GPS Question

[castlemaine]

Hi All
I've got a Hummingbird Matrix 37. I marked in the channel markers from a previous trip but on the last trip noticed that the marks were consistently around 20m away including my previous track.
Should I re-enter the marks or is this an acceptable error? Does the GPS receiver on a slight angle (on the bow) make a difference?
Cheers8-)

[Hamish73]

depending on how many satelites your GPS is locked onto will determine its accuracy. Most GPS unit will let you see this info on the satelite page.

20m is pretty good IMO. My little handheld comes up with 'ready to navigate - accuracy X m' every time it gets a fix. The 'X' varies from 5 to 50 metres

[FNQCairns]

Sometimes it take a little time for the GPS to refine the position down my handheld garmin can get near 50% closer if I wait 1 minute or so while it refines it's position before finalizing the mark.

Add a normal error + one from quick marking and 20m could be a possibility?

cheers fnq

[Noelm]

I guess it all comes down to the very simple fact, that you do NOT fully rely on Electronics for Navigation (especially at Night) nothing beats care, safe speed and a GOOD LOOKOUT!

[BaitThrower]

Yeah sounds like the accuracy on the day you marked was different to the day you went back :(

20m is a fair way though, but not sure on the accuracy of your unit?

My Garmin 76Csx handheld has accuracy down to +/- 2m, and it is very accurate. Maybe you just didnt have a good fix when you marked them. Check the fix on the satellite page (if it has one) next time you go out, and then if it is good, I'd re-mark them.

[castlemaine]

Thanks guys will have to play with it a little more.

[SatNav]

1. 20 metre accuracy would be more than expected? There would have to be other variables involved with this?

2. The number of satellites does not have a lot to do with accuracy. Just because there might be more satellites does nor mean accuracy will be better.

3. There is no standard GPS that is capable of actually knowing what the accuracy is. What it might estimate is not accuracy.

4. There is simply no civil standard GPS that is accurate down to +/- 2 metres, not even a 76Csx. Not at this point in time but things will change in the next 5 years

[steve99]

Hi All
I've got a Hummingbird Matrix 37. I marked in the channel markers from a previous trip but on the last trip noticed that the marks were consistently around 20m away including my previous track.
Should I re-enter the marks or is this an acceptable error? Does the GPS receiver on a slight angle (on the bow) make a difference?
Cheers8-)


The only way I could explain a 20 metre distance error since your previous trip,is maybe you have inadvertantly changed the datum on the GPS.

Steve99

[SatNav]

1. Datum differences in Australia are in the order of 200 metres not 20 metres

[krillx]

Changing the datum in the unit will have no effect. It will convert your waypoint and track data into whatever datum you change it to.

Could have been the Yankees playing around with the accuracy of the system on the day you were out. Antenna angle should have no effect.

[SatNav]

"Could have been the Yankees playing around with the accuracy of the system on the day you were out."

1. No

[castlemaine]

When I went out last my new track was approximately out by 20m but parallel to the old track. To me it shows that on the day I entered the points there was a consistent error. Why?
I have installed my old Garmin and when the wind stops, I'll run the units side by side. Should be interesting.
Cheers

[SatNav]

1. Can you guarantee both tracks you took (actually over the ground) were one of the same?

2. As for Why? If this is not the case? Could be several reasons and one of them is that GPS is not guaranteed 100% of the time, impossible to achieve 100% for a number of reasons also.

3. Explain how you recorded the original marks? How were these original marks stored or recorded, manually or automatically?

[castlemaine]

1. Can you guarantee both tracks you took (actually over the ground) were one of the same?

2. As for Why? If this is not the case? Could be several reasons and one of them is that GPS is not guaranteed 100% of the time, impossible to achieve 100% for a number of reasons also.

3. Explain how you recorded the original marks? How were these original marks stored or recorded, manually or automatically?

Thanks for the great comments SatNav, you obviously have an interest and knowledge in GPS systems.
Marks were taken as I leave Raby Bay Boat Ramp, hit 'MARK' as I slowly go past each channel marker but what concerns me is that if I followed that track now I'd be 20m on the other side of the channel marker. Almost in a parallel line.
I could live with a 5m error but by the sound of it 20m is not unusual. I've always known to use GPS as a guide only. Had a mate looking at it too, 'cheap GPS' was his comment. What are mate's for?;D
Was wondering if I should take it in for a check?
Cheers

[Taroona]

Would the addition of a DGPS increase the accuracy?

