Activity 1: Digital Sacnning

Objective: Use ratio and proportion to find the numerical values of a hand-drawn (or machine printed) plot.

Tools: MS Paint, Spreadsheet, image processing software

Description of procedure: We start with an scanned image of an old plot from some journal.

Figure 1. Original image of the plot.

We then take a fixed interval in an axis of the plot (which corresponds to a physical value) and note the number of pixels comprising this interval in the image. This give us a ratio of the physical quantity per pixel. For example, if the plot shows 100 units in the x axis and consists of 200 pixels in the image, we get a ratio of:


100 units/ 200 pixel= 0.5 units per pixel

We do this for both a and y axes. We then take the coordinates of the points in the plot and copy these into a spreadsheet. Using the ratios we derived earlier, we can then get the values corresponding to the physical quantities for both x and y axes. When these are plotted and the graph axes are adjusted to reflect the scale on the image, we get a plot that should correspond to the original drawing as we see below.

Figure 2. Overlay of origial plot and the plotted points from spreadsheet.

Comments: The results correspond well to the curve on the image. Note that the plotted points are represented as squares on the plot above. The main difficulty encountered is the scaling and the correlation of the axes which turns out to be a bit trickier than anticipated.

Source: Wood and Petrie, "Nitrogen metabolism of plants," Annals of Botany, Vol 2. 1938.

Acknowledgements: I would like to thank Doctors Soriano and Perez for their instruction. Also I would like to thank Misters Cabello, Dilla, Garcia, Gubatan and Villoan for their invaluable assistance.

Evaluation: For this activity I give mysely a grade of ten (10) for my output is quite acceptable and that the procedure for this exercise was thoroughly understood.

Additional Image: Trendline from spreadsheet showing logarithmic fit (compare with original scanned line)