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#107142 - 09/27/07 09:20 AM Do's and Don't for purchasing a GPS
xavier01 Offline
Journeyman

Registered: 12/02/02
Posts: 86
Loc: Phx, AZ
I just did a forum search on "GPS". No results???
I am going deer hunting soon and am looking to purchase a GPS. I also spent an hour yahooing for GPS information. Does anyone have any experience they could share on what direction I could take or lessons learned. Budget is $150. My only application (that I'm able to figure out) is hiking/hunting. From what I can tell, the Garmin Etrex seems to fit the bill. Thank you very much.

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#107154 - 09/27/07 01:05 PM Re: Do's and Don't for purchasing a GPS [Re: xavier01]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
If you click the right links at GPS City you'll get a list of handheld GPS, pages of handhelds. There are quite a few eTrex units listed, I like the Vista HCx, but on your budget, the Garmin eTrex Venture HC would work, I just don't like the bright color of the case. It would be hard to lose it though.

Take spare batteries and don't forget to take a map and compass too.
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#107165 - 09/27/07 02:14 PM Re: Do's and Don't for purchasing a GPS [Re: xavier01]
Blast Offline
INTERCEPTOR
Carpal Tunnel

Registered: 07/15/02
Posts: 3760
Loc: TX
Sidenote on the ETS search function: your search word needs to have at least four letters.

-Blast
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#107169 - 09/27/07 03:21 PM Re: Do's and Don't for purchasing a GPS [Re: Blast]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
Do a seach for "vista".
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Better is the Enemy of Good Enough.
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#107182 - 09/27/07 04:58 PM Re: Do's and Don't for purchasing a GPS [Re: Russ]
KenK Offline
"Be Prepared"
Pooh-Bah

Registered: 06/26/04
Posts: 2208
Loc: NE Wisconsin
I personally recommend Garmin GPS receivers (good hardware, software, and customer support) and specifically the receivers with the newer high sensitivity chipsets that provide MUCH better reception under heavy cloud cover, leaf canopies, and inside vehicles.

Whatever you get I'll highly recommend you buy a simple case and a PC-interface cable. Use the cable with EasyGPS - a free program that help you upload/download/manage your waypoints/routes/tracks - available at http://www.easygps.com. This allows the user to store unused waypoints/routes/tracks on the PC instead of taking up memory on the GPS receiver.

If you just need to capture and use waypoints, routes, and tracks, and don't need pre-loaded maps (or the currently available maps don't meet your needs) then I'd recommend the eTrex H ($95). You'll appreciate the High sensitivity chipset when under forest canopy.

If you want a GPS with on-board maps, then you'll pay a lot more for the GPS and the mapping software costs an extra amount. I'd recommend the eTrex Legend HCx ($215), GPSMAP 60Cx ($320), or the GPSMAP 76Cx ($320). These units have high sensitivity receivers and external memory cards.

If you also want electronic compass and barometer (not really needed by most users), then I'd recommend the eTrex Vista HCx ($250), GPSMAP 60CSx ($360), or the GPSMAP 76CSx ($360).

In general H=high sensitivity receiver, C=color display, S=sensors (electr. compass & barometer), and x=memory card, but the Vista doesn't list the "S" and the 60 & 76 series don't list the "H".

For US street maps add $120. For US 100K topos add$85. For regional 24K topo maps of the national parks add $90 (there is one for western US, one for central US, and one for eastern US).

If you're still learning about GPS technology and how to use them with a paper map, check out the free pdf brochures at http://www8.garmin.com/aboutGPS/manual.html


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#107192 - 09/27/07 06:25 PM Re: Do's and Don't for purchasing a GPS [Re: KenK]
Katie Offline
Journeyman

Registered: 08/23/07
Posts: 85
+1 for the Garmin Vista series.

Here's the novel I just sent a friend who had questions about my experience with the Vista series. Unfortunately, I don't have any data points on the HCx, but I'm guessing I'll end up buying one pretty soon.

