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.
