If you look at the top or bottom of the chart or sides you will see degrees marked in whole numbers. Along these lines are graduations of 1 minute, 5min & 10min divisions are longer like on a ruler. At the 1/2 degree point is a continuous line but no grads. Graduations in the North/South direction (Latitude) are 1nm same as your ruler. East/West they are not because the smaller circle = less per degree, You can walk round the planet at the north pole in 3 or 4 paces at Lat 90. At the equator it's a bit further but still 360degrees.
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