Did you ever have a hard copy of a graph, with large intervals but without any tick-marks? And you wanted to know the true values that the plot was indicating?
Well, I did, and these are the results I obtained after reconstructing the plot using simple ratio and proportion, knowing the pixel locations of the points on the graph. This method is very simple. After using GIMP (sort of like a free Photoshop :D) to scan the x and y axes of the graph and recording the pixel locations in Microsoft Excel, we can get a relationship of the number of pixels per fixed interval of, say, 10 units. Given this and the pixel location of the origin, we can obtain an equation to convert the pixel values to real values! Once we have computed for the real values, we could then replot the graph in Excel.
In the picture, the blue and green dots correspond to the plot of real values computed in Excel, while the black plots beneath them correspond to the original scanned image of the graph. We can see that it corresponds well and that there are practically no deviations from the original plots. I also took the average difference of the pixel locations for several intervals, giving less room for error. We can conclude that the reconstruction of the scanned graph is accurate and can be used for further analysis.
For this Activity, I would give myself a grade of 12 because besides understanding the lesson and reproducing the required one plot, I went ahead and reproduced the other plot in the graph for completeness.
This activity amazed me because I never knew that it was so simple to digitize old records (the graph I picked was published in 1923).
Hope this helps! :D
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