Quote:
I own the Garmin eTrex Vista and the Garmin eTrex Vista Cx, and I've been really happy with both. The entire eTrex series seems very robust. They're waterproof to 1 meter, so you don't need a separate watertight bag for them if you're outside. I really like the interface. Tech support is responsive and helpful. The battery life is good, and it takes 2 AAs so it's easy to get replacements while you're on the road. Using Li batteries makes it a lot lighter. Some of the handhelds have an issue with the rubber gasket separating after a couple of years, but Garmin has been good about replacing them at no cost with a week or so turnaround time, or sending you a self-repair kit.

I worry a bit about scratching the screen, so I keep mine in a neoprene case with a belt clip and a plastic window for the screen. Unfortunately, I find it hard to read the screen through the plastic, so I take it out of the case whenever I'm actually trying to navigate somewhere, but if I'm hiking it can stay in the case on my belt or pack and still get signal/keep a track of where I have been. Setting a waypoint at the trailhead or your car before you start hiking is always a good idea, but it also leaves a "snail trail" on the map to show where you've been, so you can retrace your steps if you have to.

It can up to a couple minutes to acquire satellites after you first turn the unit on, especially if you're in a car going 60 MPH. This is annoying, but once it gets an initial satellite lock it's almost never necessary to do it again until you power off the unit. If I'm moving in a heavily wooded area, a canyon, or an urban canyon, I sometimes have the satellite signal drop out. If I need to figure out where I am, usually just stopping for a minute is sufficient to get signal back. I'm guessing that this is a limitation of the antenna you can fit in a handheld unit. It doesn't have a connector for an external antenna, but I rarely find I need one.

I would recommend getting the Cx. The color makes the maps a lot more legible and information-dense, and it accepts microSD so you can expand the memory capability. I currently have a 2GB card in mine which fits all of the MapSource CityNavigator maps for the entire United States, plus some extra topo maps. On my Vista regular, I was constantly having to upload new mapsets when I traveled. It could handle the maps for driving from Boston to Portland, ME, or a state or so, but not much more than that. This may not matter to you if you're mostly going to be in the same area most of the time. My old Vista is a serial device, so I needed a serial->USB converter. My new Vista has a miniUSB connector, and comes with a USB-miniUSB cable for data transfer. Needless to say, the data transfer is also a lot faster.

The units come loaded with a basemap of the US, which I found it to be usable but pretty limited. I think if you're going to spring for a GPS, you should also get the maps unless you are only going to use the most basic features of the unit. On my old Vista I used an old version of MapSource Metroguide, and when I bought my new Vista I also bought the MapSource CityNavigator maps. The maps are much better in the CityNavigator product. I don't know if it's a limitation of the maps or the unit, but road routing on the old Vista was pretty crappy. It worked well down to the resolution of a street the size of Storrow Drive, but couldn't get you the last mile to your destination. The routing on the Vista Cx + CityNavigator is great. In head-to-head
competitions with the built-in GPS in Jeremy's Prius, my GPS always wins with the more sensible routings/better time estimates.

I also own some of the Garmin topo maps, some European maps, and some Bluewater charts. All of those work great as well. I don't think you can purchase topo software for the entire US, but the national parks, White Mountains, etc. are covered. Don't take the trail locations in the topos as gospel -- I've been on a few trails where the GPS thought I was ~1/8 mi. off the trail and running parallel to it, and in some cases I found out later that the rangers had moved the trail to limit erosion, although in other cases I think the trail locations on the topos weren't entirely accurate. The city maps are much better in this regard. They are certainly accurate enough to keep you from getting lost, however, especially when combined with waypoints and snail trails. Depending on which country you are in, the maps may be
more or less detailed. Ireland was good; Turkey was poor. I also recommend buying the Garmin maps if you go with a Garmin GPS. Other companies like National Geographic also sell map software, but you can only mark waypoints on the map on your PC and then upload waypoints to the unit, not the map itself. By the way, I think that the Garmin software is only PC-compatible, not Mac or Linux, but that may have changed.

