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    Modern Bygrave Slide Rule Emulator
    From: Bill Ritchie
    Date: 2020 Mar 5, 12:45 -0800

    I have written an emulator for Gary LaPook’s “Modern Bygrave Slide Rule”. Unlike his original, I cannot think of any practical use for it! However, I hope it will be of interest to those of you who have contributed to Bygrave threads in the past. I wonder how many of you have made their own versions (cylindrical or modern) from the helpful information previously posted on NavList.

     The emulator can be run from https://www.vigilanceofbrixham.co.uk/Astron/Bygrave/FlatBygrave.html

     Gary has posted comprehensive reference information for his excellent creation, including his pro forma, on


     In this emulator, the scales are calculated and plotted, rather than using saved images. Whilst slower to load, this has enabled me to allow choice of number and length of rows, plus allow three choices of scale limits. The default values give approximately the same laptop resolution as the original Bygrave. However, if you had (say) a Kogan 34” external monitor at 3440 x 1440, you could change the settings and have of an 1800-inch display compared to the 300-inch scale length of the Bygrave. (And, even that is zoomable!)

     I found that dragging with a mouse was imprecise, so I added single pixel fine adjustment. The original Bygrave was criticised for inability to lock the scales. Such unintended movement can also occur on this emulation, so I have added a freeze mechanism. 

    The scales used are (red) loge Cosine and (black) loge Cotangent. However, as on a normal slide rule, such scales increase from right to left. To achieve the desired left to right progression, all calculated pixel values are subtracted from the pixel scale length before plotting. Like Gary, I decided to discard displaying complementary angles beside the datum angles on the grounds of clutter. Their presence would also have confused the use of his pro forma.

     The attached note uses the same example as Gary’s document above, together with mini tutorial on mouse control and alternative keys.  

     Bill Ritchie.

    Home 50N 003W

    Presently 38S 176E





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