adkel53 Posted April 20, 2008 Share Posted April 20, 2008 I have just installed a Navman 5380i in my boat and used it for the last week at Narooma. On a couple of occasions the track showed me passing over land. Is this simply due to the basic nature of the maps provided (basically a rough outline of the shape of Wagonga Inlet - no smaller bays showed up) or due to an error in calibration of the unit itself? I was using the WGS84 map datum and turned the DGPS option off after the first "land crossing". Any advice/tips much appreciated. How does one check the accuracy of a GPS plotter? Kel Link to comment Share on other sites More sharing options...
krill Posted April 21, 2008 Share Posted April 21, 2008 (edited) The inbuilt mapping will generally be of low quality and is probably the cause of what you are seeing. Slot a mapping card into it and see how you go. Edited April 21, 2008 by krill Link to comment Share on other sites More sharing options...
netic Posted April 21, 2008 Share Posted April 21, 2008 krill is probably on the money The Basemaps that come with the units are very basic at best, to be honest they are piss poor....Purchase a mapping card for it and you will see a huge difference... Link to comment Share on other sites More sharing options...
VA911 Posted April 21, 2008 Share Posted April 21, 2008 I have also been told that some days the GPS system is out a bit. could be a rumor though... Link to comment Share on other sites More sharing options...
warnie Posted April 21, 2008 Share Posted April 21, 2008 I have also been told that some days the GPS system is out a bit. could be a rumor though... VA911...... just out of interest everytime i go out fishing i test my plotter out in relation to were the cursor shows were i am ...in relation to were i know i am..... against immoveable objects.....headlands etc... in 6 yrs and all the times i have put this unit to the test it has never wavered a fraction... whether it may be that what you pay for is what you get and cheaper units might not be as reliable i couldn't say... however i can assure you every time i have gone to fish browns mountain for excample i am yet to find it's gone missing.....so to how much it is actually is out,,,,,for my unit i would have to say by not very much..... Cheers Warnie.... Link to comment Share on other sites More sharing options...
VA911 Posted April 21, 2008 Share Posted April 21, 2008 (edited) VA911...... just out of interest everytime i go out fishing i test my plotter out in relation to were the cursor shows were i am ...in relation to were i know i am..... against immoveable objects.....headlands etc... in 6 yrs and all the times i have put this unit to the test it has never wavered a fraction... whether it may be that what you pay for is what you get and cheaper units might not be as reliable i couldn't say... however i can assure you every time i have gone to fish browns mountain for excample i am yet to find it's gone missing.....so to how much it is actually is out,,,,,for my unit i would have to say by not very much..... Cheers Warnie.... i was only refering to it being out say 10 meters at the most. should have been more specific. sorry how much more accurate do we need to be? long story short though.. i was out on a charter fishing some wrecks in a known waterway by a very experianced operator. For some reason on his raymarine plotter the marks he has fished for years were all of a sudden about 15 meters east of where they should have been. you could see the structure on the sounder clearly but the marks were off. the operator made comment on it that day. just my $0.02c Edited April 21, 2008 by VA911 Link to comment Share on other sites More sharing options...
warnie Posted April 21, 2008 Share Posted April 21, 2008 i was only refering to it being out say 10 meters at the most. should have been more specific. sorry how much more accurate do we need to be? long story short though.. i was out on a charter fishing some wrecks in a known waterway by a very experianced operator. For some reason on his raymarine plotter the marks he has fished for years were all of a sudden about 15 meters east of where they should have been. you could see the structure on the sounder clearly but the marks were off. the operator made comment on it that day. just my $0.02c VA911 no prob's i have had to rely on the accuracy of my gps on quite a few occasions mainly in the dark early morning... with well known hazards and running up a channell following a course layed out on the plotter which i can call upon when required.... off course not wanting to put my total reliance on gps but at the same time it needs to have a better than a 10 mtr tolerance to get me through with out getting to close.... not saying what you have said is wrong its only for me....... as yet a 10mtr or more variation hasn't presented its self One contradiction to that though for me is an area just north east of broughton island for whatever the reason i am not able to maintain satelite lock and the compass goes to pot whether this is any thing to do with williamstown air force base or not couldnt tell you but particularly when the fighters do there mock training all the electronics on board the boat go haywire including the radio i guess its some sort of electronic interferance they are running...... on that particular day they both broke the sound barrier right above me and fair dinkum i nearly crapped my pants..... Cheers Warnie... Link to comment Share on other sites More sharing options...
