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-   -   Navigating with GPS and other means? (http://www.sailnet.com/forums/general-discussion-sailing-related/71326-navigating-gps-other-means.html)

Boasun 01-14-2011 11:13 AM

Navigating with GPS and other means?
 
The Nov/Dec Good Old Boat magazine had a good article on the GPS and its foibles.
Now the question is; How many of you rely 100% on the GPS?
How many of you use the GPS as a tool and use a secondary means of Navigation. (Coast Piloting/celestial)

Note: I've had GPS showing me to be on the beach by several hundred yards, when all other means, including the Fathometer, showed me to be in the center of the bayou. Guess which means that I believed?:rolleyes:

justified 01-14-2011 11:49 AM

As reliable as GPS can be, and I do use it as a primary tool when sailing in my home waters, I always have my paper charts out as cross reference and verification. When in areas new to me I will primarily rely on a paper charts and the GPS as back up. There is nothing like your eyes, ears and a good paper chart.
Peter
s/v Frayed

SailingIsis 01-14-2011 11:52 AM

I use both GPS and good ole coastal navigation with a full size paper chart spread out below and a plastified chart book and hand bearing compass in the cockpit. I'm desperately missing (and planning to install this winter) a working depth sounder.

Here in the San Francisco Bay I'd think (hope) that everyone is intimately familiar with the tide tables and current charts as the ebb current frequently exceed 6kts around the Golden Gate.

(I've taken a celestial navigation course, but I'm virtually never in a situation where I can practice the art).

RhodesSwiftsure 01-14-2011 12:00 PM

Quote:

Originally Posted by SailingIsis (Post 686839)
(I've taken a celestial navigation course, but I'm virtually never in a situation where I can practice the art).

These are great for practicing:

Artificial Horizon by Davis

Just remember to half the angle and "skip the dip" correction.

jackdale 01-14-2011 12:29 PM

Offshore I do use GPS. I set the destination and then try to maintain the best VMG to that destination.

For coastal I like the ETA functions when I am attempting to get through a pass at which I need to arrive at a specific time. The Yucultas can only be transitted once a day in daylight hours when going north.

For most other activities the use SD's mark I eyeball and paper charts.

The boats I sail all have chartplotters. I have a laptop based system and I now have charts on my Android phone. I use these to discuss the differences among the various systems.

I would never use a GPS to go through a narrow channel.

Omatako 01-14-2011 01:15 PM

I have a chart plotter on a notebook, rarely use it while sailing, use it to plan routes and it is driven by a hand-held Garmin GPS76. I use the GPS to get fixes at sea which are transferred to a chart.

Coastally I navigate by visual verification using a simplified local chart that is marked "Not for navigation" and contains satisfactory detail but doesn't have the distance scales on the sides. These are published by the local Hydrographic Office and distributed by the Coastguard and are named "Recreational Charts".

hellosailor 01-14-2011 01:22 PM

"the Fathometer, showed me to be in the center of the bayou."
That's interesting. Here I thought a GPS showed *position* while a fathometer only showed depth of water under you.

It isn't that one or the other is necessarily wrong, they can both be "right" and still be in total disagreement. The question is, what is the GPS position based on (datum and accuracy) and what is the fathometer based on? (Presumably "showing" your position by depth matchig a surveyed depth on some chart, again with datum and age to be considered.

The GPS accuracy is usually excellent, the problem is the geocoding, matching up that position to where things have been put on a chart. In an area where I haven't actually been there and "done the ground conformation" I have to allow for the GPS position possibly being off 1/2 mile or more, because I've experienced that and I know the limits of the MAPPING as well as the GPS.

I've also been diving in a channel well surveyed and marked at 40' to bottom, and seen 80+ feet and descending on a depth gauge. Depths change too!

So like a good pilot I trust my instruments--but as Bush and Mao and many others have said, "Trust but verify". I look up, I see the sun against a blue-ish sky and a certain size, so my circle of position starts as "probably on earth" and gets refined from there. Although some days....

Faster 01-14-2011 01:24 PM

Much like Jackdale, I use the GPS for it's ETE function, I like the satisfaction of seeing the miles scroll down as progress is made. I have found it to be remarkably accurate here in BC/WA - including picking our way through our first passage of the LaConner 'Ditch' in the dark - but we did have radar as a backup.

However we always keep paper charts at hand too. If we had a large-screen plotter I might not dig out the charts so early.. but I still like to have one at hand for any tight passages.

Most of our sailing is "VFR" - we can usually 'see' our destination, and are often tacking up a particular shoreline or narrow channel.

We've seen ridiculous errors on plotters in Mexican waters, but I believe their paper charts are no better......

tommays 01-14-2011 02:47 PM

I kind of always saw my compass as really critical because if i don't have a landmark i am not real good at holding a course

BarryL 01-14-2011 04:27 PM

navigation
 
Quote:

Originally Posted by Boasun (Post 686814)
Now the question is; How many of you rely 100% on the GPS?
How many of you use the GPS as a tool and use a secondary means of Navigation. (Coast Piloting/celestial)

What do you mean by 'rely 100%'? Most / all of my sailing is done within site of land. I have only been out of site of land a few times when it was foggy or very hazy and I was far out, or it was dark and I wasn't familiar with the lights on shore.

Whenever I am out of my harbor, my chart plotter will be on and displaying my location, speed, heading, etc. I won't have a paper chart out or anything (but one will be close by). Since I'm not purposely going anywhere I can't really get lost. If I plan on anchoring somewhere new I certainly will look at the chart and study the plotter, but that's not navigation.

If I am sailing with a particular destination, my chart plotter is my main tool. I will have a route entered and active. I will use the ETE, Off Course alarm, and other electronic tools in the plotter to assist me. I will record my position, speed, and heading every hour in my chart book, as well as keeping my log updated. Since my routes usually take me from nav aid to nav aid, if the GPS went down it would be inconvenient, but not terrible.

Barry


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