All About Background Maps in JMP 9: Simple Earth and Detailed Earth
Nov 1, 2010 9:33 AM
In my previous blog post, I gave an overview of the different types of background maps you can add to a graph in JMP 9. Today, I will discuss two of the options for a background map: Simple Earth and Detailed Earth.
Simple Earth and Detailed Earth are two of the choices under Images on the background map dialog. They both consist of a collection of raster images that can be tiled together to make a map of the entire Earth. Both sets of imagery are bundled with JMP and are installed as part of the JMP installation process. This means that no Internet connection is necessary to use a background map -- nor will you have to find and install an extra download.
Here is a graph that uses Simple Earth as the background map.
To create the tiles for Simple Earth, my source data was Natural Earth III, from www.shadedrelief.com. The Natural Earth III data is public domain and free to use, modify and disseminate. The Natural Earth III texture maps come in different versions from the deluxe edition, which includes clouds and arctic ice, down to a basic version. I opted for a version in the middle, which included better geography but did not include either the clouds or arctic ice. It uses a green and brown color scheme for the land and a constant deep blue for water.
The image that I started with had a resolution of 16,200 x 8,100 pixels. I first increased the compression of the jpeg image, which minimally reduced the visual quality while reducing the overall size of the image (and the resultant tiles) considerably. I then wrote a script that would read in the complete image and tile it into many smaller pieces. I generated three different sets of images.
For the first set of images, each image covers an area that is 10° by 10° at a resolution of 450 x 450 pixels. The images in the second set cover an area of 30° by 30° and are at a resolution of 360 x 360 pixels. Each image in the final set covers an area of 45° by 45°, at a resolution of 540 x 540 pixels. If you calculate the pixel-to-degree ratio for each set, you’ll see that the resolution decreases with each set. The reason for the multiple sets is so that a higher resolution is used when you are zoomed in, showing the highest-quality possible. When you are zoomed out, using a lower resolution reduces the number pixels that have to be processed at render time, allowing the graph to render more quickly.
Here is an example of Detailed Earth used as the background map. This graph might look familiar as it is the same data I showed in my previous blog post. Because I used the Detailed Earth background, we now see more detail in the water. Simple Earth doesn't show any detail in water features.
The source for the Detailed Earth was Natural Earth data. This data also comes in different versions and resolutions. Since I wanted this background map to be more detailed, I started with the raster version of the highest resolution I could get, which was the large scale data, 1:10m. From the different options, I chose Natural Earth 1, with Shaded Relief, Water and Drainage. This version had a softer color scheme than Simple Earth, lighter greens and browns for the land, as well as variation in the blue for the water.
Detailed Earth is so named because it has slightly more detail than Simple Earth, in terms of resolution as well as in the geography and the water features. I created the Detailed Earth tiles in a similar way to the Simple Earth tiles. Like the Simple Earth data, I generated three sets of tiles, using the same coverage sizes and resolutions, with one exception. The highest resolution set has a resolution of 600 x 600 pixels for each tile. I was able to get the higher resolution for the first set, since the original source had a resolution of 21,600 x 10,800 pixels. Because of this, you can zoom into a graph further with Detailed Earth than with Simple Earth, before the quality of the background image begins to blur.
You can use transparency with both Simple Earth and Detailed Earth. Tip: To set the transparency, right-mouse-button over the graph and select the Customize... menu option. This will bring up the Customize dialog, where you can select the Background Map and assign a value for transparency. A valid value for transparency goes from 0.0 (completely transparent) to 1.0 (completely opaque).
In the graph below, you can see wind data plotted over a background map showing Lake Michigan. Originally, the arrows were difficult to see over the dark green of the map. After applying 50% transparency, the background map fades out, and the data becomes more visible.
Another nice feature of Simple Earth and Detailed Earth is the ability to wrap. The Earth is round, and when you cross 180° longitude, the Earth doesn't end. The longitudinal value continues from -180° and increases. The map will wrap continuously in the horizontal direction, much as the Earth does. The background map does not wrap in the vertical direction.
Simple Earth and Detailed Earth both support a geodesic scaling (thanks to Xan Gregg). In the images above, the Earth appears as a rectangle, where the width is twice as wide as the height. If we were to take this rectangle and roll it up, we would have a cylinder.
In reality, we know that the Earth doesn’t form a cylinder, but rather a sphere (or at least a close approximation to a sphere). To use these maps but get a better representation of a sphere, you can use a geodesic scaling, which transforms the map to a more realistic representation of the Earth. To use the geodesic scaling, simply change the type of scale on the axes.
Tip: You can change the axes scale by selecting the right-mouse-button over the axis and then selecting Axis Settings…. Change the Scale to Geodesic or Geodesic US. Both choices will transform the map to a geodesic scaling. Use Geodesic US if you are viewing a map of the continental US and you want Alaska and Hawaii to be included in the map. It is important to note that you must set the scale to geodesic for both axes to get the transformation. You will not see a change in the map after setting only one of the axes.
In this graph, we see Simple Earth used as the background map with the axes set to use a geodesic scale. The axes lines are turned on as well. Notice the longitudinal lines are now curved, instead of straight. You might also notice how Greenland looks here compared to how it looked in the previous maps. (Greenland is the country at a high latitude in the Atlantic Ocean, predominantly covered in ice.)
Simple Earth and Detailed Earth are very easy to use, involving nothing more than selecting the option in the background map dialog. You don't need an Internet connection. The axes define the appropriate tiles for the background map.
But both Simple Earth and Detailed Earth do have their limitations. Tune in next time when you’ll hear somebody say, “Hey, when I zoom in to see my neighborhood, those Earth background maps get pretty fuzzy. What’s up with that?”