navigating off the grid: iPhone apps

[yubert]

Maybe I should have used the word "accurate" instead of "sensitive". It has been reported online, and I have witnessed first hand through friends iPhones, that the iPhone 3g GPS is less accurate than many stand alone GPS units (in particular recreational mapping units). This is especially true when the iPhone can not pick up a cell or wifi signal.

Most stand alone mapping units have WAAS these days.

In order to pin point a location with cell towers you do need to triangulate. Nearest tower by cell strength is used to speed up the sat signal time to first fix (TIFF) location but will not place you on a map by itself.

http://gisandscience.com/2009/07/15...f-assisted-gps-wifi-and-cellular-positioning/

All of that said the iPhone 3g GPS is accurate enough for it's purposes, especially around town. I have been impressed with the iPhone performance in the open spaces of the Sierra Nevada's as well, but I would never rely on it as a mapping or off route unit.

First, using cell tower and/or WiFi for determining location is primarily for devices that do not have a GPS chip or when one is indoors. It is not accuracy. However, the footprint of a WiFi signal is smaller than that of a cell tower so WiFi will be more accurate.

You DO NOT need to triangulate to determine location using cell towers. Each cell tower has an ID, and location based services (LBS) such as Google Maps, matches the ID against a database of the cell tower IDs which has the location for that cell tower. They also maintain a similar database of WiFi network IDs and employ the same strategy to determine one's location. This is referred to as handset-based location as opposed to network-based.

Perhaps this video below from Google will clarify how they determine cell tower location without triangulation.

http://www.youtube.com/watch?v=v6gqipmbcok

Also triangulation using cell towers can be problematic, the cellular networks track the current cell tower and the next/adjacent cell tower for handing off the call. However it's possible the current cell tower and next/adjacent cell tower are actually located on the same tower. This is because to increase the call handlng capacity in a given area, macro and micro cells of varying strength are added. Your call can be switched between micro and macro cells depending on your speed of movement. So try triangulation when two datapoints have the same location!!

As for TTFF, that's what A-GPS s all about. Using cell tower location is one method but so is storing satellite orbital information to reduce TTFF. The iPhone 3G/3Gs has this feature too, just like dedicated GPS units.

Bottom line, the iPhone 3G and 3Gs has a high sensitive GPS receiver, the Hammerhead II, which features A-GPS for fast TTFF. The GPS receiver is evey bit as good as the ones found in dedcated GPS units. As a matter of fact, some dedicated GPS units such as the TomTom One XL also use the Hammerhead II.

But unlike dedicated GPS units, the iPhone 3G/3Gs can utilize cell towers and WiFi to determine location in weak signal areas such as when going through a tunnel.

 

[chet]

for those with an ipoid touch there should be two GPS solutions coming in the new year one from Magellan and one from Dual electronics. The dual one looks better but the Magellan one will be out sooner.

 

[DaveM]

First, using cell tower and/or WiFi for determining location is primarily for devices that do not have a GPS chip or when one is indoors. It is not accuracy. However, the footprint of a WiFi signal is smaller than that of a cell tower so WiFi will be more accurate.

You DO NOT need to triangulate to determine location using cell towers. Each cell tower has an ID, and location based services (LBS) such as Google Maps, matches the ID against a database of the cell tower IDs which has the location for that cell tower. They also maintain a similar database of WiFi network IDs and employ the same strategy to determine one's location. This is referred to as handset-based location as opposed to network-based.

Perhaps this video below from Google will clarify how they determine cell tower location without triangulation.

http://www.youtube.com/watch?v=v6gqipmbcok

Also triangulation using cell towers can be problematic, the cellular networks track the current cell tower and the next/adjacent cell tower for handing off the call. However it's possible the current cell tower and next/adjacent cell tower are actually located on the same tower. This is because to increase the call handlng capacity in a given area, macro and micro cells of varying strength are added. Your call can be switched between micro and macro cells depending on your speed of movement. So try triangulation when two datapoints have the same location!!

As for TTFF, that's what A-GPS s all about. Using cell tower location is one method but so is storing satellite orbital information to reduce TTFF. The iPhone 3G/3Gs has this feature too, just like dedicated GPS units.

Bottom line, the iPhone 3G and 3Gs has a high sensitive GPS receiver, the Hammerhead II, which features A-GPS for fast TTFF. The GPS receiver is evey bit as good as the ones found in dedcated GPS units. As a matter of fact, some dedicated GPS units such as the TomTom One XL also use the Hammerhead II.

But unlike dedicated GPS units, the iPhone 3G/3Gs can utilize cell towers and WiFi to determine location in weak signal areas such as when going through a tunnel.

Yes, that’s true. Cell towers and WiFi for AGPS are used primarily to speed up the TTFF but also for location when there is no satellite signal.

You do not need to triangulate to approximate location. Cell phone towers will approximate your location without triangulation. I said previously that that in order to pin point your location you need to triangulate and that is still true. If you watch your linked video closely you will notice that it says without a GPS signal you will get an “approximate” location with a larger blue circle instead of a pin point dot. This is because a single cell tower only knows your distance (and possibly your general direction) from the tower. It likely uses a form of geo-coding to guess where you are on the map by assuming that you will be traveling on a street or sidewalk, and snaps your approximate location to the map. This is for cases where there is no GPS signal.

The iPhone 3G & 3Gs GPS chip my have similar characteristics to many handheld units but there are other variables including software and antenna configurations.

My bottom line is that real world tests have shown the iPhone 3G (same GPS as a 3Gs) to be less accurate than a standard consumer level handheld unit. This is not a concern for most location based services or for traveling in and around town but once away from that AGPS network (the original question in this thread) it’s GPS only and I would not rely solely on the iPhone for back road navigation.

I don’t know exactly what the error level is on a GPS only iPhone, the article I linked mentions 8 meters. Not a big deal when on a marked road but it could be important when walking backcountry or in a complicated un marked road network like many forest services areas.

 

[haven]

How accurate does your GPS receiver have to be?

GPS forums online have a lot to say about the iPhone GPS, particularly its accuracy. While commercial GPS units costing $4000 and up are accurate down to a few cm, the iPhone GPS is often wrong by many meters.

Here's one report
http://gisandscience.com/2009/07/15...f-assisted-gps-wifi-and-cellular-positioning/

"Results indicate that A-GPS locations obtained using the 3G iPhone are much less accurate than those from regular autonomous GPS units (average median error of 8*m for ten 20-minute field tests)."

The same report says its positioning using WiFi had a median error of 74m. Cell phone median error was a whopping 600 m.

The report concludes, "[The iPhone GPS results] appear sufficient for most Location Based Services (LBS)"

In the article, the iPhone GPS usually showed where it was within 30 feet or so of the correct position. That's close enough for route-finding using a topo map off the grid, and for turn-by-turn navigation on mapped streets.

Generally speaking, dedicated handheld GPS units from Garmin, Lowrance, Magellan and others are more accurate, and resolve your position faster than the iPhone. However, the Garmin website says, "Garmin GPS receivers are accurate to within 15 meters on average."

For my use, the iPhone is accurate enough.

 

[chet]

my garmin hits 30' accuracy only on a VERY good day! Usually its more than that.

 

[IZVERG]

I have two of MotionX's GPS apps and they both work pretty well, nice features too.

 

[dustboy]

Woo! The new version of Topo Maps now can download 10 quads at a time, and it will "seam" one quad into the next, so if you're on the edge you don't have to jump back and forth.

Pardon me if this is redundant, but damn it's slick!