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Raven Maps has been giving latitude and longitude notation more attention lately. When most of our maps were “island maps” (wherein the area mapped ended at political boundaries), we included 1-degree tics and numerical values of  the mapped territory. (see US map below)

          United States map with tic marks for latitude and longitude shown in red
 

We originally included tics within the map, but that reference to the lat/long grid did not seem to be of much help to our users. We never included the full grid itself, the graticule,  because Raven maps feature shading and color which would make the conventional graticule hard to see.

The graticule is a much more important element on world maps, since it conveys the sense that this flat image is an attempt at representing a sphere. And it shows that, yes, New York on the Atlantic (74° W)  IS west of Santiago, Chile (70° W). So, we print the graticule on our world maps, but in minimal form— typically every 15 degrees. Apart from those uses, does the graticule matter?

              Tic marks of longitude on our Countries of the World (Concise) map

Well, half of it matters a lot for many reasons.Latitude is the single most significant fact about any location on earth. It is the measure of position between the equator and the poles, therefore the seasonal sun angle. That determines a host of important effects with a defining influence on physical geography, and on life. It’s also a convenient measure of distance, usable on any projection and at any map scale. One degree of latitude is about 69 miles long, with slight variations (since the earth is not quite a perfect sphere). Very handy.

Longitude is different: It matters a lot for one reason: accurate navigation on the ocean, beyond sight of land. It depends upon coordinates that define position in two dimensions, not just distance from the equator. Determining east-west position requires both precision time-keeping and reliable tables of astronomical observations. By the late 18th century both were finally available, and European governments started printing maps showing degrees of longitude based on their respective capitals (so maps of the period published in languages other than English can be puzzling). In the end, British naval and maritime dominance led in the 1880’s to the worldwide adoption of the British Naval Observatory’s location as the Prime Meridian. The location from which longitude is measured is purely historical and, with respect to the planet, arbitrary, but essential. (see world map, above)

Essential— but annoying, because longitude value varies. A degree of longitude at the equator is about 69 miles long, the same as a degree of latitude. But the meridians converge at the poles, so that length decreases with distance from the equator. At 26 degrees north (Miami), a degree of longitude is 60 miles long; at 40N (Philadelphia) it is 53 miles; at 48N (near Seattle), 46 miles; at 61N (Anchorage), 34 miles; at Barrow (71N), 23 miles. Degrees of longitude are useless for scaling unless you have the reference tables at hand.

Finally, for map design: the convergence of meridians means there are more longitude values than latitude values per running inch of map on (nearly) any projection. The discrepancy looks a little odd at scales large enough that one degree intervals are called for, but small enough that a lot of them appear. That happens to describe our Raven 1:2 million regional series. We’ve tried various solutions, including showing every other value, and, showing every latitude value but only every other longitude value. In the end we settled on very light type for each one degree value of both latitude and longitude, but larger and bolder type for 5 degree index values. The idea is to provide easily-read quick reference information, with more comprehensive but visually quiet detail.

  The Southern Plains & Rio Grande Basin map with latitude and longitude tic marks

Who would think that all these issues arise when simply choosing type sizes and weights for a minor map element? Well, that’s cartography.