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 Post subject: GPS Accuracy
PostPosted: Nov 9th, '08, 18:24 
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Able Skipper
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Hi,

I have a very small GPS logging device. It is the size of a matchbox, and logs your position every second. Then when you plug it into your computer you can download the data and see your info on a satellite from Google Earth.

I was just checking the sail yesturday, and I noticed two things (well three - i was suprised at how fast I went, but that is another story).

1. Altitude accuracy.
Now I know that the waves on Lake Macquarie can be very big, but from the image below you can see that the data shows a huge sea.
My altitude for the day varied from 15 to 35m. Even while sitting at a mooring having lunch and admiring Terrys new Catalina 320, the boat varied by 3m


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altitude.JPG
altitude.JPG [ 127.27 KiB | Viewed 2502 times ]

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RockMan
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PostPosted: Nov 9th, '08, 19:18 
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Altitude accuracy on all GPSs is very inaccurate as the satellites are the same height of orbit so the differences between measurement to do with altidude are relatively small compared to the horizontal differences to do with where you are. We are often sailing at between 2 and 20 metres underwater which can be a concern!!

So the vertical accuracy is nowhere near as good as the horizontal accuracy. It gets worse towards the pole ie where you are in Mexico as the world is not really a sphere either.

There's plenty of write ups of this in detail if you google gps altitude accuracy.

BTW, tell us more about the logger, what is it how much etc? Sounds pretty cool!

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PostPosted: Nov 9th, '08, 21:06 
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Able Skipper
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Hi Drinko,

Great little units.

Add one to your christmas list. I got mine through one of my suppliers. As a non commercial deal - i could get some for TSP members.

Here is a link
http://www.anyware.com.au/Browse/7a038c23be744a5998667f3aff0f1fd9001ItemDetail.aspx
$100 + frieight
or

http://www.anyware.com.au/Browse/58a0cbc245e540f6bceb425ef101b489001ItemDetail.aspx

$130+freight (about)

Great little units - this is not a commercial sale - but maybe i should move this to the TSP members area.....

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RockMan
Singleton - Hunter Valley NSW
Catalina 375 #54 - Rock The Boat

Living the Dream

http://www.charteryachtsaustralia.com.au/cya/whitsunday-sailing-yachts/rocket-catalina-375


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PostPosted: Nov 9th, '08, 23:28 
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I have a similar matchbox style bluetooth GPS unit, but without the datalogger function (so it was cheaper). I link my GPS via bluetooth to either my laptop or my Imate PDA 3G mobile phone.

Add Oziexplorer to the laptop and Ozi CE to the PDA and for a very low price you have a marvellous chartplotter, with track logs, waypoints, MOB, route planning, route navigation (eg spoken change in bearing at a waypoint for the next route leg) as well as dynamic speed and altitude plots displayed on maps or charts of your choice. The GPS unit also drives in-car navigation software on the PDA, with spoken street names and directions etc. (I use Copilot, but there are others).

The unit I have gives amazingly accurate position fixes but, as drikko said, there is a slight problem with altitude because of the ellipsoid shape of the earth and the angles of the satellites. Like drikko, I usually find I'm sailing a few metres under the surface of the water. GPS altitude is more useful on hills or mountains where a few metres of error are not so significant.

You can download altitude correction data for your local area from NASA. Ozi Explorer can integrate that data to give you are more accurate altitude fix, as well as 3D maps (terrain only, not under water, lol).

Overall these little GPS units are a lot of fun, but also very useful.


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PostPosted: Nov 10th, '08, 12:21 
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I have a similiar unit aswell. The advantage of the data logger is that you do not need all the other gear turned on to record your trip. But if you wanted to retrace the path - you could turn on the PC and follow the path.

With the price of these things dropping so fast, there will be GPS in everything before long!

The other advantage of the logging unit....
1. I can throw it in the car and it will tell me where my son has taken the car - and how fast he drove.

2. I can tuck it in another boat - and retrieve it after the race - it will show me how they tracked around the course - and i could compare it to my path and maybe give me some insight to the performance of another boat. As a training tool of course!

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RockMan
Singleton - Hunter Valley NSW
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http://www.charteryachtsaustralia.com.au/cya/whitsunday-sailing-yachts/rocket-catalina-375


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PostPosted: Nov 10th, '08, 16:22 
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I pity your kids .... if they get any smaller (the gps's :P ) you'll be dropping one in his corn flakes !!!! :lol:

Mat.

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PostPosted: Mar 14th, '09, 09:45 
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Hi guys, I bought one of these units and tried it with Cmaps. Unfortnately I run Vista on my computer as a result Cmaps was a disaster. I'd be interested to know how you set your GPS receiver to interact with Google. Do you have some go between software that I should get.

Getting a cheap GPS has now become an obsession rather than a necessity for me, and it is all due to the fact that so many of you guys seem to have done it. As the advert goes "How do you do it?" :lol:

Ed
Wanton C22/110

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 Post subject:
PostPosted: Mar 15th, '09, 20:04 
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The GPS unit came with some software that allows it to interface with Google. I would suspect that they have "copied" it from somewhere else and then packaged it with their product.

If you would like a copy - send me an email/PM and I will organise it.

Cheers
Simon

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RockMan
Singleton - Hunter Valley NSW
Catalina 375 #54 - Rock The Boat

Living the Dream

http://www.charteryachtsaustralia.com.au/cya/whitsunday-sailing-yachts/rocket-catalina-375


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