I haven't done much comparison shopping for GPS units. Back when I bought the original Vista, I bought it because of the map software support available from Garmin, and because Garmin has a good reputation. When I bought the Vista Cx, I did it because I had such a positive experience with my original Vista and wanted a better display. I see they also have a Vista HCx out now, but I haven't looked at it.

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#107199 - 09/27/07 07:23 PM Re: Do's and Don't for purchasing a GPS [Re: Katie]
harrkev Offline
Enthusiast

Registered: 09/05/01
Posts: 384
Loc: Colorado Springs, CO
Another option is to go for a Magellan Sportrak series.

I do admit that the Garmin offerings are better. However, given your budget, you might be able to snag a Magellan with mapping software for about that price.

Here are the relevant things to look for in a GPS (each one adds money -- you decide what is most important.):

1) Mapping. Unless you get a good package deal, maps (either road or topo) will add around $100 or so. Some cheaper units CANNOT use a map.

2) Storage for maps. Some older units have a limited amount of memory built-in. Newer ones use removable standard memory (SD or micro-SD). Removable memory is better, since you can add a 1Gb card (enough to hold the entire US) for $20.

3) High-sensitivity chipset. You can get by without it, but it sure helps when you are surrounded by trees or mountains.

4) Color screen. Makes looking at the maps a LOT nicer.

5) High-resolution screen. Less important than color, but still nice.

6) Turn-by-turn routing. Your GPS can tell you when to turn while driving (with the right street mapping sofware). Not really necessary for hunting/hiking, but it sure is nice to have when driving in a strange town.

7) Uses standard batteries (either AA or AAA). Using standard batteries, you can carry spares. With a built-in rechargable, carry a charger and a VERY long extention cord while hiking.

8) Waterproof/tough. If you drop it on a rock or in a river, will it live?

Figure out which ones are the most important, and we can help narrow things down.
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#107201 - 09/27/07 07:36 PM Re: Do's and Don't for purchasing a GPS [Re: xavier01]
Leigh_Ratcliffe Offline
Veteran

Registered: 03/31/06
Posts: 1355
Loc: United Kingdom.
1) Make sure that whatever you buy can display position data in the same format that your map uses.
In the UK that would be BNG. (Ordinance Survey British National Grid)
In the USA it would probably be UTM. (Universal Transverse Mercantor)

2) Make sure that you know how to read the display properly.

3) Remember that they don't work under trees.

4) Remember that they use batteries and batteries FAIL.

5) Learning how to use it is a GOOD IDEA. Trying to learn how to use it when you need it is a VERY BAD IDEA.

6) Keep It Simple Stupid (KISS). Always a good idea. The more complicated they are, the more difficult they are to operate. All you need is a position display. Anything else is a bonus. If your dropped into Darkness and Chaos and it's raining a storm....

7) Always check it against a known map datum. It might have gone kaput or the US Mil might be playing games with the data.

8) Take a waterproof notepad and a pencil. Write that POSITION and the TIME down. If it fails, thats your last known position.

9) Drop it at home and it will still work. Drop it in the field and it will BREAK.





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#107205 - 09/27/07 08:38 PM Re: Do's and Don't for purchasing a GPS [Re: Leigh_Ratcliffe]
KenK Offline
"Be Prepared"
Pooh-Bah

Registered: 06/26/04
Posts: 2208
Loc: NE Wisconsin
Originally Posted By: Leigh_Ratcliffe
Remember that they don't work under trees.


Actually the newer receivers with the high sensitivity chip sets DO work under trees. They are amazing!

Originally Posted By: Leigh_Ratcliffe
All you need is a position display. Anything else is a bonus.


For most people a GPS with on-board maps provides a knowledge of nearby points of interest and the distance to those points. For the James Kim family that kind of information would have been EXTREMELY valuable and potentially lifesaving.

For those who "just want to get back to where they were" or those with a paper map and the skills to transfer coordinates from GPS to map, and vice versa, a basic GPS is enough, but for all others I sincerely recommend having a mapping GPS with appropriate maps loaded.

Of course, the odds are that the paper maps James was carrying didn't have coordinates drawn on it.

