Ok I have looked thru the archives, but still stumped. I’m thinking about a gps, basic model,(too many features just confuse me…I’m someone who has a 8 yr old Nokia cell phone that won’t die, and truthfully it works way better than most newer phones in terms of recpt.and I cringe when I think about having to replace it with a new one) Is there a big difference in the etrex and the legend. I know the legend has WAAS, is this significantly better, say for instance in trees,river valleys etc.
ETrex vx. Legend
– Last Updated: Jun-30-07 2:25 PM EST –
Go here: http://www.mousetrap.net/~mouse/gps/whichgps.html
for some information. The biggest difference is that the Legend includes a base map, and the ability to load more detailed maps, so can be used to show you where you are on a map, and also to show you the way to a destination. You can (after buying the mapping software) let your computer calculate the best route from Point A to Point B, load that route into the Legend, and then follow the Legend's compass-like display to your destination. The basic ETrex has no mapping capability. I believe it only allows you to see your current location (latitude/longitude), and will point the way (as the crow flies) to any of the waypoints you have loaded into it. But it will not direct you along streets or roads. I took my Legend with me on a short paddle on a local lake, and was able to view my position on the lake, see how far I had paddled, my average speed, maximum speed, etc.
Better still, go to www.garmin.com and look at the owner's manuals for the two models.
thegpsstore.com
has employees that were very helpful to me & a good return policy, also.
Mapping
– Last Updated: Jun-30-07 3:10 PM EST –
The mapping capability and the database of the Legend is well worth the extra few bucks.
WAAS isnt really going to help you much in the woods or under cover. Mainly what WAAS does is allow you get a more accurate location but you still have to find the WAAS satelite (which is close to the horizon)for it to help.
Honestly, I have two Garmins, one with WAAS and one without. I dont notice a difference in accuracy or sensitivity. Actually, the one without WAAS is a pre 2000 (selective availability) GPS so it has an averaging feature which seems to help when satelite coverage is sparce.
The option that will help pickup signals through the trees and brush better is called a "high sensitivity receiver".
Magellan GPS
I have a Magellan Sportrak Map. I’ve used it for years. I agree you should ourchase a unit with mapping capabilities. It’s nice to know exactly where you are. Especially if your paddling on an unfamiliar lake or hiking a new section of woods.
I don’t believe Magellan distributes my model anymore. The new series is the Eplorist. Good Luck.
Legend
The Garmin Legend is a nice one, but it isn’t color. I didn’t think that was a big deal but it does seem to make reading the details on maps a little easier.
There is a color version of the Legend (the Legend Cx) but look at the Venture Cx instead. It’s the same thing without the USB cable (most people already have the one needed) or 64mb memory card (I bought a 1 gig card from NewEgg for $9) and saved about $50 by doing it this way.
I have owned 3 garmins…
an eTrex, a Legend and have a 60C now. You need to ask your self what you want a GPS for: if the goal is only to mark a spot and return to it any basic non mapping model will do. The ability to map adds some wow factor, but at the low end the tiny grayscale maps are tough to read- particularly if you are even slightly visually challenged. Going to a larger display and adding color get you up into the units that have easier to use (and see) menus, but do provide a plethora of features. WAAS is cool, but how often do you really need to locate yourself within the length of your boat? Cool, but not practically useful. gpsnow.com has lots of useful info and they are good folks.
Best-
Glenn
I have a Legend
And one of the things I like about it is that you can chose a large fonts size. The regular E-Trex doesn’t have this. When I’m execising, I like to monitor my speed and the large fonts are handy to see. Also, Garmin is coming out with updates to their E-Trex’s (they will have an H in the name) that will be supposedly significantly more sensitive so they will be more useable under trees, etc. They’re scheduled to be available the third quarter of this year. According to the tech guy I talked to at Garmin, they should be similar to the SiRF chip that’s in some of their more expensive models.
Physics of GPS and reference
GPS is a line of sight system. Almost all systems will work to some extent under canopy with varying results based on leaf density and constellation status at the time of measurement. WAAS and other SBAS corrects your results to provide better accuracy +/- 10 feet versus the 25 or more your would see for an uncorrected measurement. You can have a wicked accurate position but unless that position is familiar to you it is useless. If it only shows you a single point on a blank screen you are really no better off than you were without GPS. If there is a point relative to your location you are golden- if that is where you want to go. A background map provides a familiar moving picture that will provide you with a real time reference.
We make high end GPS for construction and surveying. $35K- need one?
