Please read - and understand - before closing; I do not yet have an answer. Thanks.
I made a mistake. I phrased my original question so that it could be answered by recommending an application. It was duly answered, I realized my mistake and awarded the answer then posted a second question explaining what I really want.
I thought that I was clear, but apparently not clear enough as my question was closed as an exact duplicate by a moderator. Oh, well, worse things happen at sea ;-)
I will try again:
I don't really need an application, but rather an API. I plan to develop my own application and don't need someone else's GUI, just their information.
I want to know the distance a vessel at sea must travel between two points. Obviously this isn't just a straight line thing, if there is land on that straight line.
In my previous question, Geoist asked "You want something that takes into account most efficient routes given currents, tides, etc?".
My answer is, yes, please, if that is possible. If it costs extra, then I would prefer to start with a free solution and add that later.
Now, I can't expect everyone to be an application developer like me, so I can't expect everyone to understand the difference between an application and an API.
Briefly, an application is a whole, complete, stand-alone program, something like a Windows .EXE file or even a browser based program.
Whereas an API is an Application Programming Interface; it provides and interface which allows a different application to get at some of its inner workings (like a Windows DLL file, or just accessing someone's database, but not using their GUI (which is exactly what I want to do)).
In this case, I am looking for an interface which will allow me to query the distance between two GPS points, possibly with route planning, etc.
I hope that the distinction is clear and that everyone can see that this is not a duplicate of my first question. If in doubt, please ask for further explanation, rather than closing this question.
Thanks a million; sorry about the long explanation and I hope that this doesn't come over as antagonistic (it's difficult with writing). I hold no beef with the moderator who closed my second question.
Thanks in advance for any help.
No-one? Btw, does anyone have an email address for the author(s) of OpenCPn? I can't seem to register at their forum
[Update]I agree with @chriserik route plotting at sea is not as simple as taking the distance "as the crow flies", because the crow in a boat which is subject to tides, winds, currents, etc
Note that I could just run OpenCpn and monitor the HTTP traffic from the program in order to reverse engineer the interface and then writ emy own program. But I won't do that on ethical grounds. Can anyone put me in touch with the author? When I try to register it says that my IP address is a known spammer (and said so the first tiem I logged in). Perhaps he and I coudl produce a joint effort which could be used by others.