光栅贴图与替代品

Question

I recently found this web page Crime in Downtown Houston that I'm interested in reproducing. This is my first learning experience with mapping in R and thus lack the vocabulary and understanding necessary to make appropriate decisions.

At the end of the page David Kahle states:

One last point might be helpful. In making these kinds of plots, one might tempted to use the map raster file itself as a background. This method can be used to make map plots much more quickly than the methods described above. However, the method has one very significant disadvantage which, if not handled properly, can destroy the entire purpose of using the map.

1. In very plain English what is the difference between the raster file approach and his approach?
2. Does the RgoogleMaps package have the ability to produce these types of high quality maps as seen on the page I referenced above that calls a google map into R?

I ask not because I lack information but the opposite. There's too much and I want to make a good decision(s) about the approach to pursue so I'm not wasting my time on outdated or inefficient techniques.

Feel free to pass along any readings you think would benefit me.

Thank you in advance for your direction.

Answer 1

Basically, you had two options at the time this plot was made:

• draw the map as a layer using geom_tile , where each pixel of the image is mapped onto the x,y axes (slow but accurate)

• add a background image to the plot, as a purely "cosmetic" annotation. This method is faster, because you can use grid.raster which draws images more efficiently, but the image is not constrained by the axes of the plotting region. In other words, you have to manually adjust the x and y axes limits to make sure that the image corresponds to the actual positions on the plot.

Now, I would suggest you look at the new annotation_raster in ggplot2 v. 0.9.0. It should have the advantage of speed and leaner output files, and still conform to the data space of the plot. I believe that this function, as well as geom_raster and annotation_map did not exist when David made those plots.