Dorado 2 Posted April 21, 2008 Share Posted April 21, 2008 (edited) Hi Kel before I did have a chartplotter I was using a gps which was accurate on most days to about a metre but when there is cloud cover and if it is thick it can change the accuracy by 50 metres. When the chartplotter came along and I did buy a mapping card I did find it very accurate but when going into some small channel and if I have the mapping is zoomed in sometimes you can see your boat on land but once I zoom out I'm back on water. Hope this might be of some help to you Cheers Edited April 21, 2008 by Dorado 2 Link to comment Share on other sites More sharing options...
bisso Posted April 25, 2008 Share Posted April 25, 2008 for your interest, nearly all GPS units you buy are single frequency systems. since GPS uses basically triangulation and resection to locate your position, numerous things come into play which affect your locational accuracy, for example, the number of satellites, the clock accuracy in the unit (Satellites uses atomic clocks)and if your clock is 0.1 of a second out of sync you will get an error in position of some 200km's, others include multipathing from "metal" objects and power sources and most importantly being a single frequency device which operates in the L1 carrier wave band, generally all GPS units are speced to a radial accuracy of abt 7m meaning you may be 14m one way of the true location. another factor is the geocoding and digitalization of the charts themselves. being spatially referenced, they are generally accurate to possibly less than the GPS unit themselves. We use GPS for surveying which are high end dual L1/L2 freuency which differenciate the signals between a base station and a portable unit, these are accurate to about +/-10mm plus 1ppm and cost $70,000+. the important point here is that you will never get exactly the same location twice with a single frequeny GPS unit unless you log the data recorded and post process the information. sorry for the dribble on, thought that might help to clear things up. Regards Bisso Link to comment Share on other sites More sharing options...
twodogs Posted April 29, 2008 Share Posted April 29, 2008 At the NSW Waterways website, http://www.waterways.nsw.gov.au/maps.html boating maps are available for download of most major waterways. These maps contain reference points for checking the accuracy of GPS units. The GPS co-ordinates of an easy to locate channel marker or other prominent structure are quoted on the map. Similar I suppose to a odometer check on the freeway. This should help out, TwoDogs Link to comment Share on other sites More sharing options...
Dreamtime Posted April 29, 2008 Share Posted April 29, 2008 I was told that the US Military control the satellites that the GPS operate from. That there is an error actually put into all domestic GPS. Only the military units are exact. During the Gulf War the US blanked out all the satelites that the GPS work from. Last year, one of my vessels was west of here at the same time a US Navy ship was there also. The plotter showed a error of some 10 miles when in the vicinity of the Navy ship. Strange? Link to comment Share on other sites More sharing options...
smokinjoe Posted April 30, 2008 Share Posted April 30, 2008 (edited) Mariner 31, i to have heard about this, and yes they would be responsible for the electronics block as well. After Iraq 1 alot of gps and actual live satellite sites were closed down national security i believe lol. NASA has a great website where you can log on and check all satellite movements in the world, its quite amazing to look at, but how many dont they show ? ITS ALL A CONSPIRACY . A m8 has just told me that when his dad operated a trawler out of coffs, they were followed by a sub, and in the late 70s sprung a fishing trawler that was actually Russian and was spying off the coast! Edited April 30, 2008 by smokinjoe Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now