Re: GPS devices

12 October 2004, 05:03

CDH

Re: GPS devices

Quote:

At this level I think one of the garmins is the best bet. The basic E trax has some good reviews but I don't know if it is WAAS enabled...WAAS allows the GPS to be even more accurate from say typically an average of 50m to maybe 3m or 4m..

When I turn off WAAS on my Garmin GPSMAP 76, it gives me a noticable increase in battery life (3-4 more hours). When WAAS is tracking (rare...only 2 WAAS satelites in orbit thus far), my 'location accuracy' as estimated by the unit averages 8-15 ft, without WAAS enabled it averages 12-20 ft. Whoopee!!! Of course I use it to navigate a boat at night in shallow coastal waters, but WAAS still isn't all that useful. ANY GPS can get you within rock throwing distance of a point, and that is good enough for 99.9% of uses.

I Do HIGHLY recommend Garmin units...between myself and a couple of fishing buddies we have owned numerous Garmins over the last 7-8 years, and nary a bad one in the lot. My GPSMAP 76 gives me at least 3 fishing trips on a set of 2 AA batteries, at least 16 hours....

If you are computer savvy, get their PC mapping software too. There are lots of varities (topo, road and highway, waterway, etc.) so pick carefully, that costs as much or more than the unit. Make sure you get a unit that can use the maps too, if the maps interest you. Not all units use the maps to full capacity.

12 October 2004, 05:12

steve y

I guess I should stay more current on my catalog reading. When I got mine there were none in blue. Now I see there are several....

The WAAS may be of zero value to some but if it offers any advantage in accuracy it's a bargain for flagging overgrown property lines from known pins and finding small pockets in the flats onshore.

12 October 2004, 12:15

CDH

Quote:

A general point about accuracy though is that the Estimated Error shown on most GPS units is just that, and may or may not be truely accurate in itself. Still for years I managed with 6 figure grid references off 1:50,000 maps and most GPS
will beat that by a long chalk except in areas of very poor coverage.

Agreed. Like I said, except in very specific cases getting to within stone's throw accuracy is good enough. If you can't find your base camp or truck, or locate yourself on a map when you are within 20 ft, you have other problems the GPS will not help with...

As for 'estimated accuracy', I have in my (too long at this point) to-do list plans for a phone call to Garmin. I want to know how their unit can give me an estimated accuracy when I am receiving a DGPS beacon of 8 ft and the same unit be showing an elevation of -50 ft!?!? I am in a boat, floating at sea level, BTW! For now I assume that their estimate is 2D only....

Oh the fun of technology!

12 October 2004, 07:39

Pete E

CDH,

I can't comment on WAAS too much as I have never bothered with it as living in the UK we don't get the satilite coverage needed.

A general point about accuracy though is that the Estimated Error shown on most GPS units is just that, and may or may not be truely accurate in itself. Still for years I managed with 6 figure grid references off 1:50,000 maps and most GPS
will beat that by a long chalk except in areas of very poor coverage.

The European Space Agency is putting up Sat's for its own version of GPS complete with WAAS and the good news is that it is fully compatable with the American system. The Russians also have a version,( Incompatable with the West?) but whether it still works and how well it works, I don't know...

But generally the good news is that GPS is only going to get better!

Megelan Units are already using data from the embryonic European system even though officially it is still undergoing commissioning, but Garmin units do not..This won't matter to most Americans but it might to say our European AR members especially those who do any boat fishing.

regards,

Pete

13 October 2004, 08:35

Rusty

I have both the Garmin Etrex Legend and the Magellan SportTrak Map.

The Garmin will accept their MapSource software to upload trip maps. WAAS will give you more accuracy.

The SportTrak I think is a bit faster in aquiring the satellites, however I agree, the Garmin more flexable and user friendly. I like both for differant uses.

13 October 2004, 08:22

jackfish

Magellan eXplorist 100 $90

13 October 2004, 10:24

Pete E

CDH,

I think you will find that the Estimation of Accuracy is simply a calcualtion (guesstimate?) done in the GPS. It factors in such things as the number and location of satilites its tracking plus any errors it can detect. As I understand it, using this information it then "predicts" how accurate its is likely to be. You can get a better idea of true accuracy if you leave your GPS switched on in one spot and use the "Average Function"...Make a note of the reading when you first do this and then see how much it varies over say the next hour or two.

regards,

Pete

01 November 2004, 19:30

WAGNER95696

Does anyone have a GPS with an altimeter function? Do all GPS altimeters require knowing the local barometric pressure or do some of them calculate altitude electronically?

01 November 2004, 20:57

AnotherAZWriter

A GPS calculates the altitude based on multiple readings from satellites.

Anyone out there know exactly how a GPS works? Each of the sats has an atomic clock, all of them synchronized. They simply send out the time and an identification number (for the satellite). The time of flight for each sat is the speed of light, so your GPS picks up very slightly different times from each satellite. Knowing the satellite ID, it then calculates your position. I think it takes four to calculate altitude.

01 November 2004, 21:17

jpb

Quote:

Does anyone have a GPS with an altimeter function? Do all GPS altimeters require knowing the local barometric pressure or do some of them calculate altitude electronically?

Garmin makes a couple of models with both GPS altitude and a barometric altimeter to achieve greater accuracy and resolution (has to calibrated at some point though).

Check out www.garmin.com and then look for e-trex Vista C. Supposed to resolve 10m in altitude if I recall correctly.

jpb