Scanning resolution

[Lester Caine]

Ed McNierney wrote:

> Lester -
> 
> Question 1:  I expect that very few users will need or require 1200 DPI
> resolution.  It is helpful, when working with line art source images, to
> scan at high resolution and then descreen/filter/clean up the image to a
> lower resolution.  Going to 400 DPI gives you excellent resolution and
> will fit on one CD.  Your numbers also suggest that you are using 24-bit
> images.  If your source is line art, that may be neither necessary or
> appropriate; that is, if there are only 8 colors used on the original
> map, an 8-bit image ought to be all you need, even allowing for some
> intermediate shades along color boundaries.  That will also greatly
> reduce the data set size; 4.3 GB is a lot for that map, even assuming
> you meant to say it's 101 CENTImeters by 90 cm!
Yep that is the size ;)
The Bartholomew maps have different colour brown contours and and blue 
shading in the sea areas, but you are right, I can look at the effect of 
decreasing the colour depth, and having has a play, 300 or 400dpi seems 
fine for the base resolution - dropping from 1200dpi gives me a few 
options to play with filtering as you point out.

> Question 2: 100MB tiles are generally manageable but a little on the
> large side.  Chopping them into 20MB tiles will help; you should also
> use the GDAL option to create "TILED" TIFF images for better data access
> speeds.
20Mb seems to be an ideal sort of size.

> Question 3: I like to create intermediate images every 2x the original
> resolution (1200 DPI, 600 DPI, 300 DPI, etc.).  The first one is the
> hardest, since it requires about 25% additional storage over the
> original source data.  The second one is only 6% of the original, and
> all the rest are tiny.  However, you do NOT want to create a large
> number of tiny files, as this will greatly slow your access speeds.
> Once you get to those levels, you should use the GDAL tools to glue the
> tiles together into reasonably-sized ones.
If I start at 400dpi it makes life easier anyway, so I think I have may 
starting point.

> Good luck!
Thanks - its been a while trying to find the time to play with this, but 
once I get started I should be able to get a 50 year spread for the 
whole of the UK :)

-- 
Lester Caine
-----------------------------
L.S.Caine Electronic Services
Treasurer - Firebird Foundation Inc.