UHL (ugly horizontal lines) when using mapImage

[richardsc]

This is probably going to be a bigger issue, as the UHL in mapPlot() has been, but I'm opening this so that we don't forget about it. Perhaps a potential fix involving the sp package can be applied to the polygon drawing in mapImage() to make a general fix.

Anyway, I noticed the problem looking at the test figure from issue #655, where there are clearly some UHL issues near the North Pole (and probably Antarctica as well):

[dankelley]

Thanks. I will be looking into how sp might help with UHL and similar problems. The sp author (email on the package-devel email list) suggests that "beyond the edge" problems are not easy to solve, but I still plan to test out one of their routines for that.

[dankelley]

I made a new test, 655c.R, and comparison with the (updated) 655a.R shows that the artifacts depend on plot geometry. Below is the result. (The fact that I've added a new test doesn't mean I am really working on this ... it's more that I'm trolling for any old and ignored bugs that might have been fixed by other work, without an update to the issue because of other work.)

[dankelley]

I will be looking into the latest R magazine to see how they solve the longitude shift problem in the mapmisc package.

update: I realize this isn't quite the right issue; the point is just to make a note about how mapmisc may have some map solutions