Ken

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#107211 - 09/27/07 11:14 PM Re: Do's and Don't for purchasing a GPS [Re: harrkev]
OldBaldGuy Offline
Geezer

Registered: 09/30/01
Posts: 5695
Loc: Former AFB in CA, recouping fr...
One more thing to consider: Do the buttons lock? My Magellan Meridian Color, which I love in every other respect, does not have locking buttons. So if I put it in my pocket, or a soft belt pouch, the buttons ALWAYS get pushed, causing it to do all kinds of wierdo things...
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#107220 - 09/28/07 03:19 AM Re: Do's and Don't for purchasing a GPS [Re: OldBaldGuy]
JohnnyUpton Offline
Journeyman

Registered: 05/03/07
Posts: 60
Loc: USA
If you’re willing to go the used/ebay route the Garmin GPS 12 is all you really need.

More waypoints than you will ever need, tracking function and good battery life.

Spare batteries are a must as is a back up compass.

The issue wrt to being able to receive a signal in cover is a function of cover interfering with signal reception and satellite coverage. If you’ve got good satellite coverage you can get a signal through pretty heavy cover, but if the satellite coverage is poor the ability to receive a signal is impaired.

Just a general comment on using GPS, I’ve always treated the unit as back up even though I’ll use it as a primary means of wayfinding. Knowing which direction the trailhead is mandatory. Batteries dying and lack of coverage follow Murphy’s law.

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#107231 - 09/28/07 01:37 PM Re: Do's and Don't for purchasing a GPS [Re: OldBaldGuy]
OutdoorDad Offline
Journeyman

Registered: 09/27/07
Posts: 76
Check out the iFinder Hunt C Plus (I used mine every time I am near the deep coastal forest)

It has hunting specific icons for creating your waypoints like deer tracks, tree stand, scrapes... It has a huge screen, too.

The plus comes with the Mapping software CDs! It not only has all the Topos in the US - It also has all the hunting units! That is a HUGE bonus when you are near the edge of your unit.


Quote:
The iFinder Hunt C Plus from Lowrance is geared to Hunters due to its rugged, waterproof design, electronic compass and barometric altimeter . The iFinder Hunt has a 2.83” diagonal display with a 320x240 high-resolution color screen with a backlit keypad. It contains a 16 channel WAAS* antenna for exceptional accuracy. The iFinder Hunt C+ has a built-in US basemap showing highway detail and a general coastal outline of waterways, lakes and rivers. The basemap can be further enhanced with the included MapCreate mapping software or an optional detailed marine chart from Navioincs. The Hunt Color has an impressive built-in memory for up to 2000 waypoints and 100 savable plot trails with up to 10,000 points. There are also 42 different icons you can use to help identify your waypoints on screen. The iFinder Hunt is waterproof to IPX7 standards, so it is perfect for outdoor enthusiasts. The map has 40 zoom ranges from 0.02 to 4000 miles and the iFinder Hunt has dedicated zoom in/ zoom out keys. Other features include RealTree Hardwoods HD camo case, Built-in Microphone, audible alarms and backtrack feature to retrace your path. 1 year warranty. * While the regular GPS signal is available worldwide, the WAAS signal is currently available in the US only in open areas or on the water.
source

You can also drop in the NAV chip which may be used to navigate/fish the North American waterways!

I'll admit it... I have used it on the way back from a trip to find the nearest 7-eleven (worked very well too).

So let's see...
  • Mapping software
  • storage of maps
  • works well under tree cover and in valleys
  • color screen
  • doesn't do the turn by turn but does have hunting units on the maps
  • uses standard AA batteries
  • tough and waterproof




Edited by OutdoorDad (09/28/07 01:42 PM)
Edit Reason: added photo
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If people concentrated on the really important things in life... there'd be a shortage of kid's fishing poles.

