Setting Your Magic Number for the GSC

Step 1 - Setup

Select the "Set Magic Number" menu item (under the "Display" sub-menu).

Temporarily set your magic number to "1". Click "ok". Note: GSC V2.1 now adds the ability to separately set Magic Numbers for the horizontal and vertical directions. For this step, one can unclick the box that says "Different X/Y Scales" - this will set both Horizontal and Vertical Magic Numbers to "1".

Step 2 - Scan Reference Images

Scan in an accurate ruler. For a "sanity check" scan in a ruler at 100% scale in both a horizontal and vertical orientation. For determining an accurate magic number, scan in a rule at a 400% magnification. What seems to work well is to capture just over 1 or 2 inches of the ruler. Don't use the "0" inch mark at the end of the ruler if it comes at the very end of the ruler. These images should be stored in "bit-mapped" format (.bmp)
Very important note:A critical element in scanning images for this program (in order to keep the measurements accurate) is to set the scanning resolution to a value appropriate for screen display (as opposed to a resolution intended for a printer). Usually, an image intended for display on the screen (monitor) scans at about 72 dots per inch, whereas one intended for a printer is often around 300 dots per inch. If you scan in using the latter (300) and call it into this program it will appear about 4 times larger than normal. The exact dots per inch is not important - what is important is this: whatever value you choose when you scan in these ruler images and set your magic number with must be used when you scan in your targets. If you scan targets with a different dot/inch your measurements will be off proportionally. My scanner allows me to define "custom" scanning profiles. I've defined one to be "GSC" just to remind me.

100% scale horizontal rule
100% scale vertical rule

400% scale horizontal rule
400% scale vertical rule

Step 3 - Sanity Check of System

On the "File" menu item, select "Open..." and move to the folder containing your images of the ruler. Open the 100% scale horizontal rule. The "Shot Scanner" window will pop up with the image showing. You can re-size the window by clicking and dragging any edge/corner if desired. On the "Cursor" menu select "Cross Hair" (this is the easiest to use for calibration). Make sure the "Scale" reads "100%".

Move the cursor over an inch mark (the "1" inch mark is used for an example in the image shown here) and click the right mouse button. Be as accurate as you can. This sets the "0" mark for the next measurement.

As accurately as possible (in the horizontal direction, we don't care about the vertical direction yet) move the cursor over 1" (to the 2" mark in this example). Click with the left mouse button. An ordered pair will now show up in the upper left corner ("(1126.466, -45.250)" in this example. The first number of the pair shows the distance in the "X" direction (1126.466 in this example), and the second (-45.250) shows the distance in the "Y" direction. If you've measured one inch on the ruler, the first number is real close to your magic number. Make a note of this value. It doesn't matter if the number is positive or negative, only the absolute value is relevant at this time - the "+" or "-" is only an indication of whether the point you measured is to the left or right of the "0" mark you set down. (go ahead and click "done" on this window).

Now do the same but using the image of the vertical ruler at 100%. Open the file, make sure the cursor is the cross hair, the scale is at 100%, right click on an inch mark to set the "0" reference point, and left click on an inch mark 1" away from the "0" mark you set. This time the right hand number of the ordered pair (-1128.409 in this case) is the one of interest. Again, it doesn't matter if the number is positive or negative as it's only the absolute value that's of interest. If you've been accurate in your "clicking", this number should be fairly close to the number you saw in the horizontal measurement - within 2 or 3 "units". This tells us that the vertical scaling and horizontal scaling on the scanner/monitor are uniform. If the number you get is significantly off, repeat the exercise to make sure it wasn't simply a "pilot error". If you continue to get significantly different numbers, there is some "problem" with your scanner / monitor configuration. You'll need to track that down prior to continuing and prior to getting accurate readings from this program. (go ahead and click on "done" on this window).

