GPS accuracy?

[gcapehart]

We Have a 82 25XF with a 225 Johnson 2 stroke. The engine is an 89 w/ 750 hours. It runs great. Our dash speedometer and old sonar fishfinder record speed slightly slower than our Garmin Map498.  As a plotter when the GPS shows us next to a can we can literaaly touch it if we wanted. Is the speed that accurate?    Gary

[John Jones]

It's more accurate than any other device.  Pretty much accepted as a standard.

[RickK]

With WAAS enabled on the GPS, I think you'll get within a 100ft.  Speed is all calcs since most GPS receive signals every second.  Pretty accurate though, like John said.

[GoneFission]

GPS speed is usually dead-on, and accuracy actually improves the faster you go.  GPS speed is most questionable below 5 MPH, but that is due to your receiver moving in 3 dimensions when the speed computer is trying to translate into 2 dimensions.  Waves and up/down movement affect speed accuracy at slow speeds.  

Most GPS receivers have a "system" or "test" mode that allows you to see what the current accuracy of the unit is in feet.  Under good conditions, you may see the accuracy at 20 feet or even less.  Under bad conditions, hte unit is not as well connected to as many satellites, and you may find accuracy of 100-200 feet.  Still - that's a big leap over LORAN, and a quantum change over dead reackoning!  

Yes, I remember the old days with a sextant...  I could usually get and accuracy at least good enough so I knew what ocean I was in...  

See ya on the water!

[gcapehart]

Cap'n,
     You really ought to write. Being a neophyte power boater, I search, ask boaters, seek text on the subject, and you express all so well. We, "newbies" like I sure could use a pocket/waterproof reference with your insights. I don't believe in the series "...................... for Dummies" that they have a title, Powerboating for Dummies. Better yet do it yourself and gain the rewards.    Gary

[John Jones]

I don't think there is a Dummies book but there is a Powerboating 101  

Not to take anything away from Capt. John because he is full of good info but try to find a copy of Chapman's Guide to Piloting.  There is almost anything you ever wanted to know somewhere in that book.  New copies are expensive but you can often find them at used book stores.

http://www.amazon.com/Chapmans-Guide-Pi ... 81&sr=8-15

[warthog5]

With WAAS enabled on the GPS, I think you'll get within a 100ft.  Speed is all calcs since most GPS receive signals every second.  Pretty accurate though, like John said.

I was getting 6ft accuracy the other day. It's usually at least 13ft.

[John Jones]

Yep, I seldom see over 12-15 feet with WAAS, many times in the single digits.  I still don't understand why they come with WAAS disabled as factory default.

[RickK]

With WAAS enabled on the GPS, I think you'll get within a 100ft.  Speed is all calcs since most GPS receive signals every second.  Pretty accurate though, like John said.

I was getting 6ft accuracy the other day. It's usually at least 13ft.

I always get some killer accuracy with WAAS enabled too but the specs always say within 100 ft (so I did too ).  We, closer to the equator, have it nice with strong signals (GPS satellite signals are very weak anyway). As you get farther north you start having problems with locking onto satellites because the signal is weaker, thus when we get 6 ft accuracy, some of our northern buddies have problems getting 100ft.

[gcapehart]

We have last year's Garmin 498 GPS. We are certainly north of you. When our GPS shows a can,etc; it is never next to the boat literally until we can touch it. We've done this ledges, beacons, etc;. It really seems that ours is within 10 feet if not better. Our 25XF as the cursor on the screen shows it(the can) at our bow, midships, stern and then gone. It seems uncanny. Maybe since we haven't been in the water yet this year I am exagerating(?). But it did knock our socks off. We feel very confident with it. Can we be so dependent? We do move slowly with poor visibility. We also have the old sonar for depth.    Gary

[warthog5]

While we are on the subject of Garmin 498.

First I have one. I use it on my Scout skiff for inshore fishing. It's OK for that. I also have a mount on my Crows nest on the Mako for it and just use it for navigation. No FF use.

I have a Garmin 2006C and Furuno 582 at the lower helm.
I also keep a Garmin 76Cx handheld as a backup.

The 498 is NOT a good unit for stand alone bottom dropping.

The screen is to small and WAY to much menu jumping.

A better setup would be a Garmin 492 and 620 Furuno to keep the cost down and still have a system that would not need upgrading in the near future.

[RickK]

I carry a Garmin iQue 3600 (PDA type, shore-based GPS) with me on business trips (best tool I've ever bought) and have never had weak signals pose a problem anywhere I've gone, be it North or South.  There are plenty of GPS forums out there though that will tell you about the signal strength problems depending on how far North you are. I, again, have seen weaker signals up North but never had a problem getting a 3D lock.  Don't remember how close the accuracy was though.  Too many trips   I carry an external antenna with me in anticipation of these problems, but have never used it. Maybe I've got an extraordinary internal antenna in it?
For my boats I have one Garmin 188C Chartplotter/FF combo that I put in whichever boat I'm going out in. I have mounting stations and transducer/GPS antennas on each boat.  I am by no means, an expert on all the features of it because I haven't really tried to use them yet or don't use them enough to be proficient. I get great accuracy though.

[GoneFission]

Amen on the Chapman's Piloting and Small Boat Handling - probably the best overall book for either a newbie or an experienced boater!

[JimCt]

GPS position accuracy is phenomenal but after a certain point worrying about better than 10 foot accuracy is a bit like fretting over your watch which looses 3 seconds a year.

[John Jones]

We have last year's Garmin 498 GPS. We are certainly north of you. When our GPS shows a can,etc; it is never next to the boat literally until we can touch it. We've done this ledges, beacons, etc;. It really seems that ours is within 10 feet if not better. Our 25XF as the cursor on the screen shows it(the can) at our bow, midships, stern and then gone. It seems uncanny. Maybe since we haven't been in the water yet this year I am exagerating(?). But it did knock our socks off. We feel very confident with it. Can we be so dependent? We do move slowly with poor visibility. We also have the old sonar for depth.    Gary

Just remember that navaids do get moved.  One of our favorite cans for getting bait just offshore was moved about 1/4 mile by hurricane Ivan and has never been put back to agree with the charts.  I have not checked the light list to see if they updated the position but that sucker will surprise you at night when you come up on it 1/4 mile before it is supposed to.  Even if you purchase updated for the electronic charts there is no assurance that they are totally accurate.