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#107232 - 09/28/07 01:45 PM Re: Do's and Don't for purchasing a GPS [Re: xavier01]
OldBaldGuy Offline
Geezer

Registered: 09/30/01
Posts: 5695
Loc: Former AFB in CA, recouping fr...
Here, here , and here are a few sites that might help you...
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#107236 - 09/28/07 02:03 PM Re: Do's and Don't for purchasing a GPS [Re: OldBaldGuy]
OutdoorDad Offline
Journeyman

Registered: 09/27/07
Posts: 76
That last one is fun and a great way to get the hang of using your GPS!
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If people concentrated on the really important things in life... there'd be a shortage of kid's fishing poles.

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#107256 - 09/28/07 04:43 PM Re: Do's and Don't for purchasing a GPS [Re: JohnnyUpton]
KenK Offline
"Be Prepared"
Pooh-Bah

Registered: 06/26/04
Posts: 2208
Loc: NE Wisconsin
Originally Posted By: JohnnyUpton
The issue wrt to being able to receive a signal in cover is a function of cover interfering with signal reception and satellite coverage. If you’ve got good satellite coverage you can get a signal through pretty heavy cover, but if the satellite coverage is poor the ability to receive a signal is impaired.


True, but a receiver with a high sensitivity chip can hear and interpret signals much fainter than a regular receiver.

The newer high sensivity GPS's are amazing. Mine can get a 3D lock from the middle of my basement - not always, but quite often.

While at summer camp with my son's troop I was chatting about GPS's with Scoutmaster in the adjacent campsite. He said rather routinely "well of course we wouldn't get a signal under these trees ...". I said "I think my Garmin GPSMAP 60CSx would." So we each got our receivers (he had a nice little Magellan unit with a color screen & maps). Mine got a real strong 3D lock fairly quickly. His did get a 3D lock after a while, but it would fade in and out of 3D as he turned around and walked a bit.

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#107288 - 09/28/07 11:38 PM Re: Do's and Don't for purchasing a GPS [Re: KenK]
sodak Offline
Addict

Registered: 03/20/05
Posts: 410
I have an older Garmin ETrex that I'm not at all impressed with. It loses it's fix in pine forests, or if you are holding it so that it's sideways, it loses it's fix. There's no good way to hold it while you're hiking.

If the newer ones have better sensitivity, that's good. I would not buy another unless it had a better receiver. And I would still bring along a map and compass, and the skills to use them.

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#107290 - 09/29/07 12:02 AM Re: Do's and Don't for purchasing a GPS [Re: sodak]
KenK Offline
"Be Prepared"
Pooh-Bah

Registered: 06/26/04
Posts: 2208
Loc: NE Wisconsin
Originally Posted By: sodak
I would still bring along a map and compass, and the skills to use them.


Good point. No self-respecting GPS thread should go without a warning that GPS's don't take the place of a good map of your area, a decent compass, and the skills to use them together.

I see a GPS, map, and compass as a set that belong together. People wandering about outdoors should know how to use a GPS with a paper map and a compass.

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#107299 - 09/29/07 02:50 AM Re: Do's and Don't for purchasing a GPS [Re: KenK]
JohnnyUpton Offline
Journeyman

Registered: 05/03/07
Posts: 60
Loc: USA
Originally Posted By: KenK

True, but a receiver with a high sensitivity chip can hear and interpret signals much fainter than a regular receiver.

The newer high sensivity GPS's are amazing. Mine can get a 3D lock from the middle of my basement - not always, but quite often.

While at summer camp with my son's troop I was chatting about GPS's with Scoutmaster in the adjacent campsite. He said rather routinely "well of course we wouldn't get a signal under these trees ...". I said "I think my Garmin GPSMAP 60CSx would." So we each got our receivers (he had a nice little Magellan unit with a color screen & maps). Mine got a real strong 3D lock fairly quickly. His did get a 3D lock after a while, but it would fade in and out of 3D as he turned around and walked a bit.


I don't doubt that for a second, but IME if I couldn't pick up a signal and get "locked" in with my 12, the guys I'm with (Mostly have H20's) couldn't pick up a signal either.


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#107300 - 09/29/07 02:52 AM Re: Do's and Don't for purchasing a GPS [Re: OutdoorDad]
JohnnyUpton Offline
Journeyman

Registered: 05/03/07
Posts: 60
Loc: USA
Originally Posted By: OutdoorDad




That looks like the H2O with a camo face plate.

