What confidence do you have in your charts?

[RaymondR]

The fact of the three decimal places in the boundary coupled with that of the UTM mapping of the entire continent and fairly extensive experience with government bureaucrats was sufficient to reasonably arrive at that conclusion. The placing of the boundaries were not constrained in any way by existing monuments or physical features do location them to the nearest whole minute was entirely possible and considering the vast areas encompassed logical.

Whilst it is true that the tectonic plate Australia inhabits is travelling northwesterly just about everything of interest to the navigator is travelling with it and (to throw a fox into the chicken pen) there's no doubt that global warming will increase it rate of migration and as the extra heat softens the entire earth and the oblateness increases a new spheroid model will be required (I'm reasonably certain there is, or soon will be, learned papers out there on these subjects soon)

[Alan Mighty]

Originally Posted by requiem

There's also the minor problem that Australia is a bit of a speed demon; it's hurtling northward at about 7 cm per year and I believe just a few years back there was a 1.8 m correction to re-sync the Australian datum with WGS84.

Yair. The Geocentric Datum of Australia 1994 (GDA94) has been replaced with GDA2020, to account for continental drift which, as requiem wrote, is about 7 cm a year on average and vaguely towards NNE (and definitely not NW).

See attached graphic (GDA 1994 GDA 2020.jpg, about 100 KB) showing rounded values (in metres) for the displacement of selected points on the Australia plate from 1994 to 2020. The directions of the vectors are not reliable.

The Aus continental plate seems plastic on this time scale. So some locations have displaced by 1.8 m, others only 1.5 m. Definitely not a case that everything is being displaced the same.

I've also attached a clip (72430.jpg) from a 2021 official survey control report on a benchmark that happens to be local to my land base (it's on a high point near the CBD of Brisbane and has been used for terrestrial triangulation work for more than a century). That permanent benchmark was established in 1980. The clip shows the latitude, longitude, easting, northing, and ellipsoidal height with respect to WGS84.

Using the 1994 and 2020 lat long, I calculated a haversine distance of 1.56 m for the displacement of that benchmark from 1994 to 2020. I neglected to calculate the true direction of displacement.

Survey control mark reports for benchmarks in Aus are published by state governments. If you're in QLD, for example, and know the 6-digit identifier of your local benchmark (and adding a leading zero if necessary), you can find the relevant report using the format: qspatial.information.qld.gov.au/SurveyReport/SCRxxxxxx.pdf.

[requiem]

I try to do similar with routes, but too often wanting to clear one object or pass close by another meant got in the way of making the numbers pretty. Of course, if it's all open water there's little stopping that. It also occurred to me that someone may have simply placed an X mile boundary around every object of interest and then simply clicked around to connect the bubbles. A number of positions do seem to be round numbers, or at least to the tenth of a degree.

Digging into the datum question, the maps available at https://www2.gbrmpa.gov.au/access/zoning/zoning-maps are using the GDA94 datum, as do the official numbers in the zoning plan, so anyone working off them would need to be careful to convert them to WGS84 first, rather than entering them directly.

[Kelkara]

Originally Posted by Alan Mighty

The Aus continental plate seems plastic on this time scale. So some locations have displaced by 1.8 m, others only 1.5 m. Definitely not a case that everything is being displaced the same.

Its not that the plate is behaving plastically ... but the nature of spherical geometry means that a simple translation of a rigid plate on a spherical surface is actually a rotation about a pole ... Places further from that pole will move faster ... It would seem that the pole of rotation for the Aus plate is somewhere to the east, it probably is close to a uniform rotation.

[Talbot]

Originally Posted by Kelkara

........ but the nature of spherical geometry means that a simple translation of a rigid plate on a spherical surface is actually a rotation about a pole ... ...........

Except the world is not a sphere but and oblate spheroid. What does oblate sphere mean?
Something that's oblate is rounded, but instead of being a perfect sphere, it's flattened slightly on the top and bottom. Although we imagine the Earth to be shaped like a basketball, it's actually slightly oblate.
The first attempt to describe this was WGS72 (WGS= World Geodetic Standard)This was reasonable for early Satnav, but not good at higher latitudes, so for GPS they needed a better description and WGS84 was created. In UK all charts are at WGS84, and I would be astonished if Australian charts are not.

If you ignore continental drift, charts from original surveys are unlikely to change much if the seabed is rocks. If it is sand, then it is only good til the next storm. So looking at the nature of the seabed on your chart is pretty important.

Running without an echo-sounder working is not a good idea, as that will probably be the only telltale for a submarine.

[Amapola]

Paper and electronic charts are an important part of our cruising. However, we are also vigilant in using other instrumentation and visually keeping watch.

[requiem]

Originally Posted by Talbot

Running without an echo-sounder working is not a good idea, as that will probably be the only telltale for a submarine.

What, you're not watching for the seagulls or a periscope?

https://youtu.be/Q0yL0ddfgWc?t=1182

Edit: I'm not sure you'd have much time if you saw the numbers dropping rapidly, or good data on which way to move. Hard to starboard and hope for the best?

