Checking Gps

[jerson]

I have a hummingbird matrix sounder gps connected to an external antenna, the units coordinance are about 500 - 600 meters different to google earth reading when the boat is stationary in the back yard. Is google earth reliable, can anyone give me a gps mark they trust for a fixed position in the river mouth to green Is. area.

Jerson.

[finga]

Check which datum's that your both using.

[jerson]

yes mate both are set at WGS84.

Jerson

[banshee]

I would be surprised if your local Warterways Authority couldn't supply exactly what your after,for example,my local office situated in Ballina has the exact coordinate for the NE pylon of the Evans pontoon as well as one for a Pylon on the Ballina pontoon displayed in their office so people can check the accuracy of their GPS.In reality none of the units we use will be accurate enough to line up with these marks, they will however easily get us back to the spots that we have put into them,a lot of units though will allow you to factor in the error for navigating purposes if you want.

[FNQCairns]

yes mate both are set at WGS84.

Jerson

But where they entered as wgs84, they don't convert very well in unit. I wouldn't trust google earth myself but 600m is a huge amount too much for either to be out without another factor in the mix.

cheers fnq

[FNQCairns]

Just checked my personal co-ords against google and I got roughly a 500m position error...google is just crap is all, expected it but this is a huge amount!!

I know my co-ords are correct...as correct as to be expected anyway.

cheers fnq

[jerson]

FNQ, just looked at beacon 2 beacon 2002 edition, coordinance given for waypoints look pretty good on google earth.
Waypoint for my house wasn't entered into the gps, it was the reading from ths unit.

Jerson.

[jerson]

The unit in question is a matrix 12 with a 15 channel, i think, antenna, not many dollars. Am i expecting to much of it.

Jerson

[FNQCairns]

I suspect the accuracy will vary with each image on google, some will be calibrated closer and some further from real world, 500m is a lot though.
I can do way better than that scanning a paper map, stitching it, then calibrating it myself for viewing on my software.

If given the choice would tend to believe your GPS before any commercial co-ordinates.

cheers fnq

[jeffrey_h]

It has been pointed out on here before that if you leave your GPS on for some hours with-out moving you will move all over the place on the screen, maybe not 600 metres?

I ran around the top of Woody Island at Hervey Bay and the GPS said that I was on the outside of the Nth beacon when in fact I was heading over the shallow water on the inside! Then at the very next beacon, towards the harbour, it was exactly where it was on the GPS, go figure.

Exactly the same happened years ago at the beacons on the west side of Mud Island, one was spot on, the other was a few boat lengths or more off, not much good if you were trusting these for black night boating!

Jeffrey

[Chine]

I have a hummingbird matrix sounder gps connected to an external antenna, the units coordinance are about 500 - 600 meters different to google earth reading when the boat is stationary in the back yard. Is google earth reliable, can anyone give me a gps mark they trust for a fixed position in the river mouth to green Is. area.

Jerson.

I would be surprised if your local Warterways Authority couldn't supply exactly what your after,for example,my local office situated in Ballina has the exact coordinate for the NE pylon of the Evans pontoon as well as one for a Pylon on the Ballina pontoon displayed in their office so people can check the accuracy of their GPS.In reality none of the units we use will be accurate enough to line up with these marks, they will however easily get us back to the spots that we have put into them,a lot of units though will allow you to factor in the error for navigating purposes if you want.

Hi Jerson,

I think Banshee has the answer with a known calibration point. Most state waterway authorities have at least one calibration point on water where you can site yourself and make a comparison between your receiver's computed co-ordinates and the surveyed co-ords at that point. To note the difference AT THAT TIME (some receivers may vary/spike by up to 30 metres over time) move your boat away from the calibration point until the co-ords match and estimate the difference.

500m~600m differential does not add up.

It would also be interesting to see what comparison you get with GE co-ords and the calibration co-ords.

Cheers

Chine

[Chine]

I have just done a Google Earth check on a surveyed mark down in NSW and it is very close.

Cheers

Chine

[Angla]

I think we should all be very wary. I treat my 2 x GPS units as an aid to navigation, Not an absolute to navigation. I use my eyes to see where I am going and to confirm the route. I probably use the map GPS less than the cheap plotter unit.

Some days the mark for Home is off by 20 odd metres at a guess and I would put this down to variation from satellite reception (cloud cover) and positioning (high above or near the horizon)

Chris

[poundalead]

Hey Jeffrey H,

I was at Hervey Bay the other day and the same thing happened to me on that North Beacon. I have a Lowrance 5200c with Navionics cartography. All the other beacons in that area are spot on, which makes me think that it may have been moved?????

[jeffrey_h]

Hi Pounda,

That's what my wife said at the time, but I only recently updated the GPS, Lowrance 339i, not sure if that updates the maps or just the system. Suppose we could ask the auth. up there.

Not going back there next Xmas though, always to windy and wet. Went to Fingal today and booked a holiday down there for July.

Jeffrey