[SatNav]

1. Being on the other side of markers is obviously a difference but is still a possibility and for reasons others have mentioned is a bit of a hint that GPS should not be solely relied on absolutely for navigation.

2. DGPS is not totally about inproving accuracy in the true sense, not these days anyway but more a system that confirms and verifies that the position is in fact reliable and not affected by outside influences.

[Taroona]

I have found that my GPS has shown me anchored up on the beach at the pin.

On another occassion I relied to much on the GPS for where I was going and found my self going the wrong way around a cardinel. I was to busy watching the GPS rather than what was happening around or in front of me. Lesson well learn't

[mickstar001]

Castlemaine, give me the gps points of your favorite fishing spot, and i can put them into mine, and we'll see if it's accurate. Thats the only feasable way i can see to make sure yours is working properly mate!!

Mickstar

[castlemaine]

Castlemaine, give me the gps points of your favorite fishing spot, and i can put them into mine, and we'll see if it's accurate. Thats the only feasable way i can see to make sure yours is working properly mate!!

Mickstar

Mickstar
My fishing spots are so secretive that even the fish can't find it.;D
Cheers
Castlemaine8-)

[BrewGuru]

Domestic GPS have a variable error programmed into them for civil use not for terrorist use.

Military GPS is Spot on, as we have seen on the TV. My GPS with max satellites has had me in themangroves and not in the channel I am in

[tenzing]

This might not be worth anything and I'm happy to be told so, But when I'm wanting a really accurate mark, eg dropping pots to be picked up in the bay in a decent wind or at night, I make sure I have the gps zoomed in as far as I can when I make the mark. It seems to have worked so far although i might just be kidding myself.
Cheers , Brendan.

This would particilarly apply if you were making a mark with or at the cursor as its size (the cursor mark ) must introduce a slight degree of innaccuracy

By the way ,I am a greedy bastard and have Two gps on the boat . It is useless to compare the lat/lon of one with the other . they just dont match, possibly due to the earlier mentioned in built innaccuraccy. But this really only affects inputting marks from one gps to another . Care needs to be taken inputting stated figures for hazards, but in a single unit the error shouldnt make much difference as it is repeatable. I even tried the exercise with a third gps and the navman. raymarine and garmin all differed from each other at the same time.
(all using the same datum but different mapping in each case.)

[SatNav]

"Domestic GPS have a variable error programmed into them for civil use not for terrorist use."

1. Domestic (as you call it) GPs has not had any variable error programmed into it since 2001. The difference today between military and civil GPS is not worth discussing as the difference is simply minimal.

"Military GPS is Spot on, as we have seen on the TV"

2. Is it?

"My GPS with max satellites has had me in themangroves and not in the channel I am in"

3. What would you say would be the problem then? Your GPS, the satellites, the mangroves, the channel or ...?

4. Zooming a chart in as far as possible will have no bearing on marking a GPS position at all

[BrewGuru]

SatNav I carry three GPS units on board the original green screen valve operated jobby. the latest navman hand held and a Garmin hand held with all the east coast maps loaded, now if you have done any long range cruising and kept watch at the helm , you would know that you should mark your GPS co ordinates every 20 minutes marked with a 2B pencil on your charts, matching your co ordinates on the chart as sailors did before chart plotters, my three never have the same
co ordinates but roughly and sometimes very roughly they show where approximately where I am and that is what they are meant to do.
The best sailors in the world will tell you not to rely on domestic chart plotters or GPS units, Military units work on a totally different system and are far more accurate than what us fishos use. So in your statement ofThe difference today between military and civil GPS is not worth discussing as the difference is simply minimal. Why have the two??
I would be doing a little bit more research before I start misleading people on the accuracy of domestic GPS, thanks for your insight.

[steve99]

Domestic GPS have a variable error programmed into them for civil use not for terrorist use.

Military GPS is Spot on, as we have seen on the TV. My GPS with max satellites has had me in themangroves and not in the channel I am in


If your in a channel,and the GPS says your in the mangroves. The datum set in your GPS is obviously different from the datum on the chart your using.

I've used a dozens of different maps ( land and sea ) and I have always found If the datum on the GPS is set to the map datum,the difference should only be a few metres max - any more and something is wrong,and its not the GPS.

