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#258088 - 03/26/13 03:49 PM Re: PLB or GPS? [Re: Glock-A-Roo]
Denis Offline
Addict

Registered: 01/09/09
Posts: 631
Loc: Calgary, AB
Originally Posted By: Glock-A-Roo
Learn how lat/lon & UTM coordinates work, carry a good gridded map, and then all you'll really need is point position from your phone's GPS.

That's a really good point. I now have an iPhone app (Tom's GPS) that will display your location using UTM coordinates. Other than saving waypoints (like the location of the car, camp, etc), I primarily use my GPS to give me the UTM coordinates so I can pin-point where I am on my map.
_________________________
Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen

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#258094 - 03/26/13 06:32 PM Re: PLB or GPS? [Re: Denis]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5265
Loc: SOCAL
Interesting iPhone App if it works. I installed the Tom's GPS App on my iPhone 4S and turned airplane mode on to simulate no cell coverage. The App would not display a location until I turned airplane mode off. Then it immediately gave me a location. The problem as I see it with relying on a cell phone for "GPS" location data, is that the cell phone is not a GPS receiver.

From http://gps.about.com/od/glossary/g/A-GPS.htm:
Quote:
...Assisted GPS, also known as A-GPS or AGPS, enhances the performance of standard GPS in devices connected to the cellular network. A-GPS improves the location performance of cell phones (and other connected devices) in two ways:

-By helping obtain a faster "time to first fix" (TTFF). A-GPS acquires and stores information about the location of satellites via the cellular network (see almanac) so the information does not need to be downloaded via satellite.

-By helping position a phone or mobile device when GPS signals are weak or not available. GPS satellite signals may be impeded by tall buildings, and do not penetrate building interiors well. A-GPS uses proximity to cellular towers to calculate position when GPS signals are not available....
From the short down the block test I just did, once I determined the system would not give a location without access to the cellular network, I gave it a signal, got a fix and then went back into airplane mode. It took about 1 minute for the Toms GPS app to go blank. IOW, without the cell network, the app doesn't seem to work.

A-GPS assumes GPS signals may become weak or not available while the places people may really run into navigation problems that are life threatening are where cell signals are weak or not available. I will not be relying on a cellphone for navigation.

If others have different experience, please weigh in and let me know what I'm missing. TIA
_________________________
Better is the Enemy of Good Enough.
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#258095 - 03/26/13 06:36 PM Re: PLB or GPS? [Re: LeeG]
chaosmagnet Offline
Sheriff
Carpal Tunnel

Registered: 12/03/09
Posts: 3265
Loc: USA
Airplane mode turns the GPS receiver off on the iPhone. Go to Settings/General/Cellular and turn Celullar Data off to simulate off-grid use with the GPS receiver turned on.

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#258097 - 03/26/13 06:39 PM Re: PLB or GPS? [Re: chaosmagnet]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5265
Loc: SOCAL
Thanks. Didn't know that, although I should have, Airplane mode turns off a lot of functions. Now I'll need to remember to turn them back on. wink
Outside, back in a bit.

Edit: Crow tastes great. The Tom's GPS app works and the accuracy with my iPhone is close enough to not matter. Outside it was showing an accuracy of 10 m; inside it's showing an accuracy of 200 m. Still, close enough for a back-up.

_________________________
Better is the Enemy of Good Enough.
Okay, what’s your point??

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#258100 - 03/26/13 08:17 PM Re: PLB or GPS? [Re: Russ]
Am_Fear_Liath_Mor Offline
Carpal Tunnel

Registered: 08/03/07
Posts: 3078
I don't have an iPhone but an iPod instead with no inbuilt GPS engine. The Garmin Glo GPS Bluetooth reciever works very nicely with the iPhone mapping system apps on the iPod.

What is important to improve GPS accuracy is HDOP and VDOP which is related to the Sky View and number of Satellites. You could for example put the Garmin Glo under your hat or in a top pouch in your Rucksack. This will improve the number of Satellites your GPS antenna/engine will see.

The Garmin Glo GPS is specified to pick up GPS and GLONASS signals from both constellations and will typically see around 12-18 satellites rather than just the 6-9 GPS NAVSTAR satellites. This could be useful if the NAVSTAR constellation system goes down.

The update rate is also 10 times that of conventional GPS engines.

Maximum Accuracy would typically be;

GPS - 8-10 metres with Selective Availability turned off

GPS + (WAAS or EGNOS) - 5-8 metres

GPS + (WAAS or EGNOS) + GLONASS i.e. the Garmin Glo - 2-3 metres

DGPS - 1-2 metres depending on baseline distance

RTK GPS (phase measurement with small baseline distances) - 0.01-0.02 metres

In the UK the mapping accuracy for OSGB36 grid mapping is typically 2-3 metres for a OSGB map triangulation point so DPGS and RTK GPS would be rather pointless for general navigation.




