Re: [Flightgear-devel] Use of old maps

[Steve Hosgood]

David Luff wrote: Steve Hosgood writes: My only comment is just that 1937 maps will certainly be before the National Grid was adopted, and will be based on the "old triangulation" done between the late 1700's to mid 1800's. I don't know the details, but it wasn't metric (possibly surveyed in "survey chains" or thousands of yards or royal Babylonian cubits). The first OS maps based on the "new triangulation" (which was metric), and with the now-familiar National Grid didn't appear to the public until after WWII. Whether or not you can get an "old survey" map to line up with a "new survey" map (or even with reality!) is not obvious. I'm not even sure if the mapping projection of the pre WWII maps is the same as today's (i.e Transverse Mercator based on the "Airy 1830" spheroid). It certainly isn't WGS84 which IIRC is what FG's terrain is based on. Hmm, I've got a feeling that the 36 in OSGB36 might refer to 1936, in which case you are probably right - post-war maps should be OK - I've got no problem converting to and from WGS84 <==> OSGB36. It seems that 1" maps from the 7th series in the mid fifties to early sixties are fairly widely available - I guess that these are probably the best to go for at the moment. I will look out for one to a hilly area to try as a proof-of-concept. Check out: http://en.wikipedia.org/wiki/OSGB36 You are right: the "36" refers to 1936, the date that they created the "OSGB36" datum, but the underlying spheroid is still the 1830 Airy Spheroid, used by the earlier survey. However, the 1936 resurvey was done with technology 100 years advanced from the original survey, and was way more accurate. They brought in the trig points, still familiar to hill walkers today, and introduced the National Grid and had the foresight to do it all in metric - still regarded as rather an alien concept back then. Regarding the rest of the thread - yes, the OS jealously guards copyright in the UK. Other agencies are just as bad - it was very difficult to find online tide tables for dates in the future last time I looked. Try "xtide". There is (allegedly) a port called "wxtide32" for windoze people.