Steve99

[BrewGuru]

You don't think that the mapping software (whoever the manufactureri s)that shows where you are in relation to realtime GPS co ordinates has anything to do with it?
Do me a favour ring lawrance, Garmin, Navman, GME and ask what guarantee to accuracy of the equipment they market, my three units never agree and I admit sometimes the difference is no that much, but enough to get you stranded on shoals as an example Jacobs Well, its very narrow and I draw 4'6, I certainly don't rely on the plotter to show where I am.

[steve99]

You don't think that the mapping software (whoever the manufactureri s)that shows where you are in relation to realtime GPS co ordinates has anything to do with it?
Do me a favour ring lawrance, Garmin, Navman, GME and ask what guarantee to accuracy of the equipment they market, my three units never agree and I admit sometimes the difference is no that much, but enough to get you stranded on shoals as an example Jacobs Well, its very narrow and I draw 4'6, I certainly don't rely on the plotter to show where I am.


I dont use mapping software - and probably never will, if its as inaccurate as people are suggesting.

My GPS is deadly accurate,I've checked it against the UTM datum in street directories and Google earth with grid section and co-ordinates which were compared against track log positions stored in my GPS. The difference in position has never been more than a few metres


Steve99

[SatNav]

1. NO two standard GPS receivers will show "exactly" the same coordinates

2. The basic difference between military and civil GPS Was/is anti-spoofing/anti-jamming, capability to counter Selective Availability (SA) and dual frequency.

3. Anti-spoofing/jamming is not relevent to civil use, Selective Availability is not longer used, selective availability will not even exist on future satellites

4. Dual frequency can determine atmospheric corrections in real-time as opposed to current civil receivers using a model

5. There are currently 5 satellites in the constellation with the new second civil frequency

6. When the second civil frequency is fully operational and manufacturers release dual freq receivers then the civil / military accuracy will close even further

7. Current military PPS SIS accuracy is <5.9 to 11.8m 95% and civil SPS SIS is < 13 metres 95%

8. So no the difference between civil and military ACCURACY is really not worth discussing and even this gap will close to almost complete similarity in due course

9. The two sysytems existed in a past world, this gap is closing even faster as the civil function of GPS gets more priority than military use

10. Lowrance, Garmin, Navman, GME? or any manufacturer have no say what so ever or can not guarantee GPS accuracy

11. GPS accuracy is under the total and complete control of the GPS control segment who control the quality and maintain the system so as to provide a minimum accuracy standard

12. BrewGuru, if I was you, you should take your own advise and yes do some research, even maybe watch a little less TV

[SatNav]

1. Mapping? Mapping is no more accurate in digital form than the scale of the original mapping

2. Zooming in does not improve chart accuracy

3. Over zooming can be quite dangerous and misleading and good plotters will restrict this capability

[tenzing]

1. Mapping? Mapping is no more accurate in digital form than the scale of the original mapping

2. Zooming in does not improve chart accuracy

3. Over zooming can be quite dangerous and misleading and good plotters will restrict this capability

My point id that whilst zooming in clearly does not improve the accuracy of the reading of YOUR position , It can certainly improve the accuracy of a waypoint placed at the position of the cursor as the cursor can be more accurately placed at the positiion you wish.(as opposed to a waypoint created at the boats position)

[Chine]

Hi All
I've got a Hummingbird Matrix 37. I marked in the channel markers from a previous trip but on the last trip noticed that the marks were consistently around 20m away including my previous track.
Should I re-enter the marks or is this an acceptable error? Does the GPS receiver on a slight angle (on the bow) make a difference?
Cheers8-)

Your accuracy will vary as the geometry of the satellites change (HDOP)...an explanation should be in your manual. Satellites low on your horizon will give an inferior element of precision or cut as compared to higher units. A cut derived from many low satellites will not be a good result.

The other question to be asked is where the channel markers fixed beacons or moored buoys?

A combination of low (read different) HDOP, intermittent GPS vagaries such as solar flares etc and buoys running with a contrary current may contribute some way towards your error.

[SatNav]

1. Accuracy will vary but generally not to the extent experienced and not for a prolonged period of time

2. The provisor is that there wasn't a major satellite malfunction at the time. There has been no such reports or occurances indicated in recent times.

3. Low horizon satellites would not give the affect experienced, not present day as the constellation is fully populated and effectively running an additional 7 to even 8 sats over spec

4. The biggest possible affect on horz error would be the reverse situation where all satellites were directly overhead but this is a long call as well.

[Last Cast]

I have a Garmin GPS72 & found it WAAS enabled, ( this feature not available in Oz apparently) Once i turned this feature off the GPS was far more accurate, thought this might help.