside, it will get a true GPS fix within a few seconds (a LOT faster than my Summit HC, which I always thought was pretty snappy to reacquire a warm fix), and I will know my actual position down to a couple of metres. Out of urban areas, there is often no WiFi in range, and presumably cell phone triangulation is of lower accuracy (with fewer base stations in range), but the aGPS still gets an outdoor fix remarkably quickly. Like I said - I am very impressed with how well it works. The only reason I can think of to turn aGPS off would be to save power / extend battery life, or else if you are doing some app development, and you need to get access to the unassisted raw GPS signal. (I have just started scratching the surface of Android app development, but I believe you can access the various "location components" independently, if you want.) This is my experience with Android - I suspect the latest generation iPhones and WinMo7 phones do something similar, but I have no first-hand experience. Hope this helps!