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PostPosted: Thu Oct 07, 2010 11:57 am 
lord high faltbotmeister

Joined: Thu Sep 06, 2007 8:05 am
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Location: atlanta, georgia
So I have tried to enter waypoints in the Garmin 76CSx, with BlueChart, and I have a big problem. I used google earth to learn the coordinates of the beaches and keys I will be navigating to. When I use those coordinates in the Garmin and open them in the map function they are off, WAY off, from where the Garmin map shows the beaches and keys to be located. The coordinates put me over .5 miles from where Garmin's map shows I should be.

I know that is a little confusing, but does anyone know the source of the error? I am doing all this from Atlanta, which is close to 1,000 miles from the intended destinations, is it possible that I need to be closer for the Garmin map to read the coordinates properly?

Sorry for the esoterica, but I sure would like to figure this out before I leave. I do not have permission to visit Cuba and, with these coordinates, I might just end up there!

Best,
g

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PostPosted: Thu Oct 07, 2010 12:47 pm 
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Are you using the same way to express coordinates?
e.g. if one sytem is degrees, minutes, seconds and the other is degrees minutes, decimal minutes you will have discrepancies


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PostPosted: Thu Oct 07, 2010 12:56 pm 
lord high faltbotmeister

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Vangelis, I think you have it. Google coordinates read, e.g. 24degrees 48' 36.97" and the Garmin coordinates read (approximately) 24degrees 48.675'. As you can tell, I am pretty new to this, how do I convert? Is it as simple as expressing 36.97" as the fraction of a minute?
g

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"There is nothing--absolutely nothing--half so much worth doing as simply messing around in boats"

1990's A1 Expedition
2010 Klepper Quattro
Kayaksailer
Balogh sail rig, 24 + 36 HP
Torqeedo outboard
1938 Sachs-Fichtel seitenbordmotor


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PostPosted: Thu Oct 07, 2010 4:30 pm 
lord high faltbotmeister

Joined: Thu Sep 06, 2007 8:05 am
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Location: atlanta, georgia
OK, just to close this out for the throngs of rapt navigators...

I discovered that you can toggle between the formats on the Garmin, so there is no need to manually convert from minutes/seconds to minutes/decimal of minutes.

Of course, I discovered that AFTER I had done all the manual calculations!

g

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"There is nothing--absolutely nothing--half so much worth doing as simply messing around in boats"

1990's A1 Expedition
2010 Klepper Quattro
Kayaksailer
Balogh sail rig, 24 + 36 HP
Torqeedo outboard
1938 Sachs-Fichtel seitenbordmotor


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PostPosted: Thu Oct 07, 2010 6:32 pm 
Even when using same formats, I sometimes find Garmin maps to have about 0.5 mile discrepancy compared to printed topo map or Google Earth. As a result, location (waypoint) of my camp measured and entered into Garmin when I'm actually in the camp, shows on the screen as 0.5 mile inland or offshore. Not to mention the height - I always mark the camp when I'm at the high tide level or higher, and it may show on Garmin screen as +15ft or - 15ft elevation. Could never resolve all this, and decided to just let it be, and treat Garmin screen map as a crude, auxiliary tool, still relying on coordinates that it shows, but not on whether it shows me being on water or on land.

One of the reasons in my case could be not very accurate Garmin Blue Chart maps for Baja - they don't have much detail, but displaying a shoreline 0.5 mile off it's actual location - c'mon, this is acceptable for free world map, not for $90 BLue Chart.


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PostPosted: Thu Oct 07, 2010 8:18 pm 
lord high faltbotmeister

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Location: atlanta, georgia
Yep, a 750meter/ 1/2mile error is pretty much a pain, especially in bad weather. I will be keeping an ear on the radio and an eye on the horizon!

Thanks,
g

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"There is nothing--absolutely nothing--half so much worth doing as simply messing around in boats"

1990's A1 Expedition
2010 Klepper Quattro
Kayaksailer
Balogh sail rig, 24 + 36 HP
Torqeedo outboard
1938 Sachs-Fichtel seitenbordmotor


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PostPosted: Fri Oct 08, 2010 6:12 pm 
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Why not just use your BlueChart in MapSource (SW which came with the GPS or with your BlueCharts) on your computer to identify waypoints, and then download them to the GPS?

Note: did not check whether your BlueChart came on a card or on a CD/DVD. If the former, perhaps you can't insert the card into your computer?

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PostPosted: Fri Oct 08, 2010 6:23 pm 
lord high faltbotmeister

Joined: Thu Sep 06, 2007 8:05 am
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Location: atlanta, georgia
Dave,

Thanks, no can do. I have MAC, and software for Garmin is not yet ready for that. It is strictly PC. Even so, I find that Google Earth is perfect for locating exact points, right down to a specific beach landing, and I can live with the pain of translating that to the Garmin map function.