Step 4 - Determining the Magic Number

On the "File" menu item, select "Open..." and open the 400% scale horizontal rule. The "Shot Scanner" window will pop up with the image showing. Re-size the window if desired. Reselect the "Cross Hair" cursor if it is not already set. Even though this image was scanned at 400%, keep the scale set to 100%. This will give us a number 4 times larger than the Magic Number, but we'll divide by 4 in the end. As accurately as possible,place the cursor on an inch mark and click with the right mouse button - again, this sets the "0". Move the mouse to the next inch mark on the ruler and as accurately as possible, click the left mouse button. Write down the first number of the ordered pair (the "X" value - "4504.988" in this example). Now divide this by 4 (400% scaling) - for the example shown, this gives us a horizontal Magic Number of 1126.25. Close this window by clicking "done".

On the "File" menu item, select "Open..." and open the 400% scale vertical rule. The "Shot Scanner" window will pop up with the image showing. Re-size the window if desired. Reselect the "Cross Hair" cursor if it is not already set. Again, keep the scale set to 100%. As accurately as possible, place the cursor on an inch mark and click with the right mouse button setting the "0". Move the mouse to the next inch mark on the ruler and as accurately as possible, click the left mouse button. The second number of the ordered pair (the "Y" value - "-4503.971" in this example) is the one of interest. Again divide this number by 4 (400% scaling) to get a Vertical Magic Number of 1125.99 (only the absolute value is important, so we can ignore the negative sign). Close this window by clicking "done".

Step 5 - Setting the Magic Number

Return to the "Set magic number" menu item (again under the "Display" sub-menu). If the number found for the horizontal measurement is very close to that found for the vertical measurement, one can stick with a single Magic Number that is an average of the two numbers (shown in the upper figure at left). If, however, there is a difference between these two numbers, you may wish to click the "Different X/Y Scales" and enter separate horizontal and vertical Magic Numbers.
Enter the "magic number(s)" and click "ok". The program will remember these numbers from here on out, so you only have to do this once - eg. when you close and restart the program your magic number(s) will be remembered.

Step 6 - Final Sanity Check

As a final sanity check re-open one or more of the ruler images above. This time set the scale appropriately (100% for 100% and 400% for 400%). Right click to set the "0" and left click one inch away. If everything has been done correctly you should see the appropriate number of the "X-Y" pair ("X" in the example) show 1.000 +/- 0.001 inches. You're calibrated!

Some other things to be aware of:

One can get more accurate by scanning in a longer part of the ruler at 400% and measuring longer distances - just remember to divide by (4 x the number of inches measured). Also, often the resolution of the screen/cursor/mouse is such that one cannot click directly on the inch marks. By setting a "0" then clicking on 1/4", 1/2", 5/8", 2-1/2", etc. from the "0" (at 400% resolution), and observing the "X" or "Y" number over these various measurements, often a pattern emerges that gives a better "magic number". In the case shown in the example above, I've found a better magic number for my system to be 1125.875 rather than 1126. In most cases this is insignificant (the above error translates into 0.0001" per inch, or 0.01%), however one may want to play with it to get what one can out of it.
If you change your monitor and / or your scanner, you will need to recalibrate your system. This change IS significant. I changed the monitor on my system and saw the magic number change from 900 to 1126. It does seem to be impervious to changing simply the display resolution of your monitor itself. Eg. if you've calibrated your system with the monitor set at 1280 x 1024 resolution and change it to 1024 x 768, there does not appear to be a need to change the magic number setting. However, I always keep my ruler images around to check whenever something like this changes.
Remember, when scanning targets in to measure, make sure the "dots per inch" on your scanning tool is set the same as it was when you scanned in these ruler images and set your magic number.
With V1.8 or later you can now get measurements in either inches or centimeters. You do not need to recalculate your magic number when changing between metric and English units - the program does that for you. You can calibrate your magic number using either centimeters or inches, just remember to select metric/English FIRST, then follow the instructions above (if calibrating in metric units, all instructions above with 1 centimeter rather than 1 inch).
Enjoy!

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