Edited by Am_Fear_Liath_Mor (03/26/13 08:32 PM)

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#258101 - 03/26/13 08:19 PM Re: PLB or GPS? [Re: Glock-A-Roo]
AKSAR Offline
Old Hand

Registered: 08/31/11
Posts: 1156
Loc: Alaska
If I feel the need to carry a GPS, my personal preference is to carry a dedicated GPS (Garmin in my case). I will sometimes also carry my iphone, but I look at it as strictly a back up for the Garmin.

My reasons for not relying on the iphone as my primary GPS are listed below. I do realize there are ways to mitigate some of these issues, and I've noted some of those.

1. Most smartphones are rather fragile and are not waterproof. In back country situations it is all to easy for a smartphone to be knocked out of action, probably when you need it most. However, one could put a smartphone into a protective case, and there are some available that are more rugged than the iphone.

2. GPS tends to use a lot of battery. I can easily replace the batteries in my Garmin in the field. Recharging my iphone requires getting back to a plug in. Again, there are options to recharge smart phones in the field, but this means carrying more stuff with no other purpose.

To summarize, in the rugged areas I frequent, I get by 98% of the time by using terrain association with a map. I always carry a compass, but seldom need it. I often carry a GPS, mostly for fun, but now and then it can be priceless (in a whiteout for example). I often carry my iphone (in a ziploc bag, deep in my pack), but consider it only a back up.

Those are my personal choices. As always, your mileage may vary.
_________________________
"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz

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#258103 - 03/26/13 08:28 PM Re: PLB or GPS? [Re: LeeG]
Glock-A-Roo Offline
Old Hand

Registered: 04/16/03
Posts: 1076
For Android users, check out "Field Compass +". It uses the phone's 3-axis compass to show a traditional baseplate compass view. You can sight on a target and essentially set the 'compass' bezel with a screen tap, then box the needle at the azimuth you've set. Works just as well as a simple Silva compass. An info pane on the bottom edge shows your GPS coords in the format and datum you select. There is no waypoint recording, routing, or GPS tracking.

The GPS phone app I use is "GPS Essentials". It has a highly configurable dashboard and full waypoint/route/track features. It has mapping functions but I haven't messed with those.

My phone is a Motorola Razr Maxx which has phenomenal battery life. And I would run these apps occasionally, not continually. Since I'm toting the phone anyway, it makes sense to have serviceable navigation apps on it as backups.

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#258104 - 03/26/13 08:42 PM Re: PLB or GPS? [Re: Am_Fear_Liath_Mor]
AKSAR Offline
Old Hand

Registered: 08/31/11
Posts: 1156
Loc: Alaska
Originally Posted By: Am_Fear_Liath_Mor
......

Maximum Accuracy would typically be;

GPS - 8-10 metres with Selective Availability turned off

GPS + (WASS or EGNOS) - 5-8 metres

GPS + (WAAS or EGNOS) + GLONASS i.e. the Garmin Glo - 2-3 metres

DGPS - 1-2 metres depending on baseline distance

RTK GPS (phase measurement with small baseline distances) - 0.01-0.02 metres

In the UK the mapping accuracy for OSGB36 grid mapping is typically 2-3 metres for a OSGB map triangulation point so DPGS and RTK GPS would be rather pointless for general navigation.


Even 8-10 meters of accuracy is probably more than sufficient for most outdoor purposes. (Exceptions might be geocaching and a few other applications.) Back in the dark ages before GPS, we sometimes thought we were doing pretty well to locate ourselves within a 100 meters (depending on terrain). Ah.... but I am showing my age smile

Of course I'm a bit biased from living in Alaska. For many areas outside town, the best available topo maps date from the '50s and '60s, and are at 1:63,360. In really remote areas some of these were done with very sketchy primary geodetic control, and there are some serious accuracy busts in some places (this shows up when one tries to splice two paper maps together). Areas closer to major towns have been revised more recently, and in a few places we are lucky enough have recent 1:25,000 maps, but those are exceptions.
_________________________
"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz

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#258105 - 03/26/13 10:40 PM Re: PLB or GPS? [Re: AKSAR]
Denis Offline
Addict

Registered: 01/09/09
Posts: 631
Loc: Calgary, AB
Originally Posted By: AKSAR
Of course I'm a bit biased from living in Alaska. For many areas outside town, the best available topo maps date from the '50s and '60s, and are at 1:63,360.

This is similar to Canada, while the maps may be newer (and are undergoing updating) we only have 1:50,000 topo maps that are widely available. I've seen a couple private 1:20,000 & 1:35,000 maps, but only for a few limited areas.
_________________________
Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen

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#258114 - 03/27/13 01:48 AM Re: PLB or GPS? [Re: AKSAR]
hikermor Offline
Geezer in Chief
Geezer

Registered: 08/26/06
Posts: 7151
Loc: southern Cal
If you really want to have fun, you should use the 1930s era 15 minute quadrangles that were the only USGS maps available in the mid 50s when I first started using topo maps. They were drawn by teams traveling by pack trains, using a plane table and alidade. Sometimes they goofed on terrain contours, but the trails were absolutely spot on. By contrast, when the new generation of 7 1/2 minute quads came out a few years later, the topography was right on, but trails were erratic, sometimes quite fanciful.
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Geezer in Chief

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