The lat/lon ruler does not fit the lat/lon grid on my map. Why not?

If it is way too long east-to-west...
Remember the lines of longitude converge at the poles. That is to say the further north or south you go from the equator, the closer together they get. You need to use the ruler on a diagonal to compensate for this. Instructions are here.

If it is way too long north to south...
Not all maps use the same size lat/lon grid. You may be using a 7.5 minute ruler with a five minute grid spacing. In this case, only use the portion of the ruler from 0 to 5 minutes.

If it is way too short north to south...
Not all maps use the same size lat/lon grid. You may be using a 7.5 minute ruler with a 15 minute grid spacing. In this case, you will want to draw in additional grid lines to end up with a 7.5 minute grid spacing..

If it is just a bit too long north to south...
It turns out that the lines of latitude also get just a bit closer together as you move north or south from the equator. This is because the earth is an ellipsoid, not a sphere. MapTools lat/lon rulers are sized for an exact fit at 40 degrees north and south of the equator. For most of our users this will make the ruler about a millimeter too long. Most folks never even notice, and the results they get are well with in their accuracy requirements. To compensate for this you can use the ruler on a slight diagonal, much like you do when measuring longitude.

If it is just a bit too short north to south...
You may be further north or south than 40 degrees. But most likely the scale on your map is not quite correct.

I've seen an issue with my oldest DeLorme Atlas and Gazetteer for California. Puzzled, I did some more research and found that the old CA atlas was not really exactly a 1:150,000 scale. I ran out and bought a newer atlas, and it was right on. I ran all of the calculations for the length of my ruler, and double checked them against a USGS map as well. The ruler was correct.

My guess is that when the DeLorme Atlas series was first published, nobody was using the lat/lon lines since GPS receivers were not common back then. And a small error in a distance measurement would likely go unnoticed. On the other hand maybe the paper has stretched a bit, with years of use and less than ideal storage conditions. I've not been able to confirm the problem with DeLorme.

Only one or two customers have noticed this problem since I started selling the rulers back in 1999.

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