measuring distances on scanned images

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

Hi ... not sure if this is the best n/group for my question
if not hopefully someone can suggest where better to post...

I have a number of land plans (showing plot boundaries etc...)...
simple b&w line images - Scale 1:250...
I would like to scan a small section, enlarge it to about a4 and then
take
accurate measurements from it...
Does anyone have any good techniques for doing this (the accurate
scaled measuring from the enlargement that is)...

EG perhaps scan a millimeter rule(r) at the same time
so that I can see what size 1mm is represented at the new scale
so I can calibrate any measurement I do on the enlargement ...

Or is there any fancy s/w that will add a scaled grid overlay to a
scanned image...?

Or some s/w that would work on the Jpeg that would automate
measurements of
lines relative to something on the image of known size on the image?

thanks for any help
(btw OS is XP, scanner is canon n670u)

AJ
 
easiest is to use a clear plastic rule scaled to 1:250 (or 1:1250) and
place it "near" the portion you intent to enlarge, so the portion and
the rule are scanned at the same time and appear together on the final
image.

be sure to save files to tiff format, not jpg
 
Hi ... not sure if this is the best n/group for my question
if not hopefully someone can suggest where better to post...

I have a number of land plans (showing plot boundaries etc...)...
simple b&w line images - Scale 1:250...
I would like to scan a small section, enlarge it to about a4 and then
take
accurate measurements from it...
Does anyone have any good techniques for doing this (the accurate
scaled measuring from the enlargement that is)...

EG perhaps scan a millimeter rule(r) at the same time
so that I can see what size 1mm is represented at the new scale
so I can calibrate any measurement I do on the enlargement ...

Or is there any fancy s/w that will add a scaled grid overlay to a
scanned image...?
Click to expand...

Yes it is possible.
Or some s/w that would work on the Jpeg that would automate
measurements of
lines relative to something on the image of known size on the image?
Click to expand...

Do not use Jpeg images for this purpose. Jpeg is a lossy format and will
distort the image. Jpeg does not do line images. Jpeg is 8 bit gray scale or
24 bit color only. Jpeg does not have a 1 bit mode.

Use TIFF.

There is software that can do accurate measurements and show the dimensions.
Good CAD (Computer Aided Design) software is capable of this, not all CAD
programs can take a scanned image and turn the scan into a CAD drawing.

This Page describes how to convert raster images (scans) into CAD drawings.
http://usa.autodesk.com/adsk/servlet/item?siteID=123112&id=2405321

Autodesk is the leader in CAD software.
thanks for any help
(btw OS is XP, scanner is canon n670u)

AJ
Click to expand...
I understand the scale is 1:1250 not 1:250.

To get the same size print from a scanned image or plan, You print the image
at the same resolution that you scanned the image at.
Including a scale in the image is a good way to ensure that you got a good
reproduction.
A small tape measure or a 6 inch (15 cm) scale is ideal.

To get a enlargement and maintain the accurate measurement, you scan at 2X
or 4X or 8X the DPI or PPI that you print with:

Example to get a 2X enlargement, you scan at 600 PPI and then print at 300
PPI.
Or You scan at 300 PPI and Print at 150 PPI. (same 2X enlargement but a
lower resolution image, the final printed image is only 150 PPI).
The printed image will then be twice the size of the original plan. There is
a practical limit to how much you can scale a plan this way. Your scanner
may not have the needed resolution.

Read about Scaling here:
http://www.scantips.com/basics2c.html

Some image file types do not store the PPI of the scan. Do not use any of
those formats.
GIF or BMP do not store the PPI of the image. Both were designed for
computer monitors or the Web which do not use DPI, just pixels.

PNG is a lossless format and if the software using PNG does it correctly,
the PPI is stored.
TIF is maybe the most universal image format. It does store the DPI or PPI
of the scan. It also has lossless compression available.
JPEG stores the PPI, but it produces artifacts in plan or map scans. It is
lossy compression.
 
SNIP
Does anyone have any good techniques for doing this (the
accurate scaled measuring from the enlargement that is)...

EG perhaps scan a millimeter rule(r) at the same time
so that I can see what size 1mm is represented at the new
scale so I can calibrate any measurement I do on the
enlargement ...
Click to expand...

If you scan at the scanner's native resolution, you may be able to
deduct the true dimensions (make sure you test *both* horizontal and
vertical dimensions).
Or is there any fancy s/w that will add a scaled grid overlay to a
scanned image...?
Click to expand...

Not a grid, but if you have a software that uses Photoshop plug-ins
you may want to try:
http://www.reindeergraphics.com/free.shtml#entermag

Bart
 
Hi ... not sure if this is the best n/group for my question
if not hopefully someone can suggest where better to post...

I have a number of land plans (showing plot boundaries etc...)...
simple b&w line images - Scale 1:250...
I would like to scan a small section, enlarge it to about a4 and then
take
accurate measurements from it...
Does anyone have any good techniques for doing this (the accurate
scaled measuring from the enlargement that is)...

EG perhaps scan a millimeter rule(r) at the same time
so that I can see what size 1mm is represented at the new scale
so I can calibrate any measurement I do on the enlargement ...

Or is there any fancy s/w that will add a scaled grid overlay to a
scanned image...?

Or some s/w that would work on the Jpeg that would automate
measurements of
lines relative to something on the image of known size on the image?

thanks for any help
(btw OS is XP, scanner is canon n670u)

AJ
Click to expand...

Some of the other replies have provided good information, but none of them
have touched on one critical aspect: Calibration!

Most scanners and printers are consumer-level devices, and are not made to
the tolerances you might need. In addition, the axies will most likely
*not* be identical ... even in a high-end laser printer. You will need to
calibrate both your scanner and your printer ... in *both* horizontal and
vertical axies.

In doing so, you will find out that the scanner does not scan at 4800 spi,
but at (for example) 4720 spi vertical and 4852 spi horizontal ... and your
printer isn't really 360 dpi x 360 dpi, but more like (for example) 357.2
dpi x 365.8 dpi!

Additionally, you should include scanning something like an engineering
scale (with 1/100 inch divisions) superimposed on your image. Preferably
two scales, one horizontal and one vertical.
 
Most scanners and printers are consumer-level devices, and are not made to
the tolerances you might need. In addition, the axies will most likely
*not* be identical ... even in a high-end laser printer. You will need to
calibrate both your scanner and your printer ... in *both* horizontal and
vertical axies.
Click to expand...

Bingo!!!

A simple test is to scan a ruler it both horizontal and vertical
orientation. Quite a shock!

Secondly, and what makes this particularly bad, is that this is not a
fixed offset but changes with each scan!

Thirdly, the amount of misalignment changes even within the *same*
scan (!) as the scanner head travels and the cogs and belts slip a
tiny amount.

Therefore, for suggested approaches to work, a ruler will need to be
placed on *both* axis and then *transform* the resulting image based
on those values. I'd actually create a frame made up of measuring tape
and put the drawing in the middle.

Don.
 
Back
Top