You also need to understand how OpenSeaMap was produced.
So OpenSeaMap is produced from the same origins as OpenStreetMap -an open project aimed at mapping all the roads in the world. Idea being people who are interested mapping go out with a GPS and map stuff and upload it. They have in quite a small time achieved a bloody good map for the UK and in doing so have created something that can be updated in relatively quick time when new roads appear. And its "free" as in speech as well as beer... ...so if you wanted to build it into a commercial application if you wanted to (of course your commericial application may not update as often as source maps do!). You can also take the raw data and re-draw it in your own colour scheme etc to suit your own needs, omit layers of data etc.
Open Sea Map has taken that data and is adding 'sea' data to it. **BUT** there are at least two snags you should be aware of:
1. If the data was put on using a GPS to locate the object it could be +/-100m from correct location as they wont be using commercial grade mapping GPSs. It is possible its within 3m, but the risk will always be there. Add to that your own +/-100m error and you can see where the risk can become large...
2. Anyone can edit it. I could edit something in or remove something sat on my sofa never having been there. There are no quality checks on individual edits, instead it is reliant on other map users identifying and reporting errors.
3. There are NO depth data for the vast majority of what you will be looking at. There is world-wide depth data for depths > 100m! It is possible to put depth data into OSM, and crowd sourcing methods are used to do it ... but there is a lack of data and I suspect even when that becomes more populated they will give you lots of data for channels etc but very limited data in the little bays and nooks that Ribbers might want to adventure because it needs someone to have ventured there before you to provide the data.
There are two further errors can arise as a result of this:
- If your sounder is not set up correctly you will mis-record depths, in addition to whatever the inaccuracy of the sounder is anyway. I've certainly known yachties set their sounder to be "slightly deeper" than it really is, to give them a slightly bigger margin of error. Lets face it if your sounder is accurate to +/- 20cm you might tell it its mounted 20cm lower than it really is to reduce the likelihood you'll scuff the bottom. Result - all data submitted is 20cm shallower than it should be. Then heel your boat and it messes it all up
- It needs to relate the measured depth to the chart datum. So it must be using an algorithm to do that from pos & time info. Great. But in some places we know algorithms aren't perfect.
Even with all of that you'll get no bottom substrate info.
I think this stuff has immense potential. Someone like Easdale Harbour Trust for instance can spend a couple of days carefully mapping out the Harbour if they choose. Its then their data, their map to do as they want with. They add a pontoon in the future it can be added for all to see.
It'll be interesting to see if the guys at Antares (Antares Charts
) share any of their data with OpenSeaMap at some stage... they are doing it properly and I have to say in the bits that you might really want to know about a hidden rock etc. Rather than trying to be a free replacement for standard charting they are trying to cover the bits where dragon may still be on standard mapping.