Tell Me About GPS Receivers

Factual Questions
1

Lately I’ve been thinking that I might enjoy having a GPS receiver. Here’s one that costs about $169. I’m not thrilled with the black & white display screen, but since a color one costs about $400 more, I’ll take what I can get.

Having never seen one of these, can someone please tell me if this model looks like it’s worth the price? Can I use it off-road (like, say, in the middle of the wilderness) to nail down my position in longitude & latitude? Will it tell me my altitude? Most importantly, can I use it for geocaching?

Anything else you can tell me about GPS receivers would be much appreciated.

TIA

2

I have had the even more basic eTrex - no built in maps at all - for several years. Works great. Yes it will tell you Lat/Long (and built-in support for coordinate systems from just about any country in the world) and elevation (though GPS is less accurate at altitude than Lat/Long). Also tells the time :slight_smile: . Would be fine for geocaching.

3

GPS receivers don’t care whether they’re on-road or off-road - if they can “see” sufficient satellites, they can calculate a position (lat, lon, altitude). They will thus work just fine in almost any wilderness but may have problems indoors or when shielded by buildings, cliffs, dense trees, jungle, etc.

The bells and whistles that you pay extra for are things like fancy color displays and sufficient memory to hold lots of detailed map data. Some have a pressure altitude sensor, which can overcome some of the limitations of the calculated altitude (which, as ticker notes, it inherently a bit less accurate than the calculated horizontal position).

4

I have the eTrex legend you’re referring to and it’s a pretty versatile unit. The built in maps are okay but I bought the optional US topo maps. They are more detailed so I only load the areas I’m going to travel in. It’s very good for geocaching as the nav functions will point you directly toward a destination you have programmed in. Consumer GPS do a poor job of altitute unless they have a barometric altimiter built in. My only complaint is that it only has an RS-232 ingterface and only works with USB with a somewhats pricey adapter.

5

The etrex is a capable little unit, but when I used one one weekend, I found I was wishing for an easier to read display. I’m over 40 now, and the little writing was hard for me to see. I like my old Magellan 315 because of its large letters.

By the way, I’ve had mixed success with Geocaching. The darn thing always tells me I’m within 10 or 20 feet of the target, so I search frantically right there, then I look again and I’m 100 feet away, although the unit hasn’t moved - I leave it sitting still to average. I went a couple of weeks ago with another guy, and both of ours told us we were within 5 feet or so, although we were about 100 feet apart. The position error on both of ours was like six feet. What’s the deal with that?

6

Gub’mint conspiracy.

7

Looks like I spoke too soon. Apparently Clinton stopped that nonsense: Office of Science and Technology Policy | The White House

8

So that would put us back to the old culprits of "GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. In short, you’ll get better accuracy in or close to the the USA if you have a WAAS compatible unit that accepts corrections relayed from ground stations: http://www.garmin.com/aboutGPS/waas.html

9

My two cents are going to kick in here. . .

I bought a Magellan Blazer reciever six years ago for $99.00, and the thing still works like a charm. However, my reciever’s accuracy is within 30 meters. I’ve asked the question and with the responses provided, I don’t think mine has the software/hardware to be accurate as to those six or seven years later.

Thus being, the crux of my post is this: learn how to use a map and compass. Before the days of GPS, I’ve navigated 10 miles without GPS. But I mention this solely as that a rudimentary knowledge of a map and compass (don’t forget knowledge of declination) will be a damned good backup to any satellite navigation you’ve got.

Tripler
There’s my ninety-eight cents in change.

10

How do the dedicated GPS units compare to a palm-top device with a GPS card inserted? I had recently seen a discussion about this but the participants were unhelpfully vague. I gather that you can get a GPS unit as a Compact Flash card and then plug it into a palm computer. It would seem more flexible in terms of usefulness, software and interfaces.

Do the price considerations make this less attractive, or would this setup be even less accurate than described above?

11

Thanks for all the replies. As to the elevation issue: I’m looking mainly for elevation as I drive up and down hilly roads in southeastern Missouri. I think it would be fun to go to the top of a very steep hill and say “Wow, we just climbed 200 feet!”

Is this type of detail in elevation readings possible on a low-end hand-held unit?

12

Yes - the unit you linked to (the same one I own) will give you that. I’s a nice little unit, and I use it exclusively for geocaching (177 finds and counting). In my local geocaching group there’s a good-natured running debate about Garmin vs. Magellan. A few of the “Maggie” folks seem to consider the Garmins to be inferior - more like toys than tools. We Garmin users respectfully disagree, of course. The one thing I wish my unit had is an averaging feature - very useful if you want a super-accurate reading when hiding a cache.

13

I carry a Palm all the time, but I decided to go with a dedicated GPS receiver. Dedicated outdoor GPS receivers are waterproof and more rugged than a PDA. They have big buttons so you can operate them with gloved hands and/or while riding a bike. And most dedicated GPS receivers run on plain AA or AAA batteries so you can easily buy or carry spares. This is an important consideration for hiking, camping, touring, etc.

But if you don’t need to use GPS under “outdoor” conditions, I’m sure a PDA-based unit is just as accurate. I have a USB GPS receiver in my car connected to a laptop (Microsoft Streets & Trips with GPS Locator). It’s just as accurate as my dedicated GPS receivers, and of course the map information is far more detailed.

14

Well, if a GPS rcvr could always take you to within millimeters of the cache, it wouldn’t be much of a game now would it? You are working with two sets of inaccurate coords. One from the original hider’s GPS and one from yours. Then you throw in loss of signal from the tree cover, time time of day, phase of the moon, mood of the Geocaching Gods, and you have a little searching on your hands.

