A1 DIGITAL SCANNING

WHAT?

The aim is to use ratio and proportion to find the numerical values of a digitally scanned hand-drawn plot using the following softwares Paint, MS Excel, and Gimp.

HOW?

First I used the Rotate function of GIMP to properly align and crop the plot.

The plot was then opened in Paint where the pixel locations can easily be found by selecting points using the mouse cursor. Note that in Paint the (0,0) pixel location is at the upper left corner of the image.

The plot origin (x0, y0) was located at the pixel point (1, 1173).

Next I computed for the ratio of the physical unit (number of tick marks) and the pixel unit (number of pixels included between tick marks). To minimize the error due to uneven grid lines and the “arbitrariness” of the pixel location (I had to decide where the edge of the grid line should be), I decided to take the average of several measurements.

x (phy unit)	y (phy unit)	x (pixels)	y (pixels)	x ratio	y ratio
0	0	1	1173
1	1	150	1025	0.00671	0.00676
2	2	300	880	0.00669	0.00683
3	3	450	732	0.00668	0.00680
4	4	595	586	0.00673	0.00681
			average	0.00670	0.00680

I can now compute the physical value of any point in the graph using the following formulas:


x_phy = (x_pixel-1)*x_ratio + x_phy_origin

y_phy=20*(1173 - y_pixel)*y_ratio + y_phy_origin

In this case the physical origin is at (0 days,80 grams). In the plot a y tick mark corresponds to increments of 20 grams, therefore the second equation has a 20 factor.

DID IT WORK?

The plot below shows the reconstructed plot (yellow) superimposed with the scanned plot.
The reconstructed plot follows the scanned plot very closely except in some portions of the grid lines.
An exact match is not really expected because the original graph is hand-drawn and is therefore has some misalignments and uneven lines.
I give myself a score of 8 because although I had a good reconstruction of the plot, I was absent during the activity and I submitted my work late. (Bigay ko po excuse letter ko sa class. =) )


Acknowledgement:
Thanks to Ms. Kaye Vergel for helping me post my blog and to Dr. Gay Jane Perez for giving us the scanned copies of our plots.

Reference:
The graph that I used was copied from the a 1923 book, Practical Chemistry 8th ed by Philip Hawk printed by P. Blakinston Son & Co, Philadelphia, USA.