GPS question

[lew]

I recentely installed my 1st GPS unit.....Lowerance Global Map 7200c.....and have enjoyed experimenting with it and figuring out all the functions, well most of them anyways, but I find one thing puzzling.

 

I'm using the Navionic Gold + XL9 and have been traveling all over the lakes and comparing my actual position to what it shows on the screen and for the most part, it's very very accurate, but at certain places it seems to be off by quite a bit, and the variance changes at different times of the day.

 

One 500' long island in particular is confusing, it has a 30' wide channel at the north end and when I go thru it, the GPS shows exactly where I am, but when I come up the south side, it sometimes shows me hitting the end of the island or even going right over the island and is sometimes off by as much as 150' in that particular location. Another time I'll go past and it's accurate.

 

Just about everything else is extremely accurate with the exception of a few marker buoys, but I think they get moved around anyways.

 

I'm assuming this is just a fault with lining up the Satellites in certain locations, but is this sorta thing common with GPS units ?? I know last year on Cameron Lake we were using Robins GPS and that also showed some descepancies.

 

I didn't have room on the dashboard for the unit because my Depth Finder sits there, so I mounted a bracket on the side of the console, then a RAM mount and it's working out beautifully. I drilled thru the side, inserted a plastic sleeve, then ran the wires.

 

Surprisingly it doesn't interfere with anyone moving around the boat at all in that location, plus it'll be easier for my partner to see what's happening from his seat.

 

The colors on the screen are excellent and I'm very happy I decided to spent the extra $$$$ and go with the larger 7" screen...next year I'll be upgrading the fish finders.

 

[Terry]

Satellites are part of the problem. many GPSs will tell you how inaccurate at any given time, so at times they will be off by 150ft.. 10 years ago 150 was the average reading

but the maps are off at times... I have a lake up north and I drive my boat around a point , the map shows that I am 100ft on shore all the time. some times the Gps was showing it was correct within 3ft, I checked the map on the computer and the map was wrong, after checking I found that maps are off, they do updated the at times, but there is little you can do about it

the garmin topo maps can be changed and reloaded onto the GPS and I have been doing that

[lew]

Thanks terry, that's sorta what I figured.

 

Top left corner of my screen tells how accurate it's sposed to be, but on that particular island it has shown I'm as close as 15 feet to where it should be, yet it's sometimes way off.

 

It could certainly pose some serious problems though if you were trying to find your way thru darkness or heavy fog and thought you were in the middle of a deep channel.........BUMP !!

[Terry]

yeah it sure could be a problem..but that's why they all have a disclaimer to not rely on them for navigation

[lunkerbasshunter]

hey lew that is a great point. i know alot of people that use them to navigate waterways. that could be a very dangerous/expensive problem. wonder how often this occurs?

 

Cheers!

[charlesn]

Lew,

 

I would guess that the mapping could be the culprit too. They keep coming out with updated and improved mapping chips, but with so many lakes I'm sure it will be a long time before they are all perfect. I remember about 5 years ago it would be common to drive around on the lake and be shown as driving around on land in your boat.

 

Charles

[lew]

Actually, I'm a little bit surprised by the inaccuracies as I was under the impression that the XL-9 Great Lakes / Maritimes was a fairly new & accurate chip.

 

Can I assume the depths shown, drop offs, ridges etc are accurate or can they also be off by 100' too ??...... Kind of defeats the whole purpose of the unit if it's not giving true info doesn't it

[Terry]

ohhhhh... no every thing can and will be off on the maps.......

and it doesn't matter who's maps you buy they all take the map info from the same place

[Smokey]

Lew I'm not sure if your machine has WAAS capabilites but if so make sure it turned on. It will increase the accuracy using land based towers along with the satellite for better operation.

 

But as said before most gps maps are off a little.

Edited May 31, 2007 by Smokey

[lew]

Thanks Mike, yes it does have WAAS , guess I'll just have to remember that's it's not always 100 % acccurate