Best,
g

_________________
"There is nothing--absolutely nothing--half so much worth doing as simply messing around in boats"

1990's A1 Expedition
2010 Klepper Quattro
Kayaksailer
Balogh sail rig, 24 + 36 HP
Torqeedo outboard
1938 Sachs-Fichtel seitenbordmotor


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PostPosted: Fri Oct 08, 2010 11:21 pm 
I like trip planning in Google Earth. But you can't transfer waypoints from GE to GPS (though you can import from GPS to GE). So copy-paste is the way to go - but without a PC even this is a pain. I don't see any problem with Garmin formats - you can set Garmin to the same format as GE, which is degrees and fractions of minutes (as opposed to degrees, minutes and seconds). 1/100 minute of latitude is less than 20 meters distance - who needs THAT much accuracy....


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PostPosted: Fri Oct 08, 2010 11:57 pm 
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I'm not knowledgeable enough about this to speak authoritatively, but I think some of the displacement may come from differences in the horizontal geodetic datum used. Maybe someone (greg?) who knows more can chime in.

Datum choices: http://geography.about.com/od/geography ... datums.htm

Pertinent excerpt: As previously mentioned, there are many datums in use around the world today. Some of the most commonly used datums are those of the World Geodetic System, the North American Datums, those of the Ordinance Survey of Great Britain, and the European Datum; however, this is by no means an exhaustive list.
Within the World Geodetic System (WGS), there are several different datums that have been in use throughout the years. These are WGS 84, 72, 70, and 60. The WGS 84 is currently the one in use for this system and is valid until 2010. In addition, it is one of the most widely used datums around the world.

In the 1980s, the United States Department of Defense used the Geodetic Reference System, 1980 (GRS 80) and Doppler satellite images to create a new, more accurate world geodetic system. This became what is known today as WGS 84. In terms of reference, WGS 84 uses what is called the "zero meridian" but because of the new measurements, it shifted 100 meters (0.062 miles) from the previously used Prime Meridian.

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PostPosted: Sat Oct 09, 2010 4:04 am 
I recorded my starting point with Garmin (WGS 84) at the BOLA camp in the last trip when standing ashore. Tide or no tide, I was within 50-70 ft (not meters) from water. OK, tides can cover 50ft in North Baja, so - the waypoint was 0-120 ft from water. And it does indeed show at roughly 60ft from water on Google Earth. But on Garmin Bluechart V.6 it shows 800ft inland! That's a bit more than 100m (and why should it be at all, if I used WGS 84, I wonder). On other occasions it was more than 1000 ft off.

The only reasonable conclusion - inaccurate drawing of shoreline in BC V.6 for this region. Makes sense to me, since this chart displays "overzoom" warning when I zoom in to 2 nm scale. Some accuracy, 2nm, eh?

PS: My bad, sorry - Google Earth doesn't import waypoints from Garmin unit (there is a button in GE, but it doesn't work). MapSource (Bluechart etc) allows import/export of data between the unit and PC. May be GPS Import button does work in the paid version of GE, just my guess...


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PostPosted: Sat Oct 09, 2010 11:34 am 
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Alm wrote:
(snip) OK, tides can cover 50ft in North Baja, so - the waypoint was 0-120 ft from water. And it does indeed show at roughly 60ft from water on Google Earth. But on Garmin Bluechart V.6 it shows 800ft inland! That's [quite] a bit more than 100m (and why should it be at all, if I used WGS 84, I wonder). On other occasions it was more than 1000 ft off. The only reasonable conclusion - inaccurate drawing of shoreline in BC V.6 for this region. Makes sense to me, since this chart displays "overzoom" warning when I zoom in to 2 nm scale. Some accuracy, 2nm, eh?
I'm not surprised, Alex. When in overzoom mode, basically, the GPS is using a small-scale chart as its reference, blown up way beyond its finest detail. Usually, for a 1:40,000 chart, overzoom does not kick in until you get down to maybe the 200 to 300 ft level (scale icon in the LL of screen). A "dot" on the original chart for that one would likely be 500 feet across!

Kind of like trying using a road map of the province of BC to guide a boat through shoals in the Gulf Islands. One solution would be to find a larger-scale GPS "chart" for that region -- a different BlueChart, maybe. Might not be available for Baja, though.

Even in areas that are "accurately" charted (Broken Group in Barkley Sound, for example), I have found rocks which were mis-charted on my large-scale BlueChart by 200 feet or so. Could be in the original paper chart or in the translation to the BlueChart. Never had time to see if the paper chart I carried (1:40,000, IIRC) had them accurately located. Be a lot of compass and chart work to do that.

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