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Activity 1 - Digital Scanning


The first activity for our AP 186 class was very interesting and quite useful. I have had problems before concerning manufacturers who give calibration curves but do not give the values. It’s really troublesome when you need them and you can’t find any way to retrieve the data. Fortunately, this digital scanning experiment resolves this dilemma.

Way back when computers were not yet easily accessible, graphs were still hand-drawn. In this activity, we went to the CS Library to find old journals or thesis papers from which we can choose a hand-drawn graph. Our chosen graphs are to be scanned as an image where data are to be extracted.

The graph that I chose was taken from the PhD Dissertation of Cherrie B. Pascual in 1987, titled, Voltammetry of some biologically significant organometallic compounds. The scanned image was tilted so I had to rotate it using Gimp v.2 (see Figure 1).

Figure 1. Concentration dependence of DPASV stripping peaks
of triphenyltin acacetate using an Edep = -1.0V and tdep = 1min.

Now that we have our image ready, we can now start the extraction of data by simply using ratio and proportion. For each axis, we take the ratio of the axis length in pixels and the axis length in actual units as indicated by the tick labels.


For the x-axis, 1180 pixels  corresponds to 20 in physical units which gives us a scaling factor of 0.016949 physical units/pixel. Meanwhile, for the y-axis, 1188 pixels correspond to 1.5 in physical units, resulting to a scaling factor of -0.00126. The negative sign compensates for the fact that the origin of an image is at the top left corner which makes the y-axis pixel values increase as you go down.

 By multiplying the x and y scale factors to any x and y pixel values, respectively, we can compute for their corresponding physical values. To reconstruct the graph, I used 20 points distributed along the curve. One important things to remember while doing the computation is to subtract the starting point of the graph from the obtained pixel values since the origin is not located at (0pixel, 0pixel).

I was so happy when I plotted the values that I calculated because I obtained a fairly good fit with the curve from the image as show in Figure 2. The generated curve is shown in red broken lines and the data points  which were represented as circles in the original graph are shown in yellow. However, there are some discrepancies with the x-axes of the plots, which I think is because the intervals of the ticks in the x-axis of the drawn graph are not perfectly equal. But still, I can say that the data extraction was successful. yey! :D 

Figure 2. Plot of the extracted true values (red and yellow) superimposed 
on the scanned image of the oriignal graph (black).  

I actually tried different ways to compute for the scaling factor. One was by plotting the tick mark value with respect to the corresponding pixel value than taking the slope of the best fit line. Another was to individually take the pixel values corresponding to a certain interval (e.g. 120 pixels bet. 0-2) and then take the average pixel interval and divide it by the smallest interval value (i.e. 2.0 and 0.5 for x and y). However, using all these methods gave me similar values for the scaling factors so I just settled for the easiest way.
I had fun doing this activity! :D 
And because I think I did a good job in reconstructing the graph and even trying out different possible ways, I give myself a perfect score of 10/10 .  :)

I would like to thank Ms. Mabel Saludares for sending me the scanned copy of my chosen graph and
Mr. Gino Borja for teaching me the subtleties of blogging. :D 
  

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