I have experienced bad locations from GPS, but not so bad that I didn't know where I was. In a fairly deep canyon (and probably less than optimum satellite geometry), the GPS told me I was in the bottom of the canyon, while I, eagle eyed archaeologist that I am, could readily see that I was well up the canyon slope at an obvious bend in the canyon.

You must think when you are navigating. Compasses and maps have their imperfections, as well. Don't trust any tool mindlessly.

I haven't decided yet.

After all, we only have her story. Maybe she was the one who was driving.

Sue

I had a funny GPS experience about 2 years ago: I drove off the edge of the world ! Yes really !

In case you're wondering, there were no sea monsters, but there is a 100m cliff and a pounding surf which sprays up, onto the top.

I encounterd this when I was heading out to Steep Point, that’s the most Westerly point of the Australian mainland, part of Shark Bay in Western Australia. I’d plotted in a few way points (from home in advance) and was expecting to come to a minor track intersection when suddenly the GPS went blank. No information. No nothing. I did a reset and fiddled for quite a while, but the maps and information just stopped ! Later on I met the local caretaker who laughed and told me than (until recently) none of the GPS manufacturers included Steep Point and even Google maps did not cover it (until a few years back). He and a few of the locals encouraged travellers to report back to Garmin etc to request it be added !

This may seem a bit odd to you people in Europe and North America, but its true ! So if any of you are planning a driving trip down here and want to visit Steep Point: check your GPS !

(Yes I had maps etc as a backup and there’s only really one road (track) so you can’t go too far wrong … unless you try of course).

GPS is most certainly a useful tool, but GPS errors are not understood well enough by most folks to risk their own lives with. Most folks assume you just stick the battery in and punch in your destination and away they go. GPS systems have only really become popular in the mainstream since manufacturers dispensed with the monochrome screens that just showed WGS84 polar co-ordinates with no maps to display and didn't ask for such setup parameters such as a map datum and magnetic/grid/true north deviations or declinations etc.

The relationship between the GPS polar coordinated and the electronic map and the algorithms which compute routes and methods to get back on the computed routes if that route is deviated from. A road type GPS system will rarely tell you to stop and tell you turn around because it has detected an incorrect turn off. i.e. there might be 2 right hand turns within 50m - well within GPS error within limited sky view and hence a large HDOP error. That wrong turn will eventually be detected but the GPS algorithm to get you back to the correct turn off may involve a long and tiring circular route back to the same turn off point rather than telling you to Stop and backup the same road you've just traveled down.

For example the variable nature of the speed of the vehicle may increase errors if the velocity filtering is poorly implemented. GPS systems also cannot detect direction or acceleration very well without filtering a number of position data sets so the update rate of the GPS is important. The GPS circuit might be updating every 1 sec or every 10 or 30 sec for example to save battery power.

A lot depends on the accuracy of the electronic map. The map may have been produced from a very old survey on a different map datum for example. It may have been produced from satellite or aerial imagery or ground survey. There is a lot of computation that is also required for datum conversion to cater for different map datum transformations i.e. WGS84 to OSGB36 for UK Ordnance Survey maps (which are very intuitive easy to use and accurate grid type maps). This conversions all have inherent errors transforming elliptical polar coordinates such as WGS84 to grid mapping types where they tend to loose some accuracy at the edges of the grid.

http://www.movable-type.co.uk/scripts/latlong-convert-coords.html

Solar storms may increase TEC atmospheric errors (most GPS don't have access to both L2 and L1 bands, then there is multipath errors where more costly GPS antennas are required to reduce this error. Turning on WAAS or EGNOS will also affect circular and spherical positional error probabilities.

Out of band interference from other microwave signals such as Lightsquared will most likey produce random additional errors.

Then there is deliberate Jamming and Spoofing.

All these error can sometimes conspire to send folks over a cliff.

That happened to a Squadron of F22 fighter jets as well when they first flew from the USA to Japan. Luckily they managed to follow the tanker aircraft in the right direction or they would have most likely have had to have ditched in the Pacific ocean.

A GPS "failure" when crawling through a sand dunes at walking speed is not really a big deal. In a jet, over the ocean would be really worrying !

And I think Australia is looking at buying a few F22s - hopefully no one will ever invade via Steep Point or we're out of luck !

I guess I'll agree that most people don't understand GPS. What most people call GPS is a GPS receiver tied to a navigation package of some sort. GPS is nothing more than a positioning system that works (very rough description) by comparing a set of timing signals against a "catalog" of satellite data. The accuracy of the timing data (and the catalog) along with the distances involved allows the position and probable error values to be calculated. Height is always less accurate than lat / lon due to the geometries involved. All a GPS can tell you is where you are or maybe some track and velocity information if you integrate position data over time. Getting access to more accurate timing data can dramatically improve the position accuracy as can some other tricks (known as WAAS and LAAS).

Every GPS I have ever used (even the horrible navigation system in my wife's car) has provided (somewhere) an indication of how accurate its positioning is. Figure out where that is on your system. As long as you know the error margins and characteristics (i.e lateral is considerably more accurate than vertical) even commercial GPS is a decent and reliable system for figuring out where you are.

Most so-called GPS errors are actually navigation system or map generation errors. I might trust the routing of a commercial avionics Flight Management System which is designed with a lot of safety checks and subject to FAA certifications and audits. I would be less trusting of a military navigation system and trusting a navigation package in an car or hand held navigator that does not require or provide documentation of similar pedigrees in development or testing processes is just not an option. Of course the car/handheld nav systems are fun to play with and can provide good situation awareness - just don't let them think for you.

-Eric

Residents puzzle over man who died near Ore. town

MARION FORKS, Ore. – Three miles. That was all that separated Jerry William McDonald from a cold death alone in the Oregon woods and a warm meal back at the local restaurant.

http://news.yahoo.com/s/ap/20110515/ap_on_re_us/us_pickup_truck_body

The search for Albert Chretien has been called off.

Rescuers have called off the week-long search for a British Columbia man stranded in the Nevada backcountry, saying they have covered the passable areas along the route he is believed to have taken.

I once lived in the same town as the Chretien's, they are very well liked and respected people. It is unfortunate that this story has to end in this way.

Three miles. Over what sort of terrain? On a decent road, any adult in reasonable shape can walk that in an hour.
Same distance in deep snow, and I define that as being too deep to walk through comfortably without snow shoes, you might as well be on the moon.