Sat Nav / GPS

[Westy]

I wonder how GPS adjusts for shifts in the Earth crust?

After the earthquake in Chile, BA has moved 3 cm West and Concepcion is now 10 feet West of where it was.

Your ordinary navigation-grade GPS might register the 10-foot shift. It would take (it took) a surveying-grade GPS to register the shift of Buenos Aires.

GPS works by time differences. The receiver starts with the difference between "time tick" signals from the satellites in view, and a mathematical model of their positions relative to the center of Earth's gravity. If you've ever looked at the "estimated position error" figures on a GPS when you turn it on, the position gets better with time - it may start out with an EPE of 150 feet or worse, and after a couple of minutes it settles down to 15 feet or better. This is because more time, and more computations, improve the math model.

There are, however, several items that change the signals enough that a navigation-grade GPS will never get much better than 10 feet accuracy. First and worst is "multi-path error," where the signal gets reflected and re-reflected from buildings or even trees. Try using one in the downtown area and you'll soon see what I mean. A similar error can be caused by rain, as the raindrops actually do diffuse the signal. Variations in the ionosphere can slow down or bend the signal (this is familiar to shortwave-radio users); and the troposphere, the lowest layer of our atmosphere, can also bend the signals enough to introduce error. And there are tiny variances in the satellites themselves; although the GPS "control segment", with its monitoring network, keeps pretty good tabs on that. They figure the "satellite almanac" that your receiver uses to predict the orbital position of the satellites on a moment-to-moment basis; it's broadcast with the GPS signal from each satellite, and the receiver downloads it every time you use it.

You can improve that position quite a bit by using "differential GPS," which means you have a GPS receiver at a known, surveyed location near the area where you're working. This broadcasts the difference between its actual position and its "GPS position," in such a way that your GPS receiver can also receive that difference and apply it to the position it's calculating. This wouldn't help after an earthquake that shifted the crust under the benchmarks too!

Surveying GPS receivers operate differently. Rather than calculate your position on a moment-to-moment basis, they save up all of the raw data over an extended period of time. Then the surveyor loads this raw data into a computer for post-processing, using far more positioning data and more sophisticated processing programs than you'd be able to use in a navigation-grade GPS. They can also download the satellites' actual position data from the GPS control segment, 24 hours after they collected their position data; with all that help, they can calculate a position to within a centimeter or better independent of local control.

This post-processing is what the "national survey" would have used to come up with that 3cm shift in Buenos Aires' position.

(Geez, this should teach you not to dangle an "instructional opportunity" in front of a retired teacher!)

[Tessan]

The gps does not adjust. It will tell you, your position on the earth surface using longitude and latitude. The maps put into the gps is what you look at. The map uses Longitude and latitude to tell you where you are in the map. If the surface moves 10 feet or more, then it's going to be off by 10 feet or more, until someone update the map with the new longitude and latitude of the street you are on. The satellites in orbit don't know the surface has moved.

Usually car gps are not supper accurate, but a move of more then 10 feet might be noticed. There are other gps that are accurate within an few inches. Those are usually used by scientist monitoring glaciers movements or fault lines, and it is also used on some very large constructions sites.

[WorldTravel69]

I wonder how GPS adjusts for shifts in the Earth crust?

After the earthquake in Chile, B. A. has moved 3 cm West and Concepcion is now 10 feet West of where it was.

[Toymann]

Thanks for the update on the GPS situation. At the time of my post I had tried several knockoffs without any success. The Garmin authorized argentine mapset was the only one that worked with the garmin when I purchased it years ago. The point is now moot for me as I take busses north to corrientes everytime. Can't beat the executive bus in argentina fellas. LOL. I know somewhere Exon is smiling. Happy Mongering All. Toymann

ps. Kevins. At the time I purchased my mapset Garmin did not sell an argentine mapset but had contracted with an authorized group in argentina, that also created the mapset. In other words I couldn't buy directly from Garmin or I would have. As with all technology these days things change quite rapidly and based on your post there are now affordable mapsets you can get that work on a garmin. Good news for the consumer for sure. For whatever it's worth the mapset I bought has worked great and was partically good at navigating in and out of buenos aires.

[Kevins]

I am certainly getting a bit of a chuckle out of this thread. Yes, there are some internet free downloads for argentina as far as maps go. As far as I know, at this time, there is only one GARMIN, navigating map set for argentina. It's very pricey, but works quite well. Good luck with the knock-offs! Been there, done that! Doesn't get it done. Happy Mongering All. Toymann

I have both the original garmin maps (bought on a sd disk from garmin) and the proyectomapear. Com. Ar free downloaded once joined free forum and the mapsource program and at least for now in the Buenos Aires and surroundings (I have gone to Gualiguachu and found hotels using it) the projecto mapear newest version (8.6) is very very good and gets you anywhere with no problem and so far it has helped me find anyplace I need & it had all the info I need and worked great with my garmin nuvi

[BadMan]

Get an unlocked smart phone: GPS Navigation, 3G, WiFi, Skype etc.

[Mr Azul]

I have used the maps from: www.proyectomapear.com.ar and they have not failed me. Its a good map and its free.

[Sinistra]

This is just to let you all know that the Tom Tom application for iPhone is now available with the Argentine / Uruguay maps. This is turn-by-turn navigation and it's on sale until January 11th for just $49 at the iTunes store. Pretty sweet deal if you ask me.

[Toymann]

I am certainly getting a bit of a chuckle out of this thread. Yes, there are some internet free downloads for argentina as far as maps go. As far as I know, at this time, there is only one GARMIN, navigating map set for argentina. It's very pricey, but works quite well. Good luck with the knock-offs! Been there, done that! Doesn't get it done. Happy Mongering All. Toymann

[Rainot]

If you own a Navigon or any other GPS that comes with WinCE, you can load "miopocket" in it, and it'll turn your gps into a PDA. With the benefit of being able to install Garmin Mobile XT with Argentina Maps (the most detailed available) among other GPS software; play movies; listen to mp3; etc.

More info at gpspassion here:

http://www.gpspassion.com/forumsen/t...OPIC_ID=126303

There's Argentina maps for Igo 8 and 8.3 here:
http://www.taringa.net/posts/links/2...argentina.html

Of course it's not as detailed as mapear's.

[Hubbster]

Its not a Garmin site. Its a site created by people in Argentina.

Found it

www.proyectomapear.com.ar

[Sportsman]

I brought a Garmin from USA and downloaded Argentina maps onto it online

From which website and for how much? I have a Garmin also and have never seen map for Argentina in Garmin website.

Two years ago before I got my Garmin, I was walking by an electronic store on Talcahuano somewhere between Cordoba and Tucuman. The clerk told me that if I bring a GPS to them and they can download Argentina (or may be only Buenos Aires) map to it for 90 pesos. Since I don't really travel too far in BsAs so I have not gotten the map yet.

[Hubbster]

I brought a Garmin from USA and downloaded Argentina maps onto it online.

[Sinistra]

TomTom turn by turn navigation is now available for download from the app store. Available are US / Canada, Western Europe, Australia & New Zealand. Argentina is NOT available yet, but is likely coming soon. The price varies, but look for it to cost about $99USD.

[Damman]

Competition is on the horizon: http://www.directionsmag.com/press.r...=Show&id=34994