Scanning resolution

[Ed McNierney]

Lester -

I forgot to mention that in terms of visual quality you should reduce
spatial resolution first, then color.  The eye is more sensitive to
color changes, so if the tradeoff is between high resolution/low color
and low resolution/high color the latter is usually the more visually
appealing.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242  

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Lester Caine
Sent: Wednesday, December 07, 2005 10:02 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Scanning resolution

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.