Re: WGS84 offset

Jason Cox wrote:

> 	I need to create scenery for australia but have found that there is a
> problem in that all scenery is of by about 1km. Is there a way of
> setting an offset  when the dem files are cut up and the layers are
> produced.

Are you sure it's the DEM that's off?  The 30 arcsec DEM can be up to 1km 
off by definition (since that's its nominal resolution), but if you're using 
the new 3 arcsec SRTM data, it should all be in the right place, aside from 
the occasional anomaly.

What is it that the elevations do not sync up with?  Is it the airports, 
shoreline data, or land use data (i.e. cities, roads, rivers, forests, 
etc.)?  Airports should be pretty accurate, though there are still some 
problems.  The GSHHS shoreline data for inland bodies of water is pretty 
bad: it is more than 1km off in the Great Lakes for Canada and the U.S. (so 
waterfront airports are either floating out in the lake or far inland), but 
the other hand, vmap0, while less detailed, is pretty-much bang-on. 
However, Norm has pointed out that vmap0 can be way off on the *ocean* 
coastline data, while GSHHS is usually accurate.

One thing worth noting is that vmap0 data we use for land use is based on 
1:1,000,000 World Aeronautical Charts (WAC) and similar publications.  On a 
printed WAC, a 1 km error is barely noticeable because of the low resolution 
(nominally 1mm=1km), so it shouldn't be surprising to find errors like 
those, especially outside of North America where the data have not been 
tested as much.  There are much better-quality free data available for the 
U.S., but the rest of the world, unfortunately, hoards its geographical data 
so that we cannot use it.


All the best,


David