WGS84

[cgull]

Interesting article in YM about charts this month, and their accuracy

I have a new (this season) Magellan Meridian Handheld GPS and found it to be excellent and always very reliabl and accurate. This works to WAAS , being Wide Angle Augmentation System. Does this relate to WGS84?

I am impressed, took it to Majorca and it came up with map of the island

 

[tcm]

Re: Magellan

Magellan kindly sent their new product to lots of journalists ,the result beng that it was "recommneded" against ...no competition.

For 300+ notes this is a mild attempt at handdheld chart plotting, and the maps are low-resolution with not much information for marine use. By contrast, the garmin 175 is 200 quid from MES whilst obsolete stocks last, miles and miles better and can show you your pontoon, and the light signals from all the cardinals etc if you get the right g-chart for your area. The later 176c can do the same faster and in colour, but 600quid. Magellan's real marine offering is the 6000, again nearer in price to the 176 buy currently a bit out of date. Buy the 176c or a cheap end-of-line 175 for handheld chart plotting.

The realibilty of the system is binary - it's either working (cos your kit is switched on with batteries and the satellites are transmiting and we aren't at war) or simply not working. It won't accidentally postion you somewhere else a bit, like a novice navigator with a crap pencil or the wrong chart.

 

[davel]

WAAS and WGS84 are different.

WGS84 refers to the datum system used to tell you where you are. Different charts use different datums and can vary by anything from a few yards to a few hunded yards (ie the GPS on one datum says you are at poition X,Y whilst the chart will show the point your at is A,B. Most GPS units will use WGS84 as their default datum system but will allow you to change it to match the one on your charts via a menu selection.

WAAS is an enhancement being provided to the GPS source signal (from the satellites) which will improve the positional accuracy still further (I've seen claims of a few meters accuracy). WAAS is not available everywhere. Latst I heard it was in use in the USA but not yet deployed in Europe and Asia.

WAAS, does nothing to deal with the datum issue.

Dave L.

 

[dickh]

I understand that WAAS is only available in the US and is so slow being rolled out that West Marine say not to bother! Good info about this in their catalogue and also that GMDSS is not much good either as that is v slow in being rolled out!

dickh
I'd rather be sailing...

 

[colvic]

Majorcas a large Island so you shouldn't be impressed with your GPS unless it shows the pontoon or finger you are tied upto. 5 metre accuracy is the norm over here at the moment.

 

[HaraldS]

WAAS seems to work. As long as I was cruising in the Atlantic this summer, I always found a WAAS signal transmitted from the Atlantic Region West geostationary Imarsat satellite. Similarly, the Atlantic Region East satellite was sending the European equivalent EGNOS. My receiver was happy with both and switched to using EGNOS when I left the differential GPS range of northern Europe.

As to the relation to WGS84:

Like differential GPS, which is transmitted from terrestric stations on LW frequencies, WAAS and EGNOS are correction signals that help improve GPS accuracy and also help monitor system health. For example they tell the receiver when a satellite is faulty and shouldn’t be used for position fixing.

Charts are surveyed relative to a position on earth and are only correct relative to that position, called a chart datum. The LAT/LON of these positions has previously been fixed by means of astronomic observations of varying accuracy, and hence the Lat/LON printed on many charts isn’t quite the same as the one that GPS calculates. If a chart is labeled WGS84, it has either been corrected to match GPS coordinates or has been newly surveyed.

The errors of non WGS84 charts can be huge, (several hundred meters), the improvement in accuracy that WAAS bring over standard GPS is only a few meters. So the bigger concern should always be the chart accuracy, not the GPS.

Where WAAS or EGNOS comes in handy is when you need accurate reproduction of positions, like when you want to take the same path back out of a difficult approach or passage. Standard GPS, (after SA had been switched off), is said to reproduce to better than 15m, differential to better than 3m and WAAS or EGNOS are said to be somewhere in between the two.