Andrew Priest said:I am seeking advice on getting a scanner which will be used primary
for scanning maps which will be printed on A4 sheets (slightly smaller
than US letter size). While recommendations on models would
interesting, I am particularly interested in advice on what spec I
should be looking at.
As far as the scanner goes, it depends on how easy you want your life to be.
On the paper maps that I've scanned, I've had most success (best overall
accuracy and ease of calibration) with scanning a small section at a time.
That is, around the fold lines.
The paper stretches, compresses and generally distorts the image around the
fold lines, so you're limited by an image that will encompass the folds a
little. Then you join them up.
Best if you get the map as one whole unfolded (rolled) sheet, but scanners of
that size are expensive (don't even ask). I had opportunity to scan some paper
maps on an A1 size drum scanner, but it was only monochrome, so I declined.
(the map was virutally invisible in mono).
Patching the pieces together is the hard part. What I suggest here works for
me, and is accurate enough for me. From what I can tell, it ends up being more
accurate than other methods anyway. I'm making the assumption the maps you're
scanning follow the UTM projection. (most printed topo maps are like this anyway).
Take all your pieces, and crop them around the fold lines so there is a
little bit of image overlap around the fold lines. These lines are going to be
distorted regardless of what you do, so you can't really help much here.
Rectify the images, that is, ensure the easting/northing lines are perfectly
horizontal/vertical. Some photo editiing packages enable you to do this easier
than others, but I'll leave that to you.
Stretch the image so the easting and northing lines are perfectly symetrical.
That is, there is the same number of pixels vertically, as there is
horizontally, for each printed grid line. In other words, unless not already,
make the grid lines perfectly square.
Save that image, and repeat for all your pieces. If you're stretching, make
sure all the images are the same. (same pixel count between printed grid lines
across all your images).
Joining is (relatively) easy.
Enable grid lines on your photo editor (I use photoshop, I'm sure others have
the same feature). Load your first (top left or whatever) image, and resize
the grid lines so they fit the image grid lines as best as they possibly can.
They *should* fit perfectly, as you've made the printed grid lines perfectly
square in the previous step, remember?
Copy and paste the remaining images, and align them TO FIT THE EDITOR GRID
LINES, do not worry if the edges do not perfectly overlap. The edges are
folded and distorted remember, so you're never going to get that part perfect
anyway.
Clean up, and crop the image. You *might* have to play with
brightness/contrast of each section if it doesn't match with the others.
Calibrating is REALLY easy.
Choose the correct datum, choose UTM projection, and enter two calibration
points, each in direct opposite corners of the image (well, as far as the
printed grid lines will go anyway). You don't need any more than two points,
since the image is correctly rectified and sized. (now you know why you spent
all that time before). Read the co-ordinates off the map, or if they're no
longer on the image, refer to the correct spot on the paper map.
Mind you, this is only one way of doing it. I've seen images patched up with
super-duper software that have only taken ten minutes to do, but they need 20
calibration points, polynomial calibration enabled, and the calibration process
has taken an hour to get the image close enough to be even usable.
And mind you, not all mapping softwares are as flexible as Ozi when it comes
to badly scanned and patched images.
But that's your call. I'm more interested in accurate maps rather than how
fast one can put them together.
But to answer you question on 'which scanner', any that fits your budget, and
has a large enough view area to cover one map segment to the fold lines. This
covers almost any scanner on the market.
If you're using unfolded paper maps, then as large as you can afford- though
I've found paper (even unfolded) is a little distorted over larger areas anyway.
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