Personal preference but I'm not a fan of color screens. Glare makes them very hard to read Vs regular LCD.


Edited by JohnnyUpton (09/29/07 02:53 AM)

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#107312 - 09/29/07 08:48 AM Re: Do's and Don't for purchasing a GPS [Re: KenK]
Leigh_Ratcliffe Offline
Veteran

Registered: 03/31/06
Posts: 1355
Loc: United Kingdom.
The newer ones may indeed work under trees but I would not personally rely on it. Its good practice to get out into the open for a position fix.

The inherent weakness of a GPS with on board maps is that if you loose the unit, you loose your maps.

Don't get me wrong here, I think that built in maps are an excellent idea. But committing yourself to only one source of information strikes me as an excellent way of getting yourself Darwined..

Belt and suspenders time.
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#107317 - 09/29/07 12:56 PM Re: Do's and Don't for purchasing a GPS [Re: Leigh_Ratcliffe]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
That's how I felt when I bought my small non-mapping Garmin Geko 301. I bought it to be used with a map and compass as primary. When I get to any given area, I mark specific locations that are easily found on the map, road intersections, whatever. Mark where my car's parked. During the hike mark points along the way -- waypoints are free. When I don't need to actively use the Geko, I turn it off and conserve the batteries -- map and compass time. It works quite well.

That said, I'm seriously looking at the Garmin 60Cx as an upgrade, they've improved immensely. The SiRF technology allowing it to take a fix under a canopy is a major plus. Still taking map and compass though, always the compass.
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Better is the Enemy of Good Enough.
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#107353 - 09/29/07 09:40 PM Re: Do's and Don't for purchasing a GPS [Re: Leigh_Ratcliffe]
OldBaldGuy Offline
Geezer

Registered: 09/30/01
Posts: 5695
Loc: Former AFB in CA, recouping fr...
"committing yourself to only one source of information strikes me as an excellent way of getting yourself Darwined..."

Great point. That is why we always buy some type of paper map wherever we are hanging out for more than a few days...
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#107517 - 10/02/07 02:59 AM Re: Do's and Don't for purchasing a GPS [Re: JohnnyUpton]
OutdoorDad Offline
Journeyman

Registered: 09/27/07
Posts: 76
Originally Posted By: JohnnyUpton
...That looks like the H2O with a camo face plate.

Personal preference but I'm not a fan of color screens. Glare makes them very hard to read Vs regular LCD.


It is pretty much the same as the h2o but with an emphasis on hunting. It has hunting specific icons and the mapping software has all the hunting units in the US.

The barometer is handy with the weather warnings.

I have tried the grayscale screen and loved it. But the areas I am in have a lot of water that enters the picture so being able to it as blue vs another shade of gray which may be BLM land or etc...


Edited by OutdoorDad (10/02/07 03:01 AM)
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If people concentrated on the really important things in life... there'd be a shortage of kid's fishing poles.

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#107527 - 10/02/07 07:19 AM Re: Do's and Don't for purchasing a GPS [Re: OutdoorDad]
James_Van_Artsdalen Offline
Addict

Registered: 09/13/07
Posts: 449
Loc: Texas
Here's a thought if battery life is a concern - a few days before departure put the GPS outdoors where it can receive a signal and turn it on for half an hour or so. Then replace the batteries with the set you intend to use in the field. The idea behind this is to do the slow "cold start" of the GPS ahead of time, on other batteries, so that any field use has a quicker start time since it's a warm start.

(this doesn't apply to some new units that I'm told never need to do a cold start, just the 30 second maximum warm start)


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#107537 - 10/02/07 01:21 PM Re: Do's and Don't for purchasing a GPS [Re: James_Van_Artsdalen]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
Why a couple days before? Do that at your car/truck and allow the GPS to get a good position of your vehicle. Then replace the batteries with a fresh set and keep a spare set in your pack.
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Better is the Enemy of Good Enough.
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#107540 - 10/02/07 02:11 PM Re: Do's and Don't for purchasing a GPS [Re: Russ]
OutdoorDad Offline
Journeyman

Registered: 09/27/07
Posts: 76
I usually leave it plugged into the vehicle's power port (cig lighter) until I arrive and carry a couple spare sets of batteries JIC.

