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Maptech digital charts
On Thu, 15 Jan 2004 13:04:49 -0500 (EST), (Ron Thornton) wrote:
As I recall, every time I have looked at the free charts there is a
disclaimer "not for navigation" on them. Is this because the resolution
and detai is not as high as the commercial charts or is it just
government. "CYA".
Regards, Ron
it's resolution, which triggers the CYA
When NOAA started researching going digital they determined
254 dpi as the minimum resolution to capture the smallest critical
detail of their charts and is / was the resolution the digital raster
charts are delivered at.
the online versions are about 85 dp and as such do not meet their
minimum use standards. maptech puts them there to help you id the
chart you want to pay them for, while plenty of people use them for
overall planning and cruising.
btw: approx : 1 pixel on a 1:40,000 at is about 13.7 ft for 254 dpi
scan on a 85dpi each pixel is 41 ft in size.
for real CYA , if I remember correctly from the CRADA, Maptech is
protected by the US Gov't the same as the Gov't charts
they are an aid to navigation and the prudent mariner watches where
he's going (as they say on every chart)
before they had the CRADA, BSB/Maptech scanned the paper charts just
like everyone else. The bid they won got them access to the color
separations which they scan at 3x the delivered resolution.
Now they split between the color separations and digital data.
I haven't kept up with the ratio ...
disclaimer: most of this is from memory from the CRADA and NOAA
followups and explanations of the precess, so buyer beware on trusting
any number data
- Ed
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