Can we trust DIY charts?
How can you make your own bathymetric charts, and are they any good? Duncan Kent heads to Portsmouth to test Navionics’ SonarCharts
You may have wondered, when out sailing, why your chartplotter can’t take readings from your echosounder and plot them on your charts. Well, Navionics SonarChart (version 7.4+, released last year), does just that. With a compatible plotter and Navionics’ mobile ‘Boating’ app, you can record your soundings and apply them to a ‘user-generated’ chart overlay by uploading them to the SonarChart database. These user-generated charts can then be accessed by any mariner using Navionics+, Platinum+ or HotMaps Platinum cartography in their plotter, or via the Navionics Boating app.
Six years ago Tim Thornton, a digital cartography expert, started a project called TeamSurv (www.teamsurv.eu). The idea was for leisure sailors to collect depth data using software or hardware supplied by the company, recording the sonar readings from the contributing vessel’s echosounder. At the end of the trip you could go online, upload the data to TeamSurv’s database and view it sometime later, after it had been recalibrated to take account of tidal heights, barometric pressure and so on.
Six years is a long time in technology. Now user-generated charts look like they’re here to stay, but are they reliable?
How we tested it
The plan was to check how SonarChart worked at sea on a mobile device, and to see if there were any marked differences between its user-generated bathymetric charts, the standard Navionics Boating app charts, and a Navionics chart package available for most modern chartplotters. For this, we invited Digital Yacht’s chief technology officer, Paul Sumpner, on board to help us out. We used two iPad 3s, an iPhone 5 and a B&G Vulcan 7 plotter loaded with Navionics’ Platinum charts.
Our research identified three areas just outside Portsmouth Harbour in which the user-generated data appeared to contradict the standard charts: the inner swashway, Horse Sand Fort and the main passage through the submarine barrier. We took our own readings (which were different to either version) and uploaded them to the Navionics SonarChart database.
What do I need?
To capture your own bathymetric data, you’ll either need to link your boat’s echosounder and GPS to the Navionics app on your phone or tablet, or to use a compatible chartplotter that can record the sonar data and store it on
‘Our research identified three areas, just outside Portsmouth, where user-generated data contradicted the standard charts’
a Flash memory card. Those without a Flash card will need an interface on the yacht’s instrument network, such as Digital Yacht’s Sonar Server (about £100), which can send the data wirelessly to your portable device. For yachts with NMEA-2000 (N2K) networks, Digital Yacht’s NavLink wireless NMEA server (about £425) can be used instead, although this is considerably more expensive and not striclty necessary for sonar charting only.
There are other wireless servers available, such as Navico’s GoFree WiFi-1 and Raymarine’s Dragonfly 4 fishfinder module, but some marine Wi-Fi routers don’t have the ability or codes to connect to the system, so check with Navionics first. Another alternative, which is ideal for tenders and dinghies, is to use a SonarPhone portable transducer (see p81).
Installing the server
Our test yacht had an N2K network with Garmin instruments and a B&G Vulcan plotter. We connected a NavLink device to the N2K network to act as our Wi-Fi router, which has a built-in Actisense NGW-1 converter to ‘translate’ signals from N2K to NMEA-0183. It communicated fine, but we got different depth readings on the Garmin instruments and the N2K network, due to the app not receiving keel offset data. Apparently this was a ‘NMEA priority problem’ (don’t even go there) with the Actisense software, this has now been fixed.
Calibration and tidal offset
When processing the sonar data from your boat’s depth transducer, a couple of important factors ensure the accuracy of your soundings. The first is the tidal height where you are sailing. When SonarChart Live was first introduced, it had no means of taking local tide height data into consideration, so the resulting data was pretty much worthless. Now, however, it offers the user three options: set the tide station manually; allow the app to choose a default tide station; or manually cancel the tide selection and apply none (leaving the Navionics main database algorithm to calculate the variations from its own known water depths), which, in my opinion, is fairly pointless.
The second factor is your echosounder’s keel offset, which Navionics’ database needs to deduce the correct depth from sea level, and thereby the Lowest Astronomical Tide for the chart. One of my concerns is that sailors tend to add a bit on to their offset as a safety margin, which in turn creates an unknown inaccuracy in data transmitted back to Navionics. Can we trust all SonarChart contributors to reset their sounders accurately before they start recording data?
