The Corrib GPS Chart, and all other GPS charts and fishing maps available through through AnglingCharts, are created as Marine Charts. I realise that many people using the chart have never had access to a marine chart, their only previous use of a GPS being for automotive use, and they are unaware of the symbology and capabilities of a marine chart.
To try and help clear up some of the confusion here are some guidelines.
1. Marine Charts are not routable. You cannot instruct your Nuvi to“ show me the route from Oughterard to Maam”. The unit has no knowledge of your draft, the weather, where is safe for you, and where isn’t – no more than your car GPS will tell you it’s OK to go off-road and there’s a herd of cows around the corner . Marine charts have recommended routes drawn into the chart, but it is entirely up to the user to follow them if they so desire. The reason you can route a road map is simply because everyone stays on the thin grey bits (the roads) and if you ask the map to take you from A to B then it will use the roads, and only the roads to get you there. It will also assume you are already on a road in the first place, and that your destination is on a road. If you tell your GPS to plot a route from Waypoint A to Waypoint B on a marine chart it will do so as a straight line between the two points.
2. You can create your own routes (on most GPS’s). By looking at the chart and using your knowledge of the capabilities of the boat (draft, manoeuvrability, depth of water etc) you can decide upon a route, and then enter that route into the GPS. The GPS will then give you guidance in following the route – alerting you when you approach a waypoint etc. If you draw your route over a dangerous shallow and you have more draft than the available water - you will hit it. It is entirely up to you to check the depth of water in the lake before you set out, and to know the depth your craft requires. There are still thousands of uncharted objects – so a safe speed is also a must.
3. The Blue Bits. The areas marked in light blue on the Corrib GPS chart are areas where the depth of water at normal lake level (0.6m on the gauge at Lisloughrey) is 10ft or less. The darker blue areas are SHALLOWS. They can be anything from 5ft to zero. The Shallows are more or less complete for the whole lake, but are only as complete as I can make them - this is not a professional hydrographic chart. Shallow areas are areas I consider dangerous. They are frequently a mass of large boulders, many of which will take your propellor off or set you aground. It is not possible to chart evry boulde, nor have I made any attempt to ! I've simply charted the ones I have come across. If you want to minimize your chances of hitting a rock avoid the blue bits.
4. Charted rocks are marked with a small cross on Non G2 GPS units (Nuvi, GPSMAP 60csx etc) and as a cross in a dotted circle on G2 units. These rocks may be visible or below the surface – they are simply dangerous. It is entirely up to you if you want to hit one.
5. The old marking system on the lake was unique in the world – and has been replaced with IALA standard markings. The marker positions should not be relied upon until the Corrib Trustees issue the final positions, which, as of April 2014 had not happened. Even when the positions are confirmed - there have been no floating markers on the lake until now. Possibly for good reason. Floating marks have a tendency to move and should always be regarded with caution.
6. The depths are being updated all the time with new accurate data. Many holes in excess of 100ft have been found, particularly in the area around Oughterard and there are a good few more areas of less than 10ft that don’t appear in earlier editions of the chart. These updates were contained in revision 3.00 and the newer charts have substantially more information than the older editions, including launch sites and slipways. This does not in any way guarantee that all shallows and rocks are charted - they are not. I've simply done the best I can to chart as many hazards as possible.
7. The Corrib GPS chart is designed as a fishing map, for guidance and situational awareness. NOT NAVIGATION. If it helps you get from A to B with your prop intact and the water mainly on the outside then that’s brilliant. If you hit something, then as you are probably already well aware, it’s a very dangerous lake and you venture onto it entirely at your own risk.