GPS Accuracy - how much is actually needed - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Gear & Maintenance
 Not a Member? 


Like Tree7Likes
Reply
 
LinkBack Thread Tools
  #1  
Old 03-04-2014
Senior Member
 
Join Date: Sep 2011
Location: Massachusetts
Posts: 132
Thanks: 1
Thanked 0 Times in 0 Posts
Rep Power: 3
mm2187 is on a distinguished road
GPS Accuracy - how much is actually needed

How accurate is your GPS. I have a new Raymarine A67 chart plotter which I love. When mounted on deck without an external gps sensor has an accuracy of about 4-10 feet. When mounted below deck it has an accuracy between 8-24 feet without an external gps. I plan to mount it below deck without a external gps sensor. With an accuracy of +/- 24 feet on a 40 foot is it worth the additional money to get another few feet of accuracy?
__________________
Schock - New York 36
Portsmouth, RI
Reply With Quote Share with Facebook
  #2  
Old 03-04-2014
Senior Member
 
Join Date: Dec 2012
Posts: 834
Thanks: 2
Thanked 21 Times in 17 Posts
Rep Power: 2
benesailor is on a distinguished road
Re: GPS Accuracy - how much is actually needed

Yes, mount an external sensor.

Here's the deal. I'd never navigate per the charts. So accuracy is not that big of a deal. The charts don't match reallty.

But; once you establish way points to all the places you go. It pays to hit them accurately. 40' could be HUGE. Could be BIG rock or open channel. That's why you want accuracy.
Reply With Quote Share with Facebook
  #3  
Old 03-04-2014
Alex W's Avatar
always busy with sailing
 
Join Date: Oct 2012
Location: Seattle, WA
Posts: 2,071
Thanks: 2
Thanked 121 Times in 116 Posts
Rep Power: 2
Alex W is on a distinguished road
Re: GPS Accuracy - how much is actually needed

Why do you plan on mounting it below decks? It is well sealed and designed for outdoor mounting.

For about the same price as the external NMEA 2000 GPS sensor you could buy a tablet that would give you a remote display from the A67 that is usable below decks. Any Android or iOS tablet will do it.
__________________
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior
Seattle, WA
Reply With Quote Share with Facebook
  #4  
Old 03-04-2014
Senior Member
 
Join Date: Dec 2012
Posts: 834
Thanks: 2
Thanked 21 Times in 17 Posts
Rep Power: 2
benesailor is on a distinguished road
Re: GPS Accuracy - how much is actually needed

Yea; why do you want to mount it below deck? It won't do you much good down there. Are you talking about putting it on a swing out bracket in the campanion way?
Reply With Quote Share with Facebook
  #5  
Old 03-04-2014
RichH's Avatar
Senior Member
 
Join Date: Jul 2000
Location: Pennsylvania
Posts: 2,742
Thanks: 9
Thanked 63 Times in 56 Posts
Rep Power: 15
RichH will become famous soon enough
Re: GPS Accuracy - how much is actually needed

NO,
GPS (chartplotter) 'accuracy' is totally dependent on 'resolution accuracy' of the original charting from which it is based. Problem is that for most areas that do NOT have large scale commercial marine traffic, the base chart 'may' still be based on 1927 or earlier surveys. Plus and with respect to 'resolution errors', using GPS charting based on 'old' lead-line surveys may also induce EXTREME errors especially when the GPS chartplotter MAGNIFICATION greatly exceeds the 'magnification' of the original hand surveyed charting.

FWIW .... most USA marine charts are only upgraded if and only if there is large commercial boats constantly using the area of the chart.

OEM Charting error trumps GPS accuracy; 'over-magnification' by chartplotters MAGNIFIES the original errors.

;-)
jackdale and TakeFive like this.
Reply With Quote Share with Facebook
  #6  
Old 03-04-2014
paulk's Avatar
Senior Member
 
Join Date: Jun 2000
Location: CT/ Long Island Sound
Posts: 2,533
Thanks: 4
Thanked 20 Times in 19 Posts
Rep Power: 15
paulk is on a distinguished road
Re: GPS Accuracy - how much is actually needed

"Repeatability" of GPS postitions is not as good as Loran was. You may get the same reading from your GPS unit the next time you get near your favorite fishing spot (or wherever) but you may not be exactly in the same place. There may be several more (or many fewer) satellites over the horizon providing data, or the tide may be at a different level, or the military may have degraded the signal for some reason they didn't tell you. Tie up in a slip and watch how your GPS position changes because of these (and other?) variables. Mix that in with charts that may have last been sounded with a lead and line in 1840, and it becomes obvious that GPS is nice, but not necessarily definitive. I consider it pretty accurate, and then I look around and watch my depthsounder.
Reply With Quote Share with Facebook
  #7  
Old 03-04-2014
WoobaGooba's Avatar
Member
 
Join Date: Oct 2012
Posts: 83
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 2
WoobaGooba is on a distinguished road
Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by paulk View Post
"Repeatability" of GPS postitions is not as good as Loran was. You may get the same reading from your GPS unit the next time you get near your favorite fishing spot (or wherever) but you may not be exactly in the same place.
That was indeed the case in the early days of consumer GPS and Selective Availability, but I believe today that WAAS addresses the repeatability issue.

