Scanning resolution

[Ed McNierney]

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!

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.

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.

Good luck!

	- 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 4:06 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] Scanning resolution

OK - I have probably gone OTT on this, but I've got the first map for my
archive scanned and the raw tiff's are looking good. It's the Contour
Motoring Map of BRITISH ISLES published by the AA in 1920. (16miles to
Inch) A single page covering the whole country, 101mm by 90mm and I have
a scan of it in 40 tiles approximately 5200 by 9600 pixels - 1200 dpi. 
The resulting data set is 4.3Gb ;) So I think we are talking a DVD
rather than a CD :)

Question 1
I will maintain the raw scans at that resolution, as it is easy to
achieve, and allows me to adjust alignment so that all the tiles are
'vertical', but should I be looking to scale the raw images to a lower
resolution for general use (CD size rather than DVD)

Question 2
The next step is to produce the geotiff data so that the map will appear
as a single layer in MapServer. With reference to the resolution, will I
be better off splitting the 40 tiles (currently around 100Mb each) into
small chunks of say 20Mb each (2600x3200 approx)

Question3
I currently have an overview of the whole map which is 800x900 pixels
(40 108x183 tiles) what would be the best method of scaling to cover
intermediate scales, produce just one more 'scale', or a couple?

What ever I do now will determine how I handle a stack of some 300 other
OS and Bartholomew maps I have that are already out of copyright, and
about the same again that will 'become available' over the next 10
years. Given that the data sets are going to be large, I will have to
charge for CD/DVD copies, but the plan is to have a low resolution copy
of each map available for free download.
http://home.lsces.co.uk/Maps/AASeries/BritishIsles1920/AABritishIsles192
0Index.html
Be warned I've only got a 256k uplink so the high res sample of the Isle
of Map will take a time to download :(

As a slight aside to the main questions - can someone point me to a good
'beginners guide' to building the geotiff data - flat tiles first, I'll
worry about projection later ;)

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