_________________________
If people concentrated on the really important things in life... there'd be a shortage of kid's fishing poles.

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#107547 - 10/02/07 03:31 PM Re: Do's and Don't for purchasing a GPS [Re: OutdoorDad]
James_Van_Artsdalen Offline
Addict

Registered: 09/13/07
Posts: 449
Loc: Texas
GPS "almanac" data is valid for a couple of months. So if a GPS receiver is left on long enough to receive an almanac a few days before a trip then it won't be necessary to wait for it in the field. No big deal, but it a ~ 15 minute delay that can be avoided via pre-trip preparations.

A GPS is that is used from time to time probably already had a valid almanac. I was thinking of the more-casual case of one that hasn't been used since the last vacation.

All GPS receivers collect and store almanac and ephemeris data whenever they're on. You don't have to do anything other than leave the receiver on longer enough.

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#107552 - 10/02/07 05:21 PM Re: Do's and Don't for purchasing a GPS [Re: James_Van_Artsdalen]
OutdoorDad Offline
Journeyman

Registered: 09/27/07
Posts: 76
~15 minutes?!?!?!?

Are you kidding? I don't think it ever took that long. The longest it has taken was maybe 1 minute to boot up and 2-5 minutes to acquire its location. That is with any of the three brands I have used.
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If people concentrated on the really important things in life... there'd be a shortage of kid's fishing poles.

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#107557 - 10/02/07 06:07 PM Re: Do's and Don't for purchasing a GPS [Re: OutdoorDad]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
I agree, from a cold start -- 5 min max.
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Better is the Enemy of Good Enough.
Okay, what’s your point??

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#107566 - 10/02/07 07:00 PM Re: Do's and Don't for purchasing a GPS [Re: Russ]
Frank2135 Offline
Enthusiast

Registered: 04/26/07
Posts: 266
Loc: Ohio, USA
Hmmm...when I first fire up the boat GPS in the spring, after winter storage, it does take a few more minutes to give me a position than later in the season when I'm using it weekly.

Almanac downloading?
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#107567 - 10/02/07 07:11 PM Re: Do's and Don't for purchasing a GPS [Re: OutdoorDad]
Am_Fear_Liath_Mor Offline
Carpal Tunnel

Registered: 08/03/07
Posts: 3078
Hi OutdoorDad,

The GPS almanac takes around 12-13 minutes to download no matter the GPS reciever chipset. The almanac data is valid for 2-3 months so if your GPS has been powered down for more than 2-3 months it will take at least 13 minutes to get a fix.





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#107574 - 10/02/07 07:55 PM Re: Do's and Don't for purchasing a GPS [Re: OutdoorDad]
Brangdon Offline
Veteran

Registered: 12/12/04
Posts: 1204
Loc: Nottingham, UK
Originally Posted By: OutdoorDad
The longest it has taken was maybe 1 minute to boot up and 2-5 minutes to acquire its location.
That's how long it took to find the satellites and download the ephemeris for each one. The ephemeris is the data it needs to actually get a location fix. It's a very precise description of the satellites movements and so expires after a few hours. Once you've got one, switching off and on gives a "warm start" which takes a second or so.

The almanac is different to the ephemeris. It's a less precise description and so doesn't expire for after several months. It takes longer to download because it covers all of the satellites, rather than just the one broadcasting it, and it is broadcast more slowly. However, you don't need the almanac to get a fix; you need the ephemeris for that. The purpose of the almanac is to give the unit a good initial guess for where the satellites are, so it can find them and download the ephemeris quicker.

If the unit has been switched off for a long time, then the almanac will be out of date and it will take longer to get an initial fix. Once you have that fix, you have the ephemeris so you can switch off and on again for a "warm start". But you won't have the almanac for another 15 minutes or so. If you switch off for about 6 hours and then switch on again, it will again be slow to get the initial fix. If you leave it on for half an hour, then switch it off for 6 hours, you should get a quicker initial fix using almanac data even though the ephemeris is expired.