Also, more accuracy in heading and speed readings is valuable to relatively slow moving sailboats.
Reply With Quote Share with Facebook
  #8  
Old 03-04-2014
Senior Member
 
Join Date: Dec 2010
Posts: 3,397
Thanks: 0
Thanked 113 Times in 101 Posts
Rep Power: 4
JonEisberg will become famous soon enough
Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by RichH View Post
NO,
GPS (chartplotter) 'accuracy' is totally dependent on 'resolution accuracy' of the original charting from which it is based. Problem is that for most areas that do NOT have large scale commercial marine traffic, the base chart 'may' still be based on 1927 or earlier surveys. Plus and with respect to 'resolution errors', using GPS charting based on 'old' lead-line surveys may also induce EXTREME errors especially when the GPS chartplotter MAGNIFICATION greatly exceeds the 'magnification' of the original hand surveyed charting.

FWIW .... most USA marine charts are only upgraded if and only if there is large commercial boats constantly using the area of the chart.

OEM Charting error trumps GPS accuracy; 'over-magnification' by chartplotters MAGNIFIES the original errors.

;-)
Exactly...

How accurate are our charts? - Ocean Navigator - January/February 2003

Quote:
Originally Posted by mm2187 View Post
With an accuracy of +/- 24 feet on a 40 foot is it worth the additional money to get another few feet of accuracy?
Such accuracy within a dimension smaller than the size of your boat is really only important if you care which PORTION of your boat is at a particular geographical position on the earth's surface... in other words, with such precision on a 40-footer, you really have to decide: Do i want to know where my KEEL is, or the NAV STATION, or the BOW, or the HELM ? And then, position the antenna accordingly...

When you think about it, such accuracy really is more valuable for your 8' tender, than for the 40' mother ship... Not to mention, far less important on a $80 million 200' megayacht...

Most of which will have multiple GPS receivers both fore and aft, anyway, and at other critical locations like helo pads, and so on... :-)

As for the 'repeatable accuracy' of GPS for waypoints, LORAN was probably superior in that regard... And, if I need to rely on GPS accuracy within a few feet to keep me from hitting a submerged rock, I'm putting WAY too much faith in my gizmos, and it's probably time for me to think about taking up another hobby... :-)
tdw likes this.

Last edited by JonEisberg; 03-04-2014 at 09:34 PM.
Reply With Quote Share with Facebook
  #9  
Old 03-04-2014
Senior Member
 
Join Date: Jan 2011
Posts: 666
Thanks: 0
Thanked 17 Times in 17 Posts
Rep Power: 4
Frogwatch is on a distinguished road
Re: GPS Accuracy - how much is actually needed

Good God! If you need your GPS to be more accurate than 1000' you are doing things wrong. If you are in a situation needing better than that you better be using sights on objects using a hand compass cuz GPS can be seriously wrong at times. If you cant take sights then you shouldnt be there or should be dead stopped until visibility gets better. A prudent navigator relies on what he sees, not something a black box tells him.
For those who say charts are wrong, well, even with gps they would be wrong and you are still screwed.
OK, fishermen need gps to get coords of good spots and I use mine to give me good speed measurements and a position measurement when out of sight of land but I tend to rely more on my dead reckoning first and if they don't agree, somethings wrong.
In 27 yrs of extensive sailing, I've never needed "waypoints" and still dont get why people do. Instead, for example exiting Sea of Abaco near Little Harbor, I get to where the "white house" lines up at X degrees and follow the reciprocal course until my GPS Lon. reads zz.zzz to indicate that I am well clear of all the hazards. Then I can head on a course of 170 to Spanish Wells. Waypoints? Why?
Even going from Marsh Harbor down to Little Harbor where there are numerous turns, reefs and other hazards, I better be using my compass to sight on known points cuz that also alerts you to stuff that isn't on the chart. Why would waypoints help?
Reply With Quote Share with Facebook
  #10  
Old 03-04-2014
Senior Member
 
Join Date: Dec 2010
Posts: 3,397
Thanks: 0
Thanked 113 Times in 101 Posts
Rep Power: 4
JonEisberg will become famous soon enough
Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by Frogwatch View Post

If you cant take sights then you shouldnt be there or should be dead stopped until visibility gets better. A prudent navigator relies on what he sees, not something a black box tells him.
You rarely sail in places where fog can occur, it would appear...

Quote:
Originally Posted by Frogwatch View Post
In 27 yrs of extensive sailing, I've never needed "waypoints" and still dont get why people do. Instead, for example exiting Sea of Abaco near Little Harbor, I get to where the "white house" lines up at X degrees and follow the reciprocal course until my GPS Lon. reads zz.zzz to indicate that I am well clear of all the hazards. Then I can head on a course of 170 to Spanish Wells. Waypoints? Why?
Well, waypoints can be be pretty damn handy, at times... Not everywhere has conveniently placed white houses, after all... :-)

To each his own...
fryewe and benesailor like this.
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
iPad GPS accuracy? groundhog Gear & Maintenance 23 09-01-2013 03:26 PM
GPS Accuracy and Price Mark Sloan Gear and Maintenance Articles 0 10-05-2000 08:00 PM
GPS Gets More Accuracy Jim Sexton Seamanship Articles 0 05-21-2000 08:00 PM
GPS Gets More Accuracy Jim Sexton Gear and Maintenance Articles 0 05-21-2000 08:00 PM
GPS Gets More Accuracy Jim Sexton Cruising Articles 0 05-21-2000 08:00 PM


All times are GMT -4. The time now is 02:26 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012