[Colin Stone]

A small RN survey vessel was undergoing an inspection. The inspecting staff asked where an inshore chart was and proceeded to mark the Survey Department down.
The ship's Hydrographer replied "We haven't surveyed that area yet".

[Jon Hacking]

I'm really surprised that nobody on this thread has mentioned satellite imagery for their charts. We found that virtually ALL commercial charts for SE Asia were pretty poor - often low-resolution & displaced, especially when we got off the beaten track (exactly where we like to cruise).

So, since 2014 we've been making our own charts, mostly from satellite imagery. We started with KAPs made from Google Earth, but now we've moved on to mbTiles made from GE, Bing, & ArcGIS, in addition to nautical charts from CMap & Navionics. The nautical charts aren't as accurate, of course, but they have depths, which are usually pretty accurate.

With OpenCPN, I can put these into different Chart-Groups, so I can switch between them with a single key-stroke, allowing me to choose the best chart for my current situation in about 2 seconds.

Satellite imagery is amazingly accurate, with max errors on the order of 10' or 3m. You have to see it to realize just how accurate it is. You can usually see into the water down to a depth of ~30' or 10m, & often much deeper. Coral heads & reefs can all be seen & they're exactly where they're plotted. It's also easy to tell between sand & reefs, so very easy to choose good anchoring locations.

To answer the OPs original question about confidence in my charts, I'm EXTREMELY confident in them. As in, I could maneuver through reef-strewn Morovo Lagoon in the Solomon Islands on a moonless night, & have absolute confidence in my location relative to the reefs around me (see attached image).

BTW, I've now published a page so other cruisers can download our charts (mbTiles). They're designed to work with OpenCPN, which works on many operating systems (Windows, Mac, Unix, Android, etc) but generally not chart-plotters or iWhatsIts. We use a tablet at the helm with screen sharing SW (like TeamViewer) to duplicate the screen of our nav-computer (at the nav-station).

My published mbTiles are made from ArcGIS, Bing (MSFT), & Google Earth satellite imagery, as well as nautical charts from CMap & Navionics. They go from zoom-levels 10-18, which is detailed enough to see individual coral heads. The current list of countries I've charted (from east to west) includes:
Eastern Caribbean, Galapagos, Micronesia, Solomon Islands, PNG (mostly islands), Palau, Indonesia (& East Timor), Philippines, Malaysia (including Singapore & Brunei), Thailand (west coast), Sri Lanka, Maldives, Chagos, Seychelles, Madagascar (west coast), Comoros, Kenya, Tanzania, & Mozambique.

[Jim Cate]

^^^^^
A worthy and generous offer to the CF sailors, Jon! I just wish you had had a go at Vanuatu on your journeys, for the charts there are pretty poor. If we ever go back, we'll try to make our own, and if successful, will make t hem available as you have done.

Thanks for your efforts.

Jim

[Jon Hacking]

Originally Posted by Jim Cate

A worthy and generous offer to the CF sailors, Jon! I just wish you had had a go at Vanuatu on your journeys, for the charts there are pretty poor. If we ever go back, we'll try to make our own, and if successful, will make t hem available as you have done.

Thanks for your efforts.

Jim

Thanks for this, Jim

We're constantly adding to our library of charts (& we're hoping others will help) & I'll be uploading & posting them when I can. The many island nations in the south Pacific are high on our list, as many cruisers cruise there & we know the charts are sub-optimal there. But it takes us a LOT of time, & a huge amount of internet bandwidth, which can be prohibitive in many countries. We're considering Starlink to help here but it hasn't been available in SE Asia. We'll have to see how it goes...

Back in 2015, we were cruising Indonesia with an Oz trawler who believed their Navionics charts too much & went aground. Their Navionics charts were only out by 150m, but that was enough to miss the channel. They sprung a plank when the tide went out, & never floated again. Their insurance never paid out, so they lost everything.

Since then, we've been doing what we can to get our charts (which correctly show things like channels) into the hands of as many cruisers as possible.

[jo_sail]

Originally Posted by Colin Stone

…

The ship's Hydrographer replied "We haven't surveyed that area yet".

Following this short RN story and reflecting 21st century data culture: I can suggest to look for the following opportunity for supporting and contributing to better availability of detailed open source depth information in all those interesting coastal areas and nice anchorages, which are so important to cruisers…

https://depth.openseamap.org/

[twig]

I found it all depends where you are. Great Lakes and they are very acurate. Coastal Florida pretty good but do your better be careful

[RaymondR]

I doubt that I'll ever trust any chart enough to feel the need to adjust for continental drift.

[Mike OReilly]

Originally Posted by twig

I found it all depends where you are. Great Lakes and they are very acurate. Coastal Florida pretty good but do your better be careful

Not all the Great Lakes. North shore Lake Superior charts are based on some pretty old soundings; outside of the few urban areas, or commercial/industrial ports.