Once you get to the cache area, stop and look around. If you were a cache, where would you be hiding? More importantly, if you were hiding a cache, where would you hide it? Look for the thing that is a little out of place. If you can’t find it without tearing up the landscape, skip that one for now and go look for another one.

15

I have a Garmin 760c which I love. It has the color screen, auto-navigation, pretty much all the bells and whistles. There are additional expenses that come along with the auto navigation stuff, mainly buying the latest and greatest new map package once a year or so.

I use it for geocaching all the time. It actually has a whole geocaching feature menu built in for that purpose and will show you anything you have uploaded into it as a geocache on the main map, so you can see them as you are driving by on the way to work or whatever and decide to pick one up.

I use the auto-navigation stuff all the time in the car. You can type in an address and it’ll take you right there, you can also select from a database of things like gas stations, restaurants, etc and it’ll show you which ones are available in the area, how far you are away from them, etc. Handy when you need gas in a strange place and have to have a shell station, or want to know where the nearest mcdonalds is.

It’ll definitely work offroad, in fact in a lot of cases they work the best there. You’ll get your best accuracy in wide open spaces, and will see drop offs in accuracy when you get into heavy tree cover, lots of cloud cover, or around tall buildings. Trying to use it in downtown Denver is nearly impossible… the buildings will block the signal. There are a few caches downtown that have elluded for quite some time because of this.

When they are not getting great reception what you’ll find is things go wacky. It’ll say the cache is 50 feet in front of you, you’ll walk that way and then it’ll say its 200 feet behind you. Such situations are rare though. My Garmin puts me within 20 feet of nearly any cache I got for. Any closer and it wouldn’t be any fun!

IMHO (I am not a doctor, your mileage may vary, plus tax and dealer prep, not available in all locations) if you are going to get one, save up the money and go for the higher end model!

16

I think the most important thing that varies with cost is the antenna. I borrowed an eTrex for a trip to sweden and the thing flat-out refused to lock onto a satellite at all, even though I tried it in relatively open spaces (airport car park, farmyard, etc.).
The person I borrowed it from says it refuses to work in his garden, despite there only being a few trees around it.

The more expensive units have a quad helix antenna rather than a patch, which apparently gives better reception, but mileage does seem to vary significantly from user to user. I have certainly been considering a Magellan Sportrak rather than an eTrex since my experience.

17

The eTrex Legend comes with 8M of RAM for maps. That’s not much if you’re using Garmin’s Metroguide USA software. I had an 8M memory card in my eMap at first, which was plenty for US Roads and Recreation, but Garmin informs me that they’re not updating that any more, and it’s really for casual use. I live in a part of Florida that’s undergone heavy development in the last 10 years, so a lot of roads have changed and I wanted updated maps. When I got the Metroguide USA software the maps were far more up-to-date and detailed, but that came at a price - they ate a lot more memory (even without transferring the autorouting data, for those of you who might suggest that). I ended up getting a 128M card for about the same price I paid for my 8M card several years ago, and it’s now way more than enough.

18

I started off with a Legend, lost it and replaced it with a Vista - basically the same thing with a barometer, magnetic compass and more memory.

The accuracy was very acceptable to me - generally less than 9m, and as low as 4m (or 2m with WAAS). People complain about how sensetive the eTrex aerials are to orientation, but I’ve never had problems - in fact, I doubt I would have figured out the right way to hold it on my own. It does help to leave it lieing facing the sky for a few minutes before moving with it.

I’ve been told the rule of thumb for GPS altitude error is double that of the horizontal error. This seems to be borne out by experience - it was never more than 15m off the altitude markers at ski resorts.

Getting a signal can be irritating in a forest, but it’s more intermittant than useless. The same applies for cities. You might be suprised at how easy satellite signals can be blocked, so don’t expect a reading inside your house. On the contrary, it excels anywhere with a clear skyline - countryside, ski slopes, etc

The Legend does support WAAS, but it drains the batteries and isn’t really worth it for a few metres more accuracy. You also need to leave your GPS facing one of the WAAS satellites for a couple of hours the first time you use it.

No matter what the error in position, a GPS receiver is very accurate at measuring your ground speed. Mine comes in handy as a speedometer for things like cycling and skiing. In fact more than any other ‘toy’, the magic of a GPS never seems to get old. I’m sure you’ll continue to find uses for one you never thought of when purchasing it.

In case you’re wondering, the only reason I switched to the Vista is to generate more detailed elevation profiles from ski trips. However, I must admit that more memory is also a bonus - transferring several megabytes of data over a RS232 connection is not something you’ll want to do often. The electric compass is simply inferior to a real (and cheaper) compass.

You’ll get more for more money, but your extra dollars are spent on increasingly trivial features.

19

Resurrecting this thread to ask:

One feature I’m looking for is a street-mapping feature. If I’m standing at, say, 209 N 6th St in Springfield, IL, and want to get to, say, 2700 S. MacArthur in Springfield, IL, can I input those addresses and get turn-by-turn directions (like I’d get from, say, Mapquest) on a low-end receiver?

Also, I notice that Best Buy has mapping software for GPS units. Are those updates, or do you have to buy one when you buy your receiver? Do you load them into your PC and then transfer the data to your unit in some way?

20

That feature is called “autorouting” and while the more expensive mapping GPS units support it themselves (i.e. without external software), the lower-end ones don’t. I use Garmin’s Metroguide USA software with my eMap. I pick my start and end points and Metroguide figures out the optimal route (which is usually, but not always, the best route). Then I download that route to my GPS through a special cable. You can also use the same procedure to transfer individual waypoints and maps to/from the GPS.