Friday, December 19, 2014

Square Pocket Protractor

A grid map tool that is modified with 1/16" drilled holes makes for the perfect pocket chart plotting protractor. Drill the holes by hand using a 1/16" bit spun with the index finger and thumb. Make a pilot hole at the spot to be drilled using the point of a divider leg.


  1. Hello Greg,
    Can you explain the use of the holes in the Protractor. With two pencils you can use it as een compass for drawing a circle. But for the rest??

  2. Hi Herman,
    I use each square on plotter as 1 nautical mile. Put center of plotter over assumed position. Dot azimuth. Rotate plotter to dot with pencil in center then shift plotter toward or away the number of minutes and tenths of the intercept. Dot left and right. Connect the dots for a line of position. Easier to see done than to discribe.

  3. Ah, that works, but to construct the plottingsheet I have to mark each minute on the meridian, then from the center a line on angle equal to latitude. Mark this line with each minute and pull each point down on the latitude line. Then I have the basis plottingsheet. Correct?
    But to mark the minutes is difficult because the distance of each rectangle is 4,2mm. I have drilled 11 holes beside the vertikal 0-9 scale, now it is easy to mark each minute with twisting the pencil in each hole. So it works for me.

  4. You don't need a plotting sheet. Just a blank piece of paper. Longitude minute length is the cos of the latitude. Example: 1' at 50 north latitude will have cos 50 = .64' (length of 1' of longitude). I like using the a blank index card to do plots with the mini square protractor. Again it would be easier to show then describe.

  5. The construction I ment is op blank paper. With the holes drilled on 1' interval it is easy to construct the "homemade" plotsheet without further calculation.