Within the last month, I have had three requests from clients to merge two chart areas together into one image for a mural.
This usually is far more difficult than you might think.
It is hard to explain why that is, but let's try it here. What our problem entails is a problem built into the charts themselves. Any given chart is an attempt by the cartographer to make a curved surface or a slight dome (the curvature of the earth) into a flat surface.
Distortions in map-making are as old as the history of cartography. Here is an example.
On a Mercator Map, longitude lines run straight up and down as shown in this map. This cylindrical projection of the earth is more accurate the closer you are to the equator but distorts significantly as you approach the poles.
In this first map, note that Greenland appears to be a little larger than South America, when in fact, Greenland is one-eighth the size of South America.
In a projection where the longitudinal lines converge to the North Pole, as shown in the second map, Greenland appears more closely to its actual size.
Back to our nautical chart merge problem
It is safe to say that there is distortion in every chart you see because of curvature of the earth. R. Knippers in his article has a good illustration of why that is, and curvature of the earth is at the center of the problem.
As you can see, depending on the technique used distortion can be found more at the center of the map in one method, and more along the edges in another.
That's why it is quite difficult to merge charts together.
Here's an example we worked on recently, involving two charts in Maine and a preliminary attempt to merge them.
As you can see, it is possible to get part of the two charts merged but have very noticeable distortion occur elsewhere.
The good news is that we can overcome the distortion, but it takes our graphic specialist Kathryn Tyler a little time to do this. She is magnificent at this skill and can make it impossible to see where the two charts are joined. It does add a little bit to the cost, but when you need to merge two charts, Kathryn Tyler is the best option.
Take Home Lesson: When you need to "sew" two chart edges together, Tyler is your Tailor!