Getting a fix when you have an almanac already downloaded should take under a minute. If you don't have the almanac it's partly a matter of luck whether the unit looks for the satellite in the right places, and that's when it takes several minutes. However, it doesn't necessarily take 15 minutes to get the first fix because you can get a fix without the almanac, if the unit gets lucky.

I hope that makes sense.


Edited by Brangdon (10/02/07 08:02 PM)
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#107594 - 10/03/07 12:35 AM Re: Do's and Don't for purchasing a GPS [Re: Brangdon]
James_Van_Artsdalen Offline
Addict

Registered: 09/13/07
Posts: 449
Loc: Texas
RAS: the almanac is a description of the satellites in the GPS constellation and has nothing to do with your location. If you're going to hike to the South Pole it would make no difference to let it download the almanac from your back yard.

Outdoordad: about 15 minutes to download an entire almanac, technically 30 seconds per satellite so it changes a little from time to time. This doesn't mean you're going to have to wait that long, but rather that you *might*. If the satellites you need info about are at the front of list it may only take a couple of minutes.

If your GPS is turned on when plugged into the car's cigarette lighter then it's downloading an almanac already. In your case the stored Almanac may never get stale and you may never see that long cold start delay – the delay you do see might be only a warm start, doing the 30-second-max ephemeris downloads for each visible satellite. I was thinking of a handheld GPS used only on vacations that might be off all winter, long enough to have the almanac go stale.

Frank2135: yes, that sounds like it's waiting on a fresh almanac. If it's used weekly all spring it shouldn't have that long wait any more since it's always refreshing the almanac when it's on, and it takes a couple of months for the last download to become stale.

Brangdon: yeah, but I wasn't that brave with the details. grin I think the almanac is the only place to get drift and error rates, not to mention poisoning info, so an ephemeris-only fix may shift a little once that satellite's current almanac entry is available.

I didn't mean to blow this out of proportion. I was just thinking, once I get in the country on vacation why wait on almanac when turning it on ahead of time is painless? And if you want to avoid unnecessary battery consumption in the field, then do the almanac download ahead of time, before putting in fresh batteries. No big deal, it's going to work either way.

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#107599 - 10/03/07 01:03 AM Re: Do's and Don't for purchasing a GPS [Re: James_Van_Artsdalen]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
Originally Posted By: James_Van_Artsdalen
RAS: the almanac is a description of the satellites in the GPS constellation and has nothing to do with your location. . . .
I don't understand why this is important. Even a cold start when the GPS hasn't been used in a long time never takes more than a couple minutes to get a fix. It boots up, satellites start appearing in their status window and next thing I've got a <20' fix.

I've never taken out a stopwatch and timed it, but next time I will. Turn it on from a cold start, let it get to <20' and then give it enough time to download the almanac. From what you're saying the next time I boot it up I should get to that <20' fix quicker (minutes/seconds?). We'll see.
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#107612 - 10/03/07 02:46 AM Re: Do's and Don't for purchasing a GPS [Re: Russ]
James_Van_Artsdalen Offline
Addict

Registered: 09/13/07
Posts: 449
Loc: Texas
I have been told, but don't know myself, that some newer GPS receivers will give a fix using only ephemeris data if the almanac is stale. That will seem like a warm start.

The problem is that once a current almanac is available the fix may mysteriously shift once current drift and error are available a few minutes later, and the fix may shift a lot if it turns out that satellite was in testing or broken and not really part of the constellation at all.

What make & model GPS are you discussing? The manual may be online and indicate how it handles cold starts. Remember that it needs to be left off for *months* to force a cold start.

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#107617 - 10/03/07 04:35 AM Re: Do's and Don't for purchasing a GPS [Re: James_Van_Artsdalen]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
I'm using a Garmin Geko 301 for this exercise. My older Garmin GPS V is powered from the car, it's turned on stays on so cold starts are really irrelevant.
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