NavList:

Hello Tony,

You wrote:
"Is it legal to access your USNO Clone page to obtain the data for a CN simulation software? I mean - may be Eric van der Veen or somebody else could use it "under the hood" by parsing the page contents at certain moments of a flight simulation?"

Yes, it's legal, and pulling data from web services (sometimes charmingly known as "scraping" a web page, especially so in the early days of the internet) is a common solution for scenarios like that. Sometimes this is abused, and there was a major US court case recently affirming that it's legal. Of course there are better ways, and there are other services besides the "USNO Clone" app including my Nautical Almanac app which is using the same engine but with more display options (the case I have linked here is an interesting example of the options available). And there are lots of other tools both from NavList members and other sources.

While it's legal, there are downsides to using web sources like this. The biggest is that you have no contract with the creator (me or otherwise), and the source could go off-line the day after you complete your project. Or just as annoying, the source might change the format or the data access method. In fact, this is a standard tactic used to reduce unauthorized access. Sometimes folks suggest that this is a problem when using minor resource providers, like me, but in fact the biggest example, by far, of a service going down in our field of interest is the US Naval Observatory Astronomical Applications department itself, which has been down for about a year and a half (see screen capture below). Another downside is scalability. For the scenario you're describing, you might never see more than a dozen users and no more than a hundred hits on the web service in an hour, which would barely register as activity. And one user is mostly indistinguishable from another. But if you think you might create something that could one day have a thousand users and hundreds of thousands of hits in an hour, then you would probably overwhelm a small web service (or cost the provider some money) and in that case you would probably be blocked since your activity would have a signature (IP address or otherwise). Any problems like this can usually be handled with a simple contract covering appropriate use. You don't have to worry about that for small numbers of web hits. I'm just filling in a few details... just so's you know.

Frank Reed

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