P
Line of sight
I understand that. Help me explain this. Once I rented a car in Germany. It was a Mercedes with a built-in GPS. The car was parked deep inside on the lowest floor of a parking garage. There was no line of sight to the sky. However, the guy turned it on and showed me how to use it right there without a problem. Similarly, when I turned on my Garmin GPSMAP CX in my living room (it has a roof), I get all the satellites plus my clear location in a few seconds.
Good question
Some systems will display the last good position as the current position until it achieves convergence or a new position. If your living room has windows and it can see three low sv’s it could compute a position albeit an inaccurate one. It typically takes three sv’s (sats) to determine a position and a fourth one to reduce the timing error. The position is determined with very easy H.S. math formula D=V*T; V=speed of light. If we determine the distance from a number of sv’s that have a known location we can converge on one position and one position only.
There is also a latency in some systems between loss of signal and notification of LOS. I have a garmin GPSV that I really like and I have had it since it was the bleeding edge of handhelds. When I travel through a tunnel such a the Chesapeake Bay or the Big Dig I am almost out the other side before it decides to tell me I’ve lost signal. A re-radiating broadcast would not work inside a tunnel because all it would display is the current position for the static antenna outside.
The signals for L1 and L2 are very weak. Most handhelds work on the stronger of the two- L1. Most survey grade units that are capable of sub centimeter and even lower accuracies depend on both frequencies to remove the atmospheric disturbance of the two signals as they travel 11,000 miles from the sv.
This note should be read just before bedtime as it was boring and long winded.
pat
Good answer
"Last good position as the current position" - that explains the Mercedes.
I do have a large window in my living room facing west. I believe line of sight can be deflected a little and still get a signal. The new Garmin GPS is much more sensitive than the older models. A short tunnel or the sky scrapers in New York City usually present no problem.
Thanks!
Pat, you say “long-winded and boring”. I say, intelligently written and informative. Thanks for sharing your expert knowledge. Now, if only I had an extra $35K laying around . . . . .
Jeff (owner of a 6 year-old Legend that is now being held together with scotch tape but still works like a charm!)
New algorithms are derived
frequently that help clean things up. Reflected signals cause an error named multi-path. This usually lengthens the distance a signal travels and it causes the position derived to be inaccurate. A great way to visualize this; if you remember TV before cable? You are sitting in your living room fiddling with the rabbit ears and you still see Joe Mannix but there are two Joes. The second Joe is a fake and false Joe. You want a GPS unit that will remove that second Joe- filter him right out. That is what a lot of the new HH units can do. You will see multipath in canopy, wet and dry and in urban cannons. Anything reflective can cause it.
Precise and accurate are two separate subjects. If you want to collect a point you will navigate back to average a dozen measurements or so to really get it it narrowed down that is precision- accuracy is the statement on the side of the box that has a long disclaimer next to it saying that it is really great in the lab.
pat
trimble?
not a chance
Too big and too unfriendly. I have a few surveying friends over there. Sokkiadotcom. I started using GPS for surveying in 1986 while assigned to DMA now called the Geospatial Intelligence Agency. I have been involved with it ever since.
WAAS
It’s hard to find a GPS these days WITHOUT WAAS capability. Might as well assume that you’ll get that feature no matter what.
I agree that a bare minimum basemap is a huge help in some cases, but in many places, it’s still useless. More detail is a HUGE help. Having the memory to upload lots of topo or other data onto the GPS is great. With the removable chips, you can often store road map data for the us AND canada as well as a very large chunk of topo data on one 2gig chip. Saves the hassle of having to upload new maps every time you want to go someplace new.
If you want something basic, but with a big screen, take a look at some of the low end 60/76 models.
You really need to decide exactly what you want to use it for so you can narrow down which features will be useful and which will be extraneous. Do you only want to see coordinates that you can refer to a paper map? If so, then a basic etrex will be perfect. If you want mapping ability, then your options are MUCH bigger. Fixed memory or removable? Fixed memory receivers from Garmin won’t be around much longer. Color screen or b/w? Again, b/w screens won’t be around much longer. Barometric altimeter/electronic compass? Most folks don’t need these features, so a number of receivers can be cut off the list.
Once you get those features narrowed down, there’s still going to be quite a few models in contention. You’ll then have to choose based on screen size/resolution, button layout, receiver sensitivity, and capacity for data storage.
WAAS killed the USCG Beacon system
USCG Beacon was the first widely utilized LBAS along with similar beacons from the USACE. The problem was spatial decorrelation or the degradation of accuracy over the distance from the beacon broadcast position. If for example I had a beacon corrected GPS measurement in my yard here in NEK VT. I am at best 250 miles from the nearest beacon, my sub-meter unit just went meter plus because of the distance. WAAS does not suffer from such frailties. Just wait until LAAS comes along, or the L5 freq. Those will be exciting. SBAS is the way to go.