Activity 1 - Digital Scanning Report

The objective of this activity was to create a digital plot from the scanned image of a plot by getting the pixel coordinates of the data points on the image and getting the corresponding physical values with the use of ratio and proportion. This method can be used when for example, only the image is available and the actual physical values are needed.

A hand-drawn plot was photocopied from The Journal of Experimental Zoology, Number 1, Volume 20 pp 431 (1916).

Figure 1. Scanned image of the plot.

Procedure
The scanned image was opened in GIMP 2, where the graph was cropped so that the unnecessary parts were removed. Using the Measure tool, 2 points on the x-axis were selected to measure the angle of rotation of the image. (The image was scanned so it might be tilted, and making sure the x-axis was horizontal would make replicating the graph easier.) The tilt was then corrected by rotating the image.

After the image pre-processing, the pixel coordinates of the origin and the data points as well as the points on the trendline were obtained. GIMP 2 displays this information of the lower left of the window. It is important to note the pixel coordinates of the origin for proper conversion of pixel counts to the actual physical values of the graph.

The pixel coordinates of the graph was corrected by subtracting the pixel coordinates of the origin to all of the pixel coordinates of the data points.

To convert the pixel coordinates to physical values, a conversion table was needed. The conversion table for the x-axis was made by dividing the physical length of the x-axis (in this case, 160 grams) by the number of pixels that span the whole x-axis (in this case, 1126 pixels). This means every pixel in the x-axis is equivalent to 0.14 grams. Doing the same thing for the y-axis, it was found that every pixel in the y-axis is equivalent to 0.42 cc/min.

By using the conversion table, the pixel coordinates is now converted to actual physical values. We now have the digital plot.

Figure 2. Digital plot of the scanned image.

To check if the plot is right, we superimpose the digital plot with the scanned image. This can be done in OpenOffice in a few steps:

1. Crop the scanned plot in GIMP, using the plot boundaries as the crop boundaries and save the image as .bmp.
   
2. In OpenOffice Draw, select Format>Area and select the Bitmap tab. Click Import and import your scanned plot.
3. In your plot in OpenOffice Calc, right-click on the chart and select Object Properties. Select the Area tab and in the drop-down menu, select Bitmap.
4. You should be able to find the imported scanned plot and use this as the background of your digital plot. Uncheck the Tile and check Autofit.

Figure 3. Digital plot (colored red) with scanned plot as background. Notice the original unfilled circles encircling the points on the digital plot.

I give myself 10 points for this activity. The digital plot and the scanned plot are well matched with only very small deviations from the scanned data points, due to difficulty in accurately pointing the cursor and in selecting the boundary for cropping (in the image, the boundaries are more than one pixel in width). I was also able to use the original scanned image as the background for the plot.

:)