Testing the Wi-Fi
The NavLink Wi-Fi server transmitted soundings and GPS data from our boat’s instruments network, which could then be received by any phone or tablet running a compatible navigation app (in our case two iPads and an iPhone). Depending on how your instruments are set up, the Sonar Server or NavLink can either be wired to your plotter directly or to a network via a simple, two-wire NMEA-0183 connection.
Once power is applied, you simply connect to the server by going to the Wi-Fi settings on your phone or tablet and selecting ‘Digital Yacht Sonar Server’ from the list of Wi-Fi networks available.
Using SonarChart Live
Once a wireless sonar device is detected, SonarChart Live will immediately start to record bathymetric data. This is seen as a trail of clearer, more detailed contour lines and depth values around the vessel and along her track. Contours and depth area colour shading displayed on the SonarChart reflects the depth values, after subtracting the current tide height. Your tablet, smartphone or chartplotter will need some free memory capacity to store the sonar data until you are next connected to the Internet.
Once the phone or tablet is online (or your Flash card is taken out of the plotter and plugged into a computer that is logged on to Navionics’ website), it will automatically and anonymously (unless you have registered your device) upload your data up to Navionics’ own server, were it will be added to the user database within a week or so and made available to view by others using the latest version of SonarChart.
If you’re going to the trouble of setting your boat up to take part in the SonarChart project, you might as well go the whole hog and register so that you can see which updates are yours when they are processed.
Test results
To view the results of other contributors, we used the Navionics HD Boating app on iPads and an iPhone. When we first booted up the app it opened, as usual, with the original chart. We then overlaid this chart with the latest user-generated sonar data, simply by pressing the Options icon in the bottom lefthand corner of the chart and selecting the SonarChart button.
If you are navigating in a busy area, as we were (Portsmouth Harbour approaches), it will be immediately obvious that you are using the sonar charts as there will be a much denser projection of depth contours along frequently travelled routes.
When using SonarChart Live, the data from your instrument network will appear immediately on your tablet or phone in a new window, but also remain in store until you return to an Internet connection, before uploading to Navionics’ database. Your data will not appear on the worldwide SonarCharts database, however, until it has been filtered
and verified by Navionics, which usually takes around a week. Those with registered accounts can view their log updates online to follow the status of their latest uploads.
Once the data we (and other users) had collected was uploaded, it was clear to see the areas where our data had affected and changed the previous Sonar layer. The chart seemed more consistently accurate and more like the official charts currently in use.
By using computer software to overlay the screen grabs of the new Sonar layer onto our boat’s track on the day, we could see where our bathymetric data has caused the depth soundings and contours on the Sonar layer to be corrected and redrawn. One of the biggest differences was the position of the inner Swashway and the shallows to the south of it, which, according to our data, are smaller and slightly further west than they appear on standard Navionics Platinum charts.
Important updates
The SonarChart database is updated daily, so it is important to ensure you connect your Wi-Fi device to the Internet to download the latest data before you leave port. During our test, the three of us forgot to update and synchronise our phones and tablets before we set off, so none of our SonarCharts looked the same! In fact, some had wildly differing depth contours and non-existent channel and sandbanks, most of which disappeared when we got back to shore and updated our apps.
While you don’t need a current subscription to use SonarChart Live, you do need one to download updates from Navionics’ usergenerated database to your SonarChart, whether they originated from yourself or from other contributors.
Is it worth having?
The marine electronics industry is pretty slow at reacting to trends – most likely because it’s a small market sector. Once one of the big boys gets hold of an idea, the others tend to quickly follow. The more I know about the depth and seabed in a shallow channel or anchorage the better, but that’s about it.
What really concerns me is that usergenerated charts are an opportunity to introduce dangerous errors into a system that, in itself, is already susceptible to a number of possible inaccuracies. There have been rumours of dodgy soundings allegedly finding their way onto Navionics’ main database, although Navionics robustly stated this was impossible when we enquired. In our own experiences on the water we were surprised at the dramatic differences between the sonar charts on our three devices. Yes, they were all at varying update levels (all within a few weeks of each other) – but isn’t this likely to be the case amongst users? Many sailors use their old laptop and tablet on board, which can be difficult to keep updated.
Add to this concern the possibility of offset error and I’m afraid I can only recommend you take this crowd-sourced bathymetry with a pinch of salt and add it to the ever-growing bag of back-up navigation tools.