In a weather app, maps are important. So important, that as a developer of weather apps, I’ve learned far more than I ever care to know about topography. When I originally created the maps for Seasonality, I had to balance download size with resolution. If I bumped up the resolution too far, then the download size would be too big for users on slower internet connections. If I used too low of a resolution, the maps would look crappy. I ended up settling on 21600×10800 pixel terrain map, which after decent image compression resulted in Seasonality being a 16-17 MB download. At the time, most apps were around 5 MB or less, so Seasonality was definitely a more substantial download.
That compromise was pretty good back in 2005, but now that a half-decade has passed it is time to revisit the terrain I am including in the app. Creating a whole new terrain image set is a whole lot of work though, so I thought I would share what goes into the process here.
First, you have to find a good source of map data. For Seasonality, I’ve always liked the natural terrain look. The NASA Blue Marble imagery is beautiful, and free to use commercially, so that was an easy decision. For the original imagery I used the first generation Blue Marble imagery. Now I am using the Blue Marble Next Generation for even higher resolution.
Next you have to decide how you are going to tile the image. I’ve chosen a pretty simple tiling method, where individual tiles are 512×512 pixels, and zoom levels change by a power of 2. Square tiles are best for OpenGL rendering, and while a larger (1024, or even 2048 pixel) tile would work, 512×512 pixel tiles are faster to load into memory and if downloading over the network it will transfer faster as well. From there, you have to figure out how many tiles will be at each zoom level. I’ve chosen to use a 4×2 tile grid as a base, so the smallest image of the entire globe will be 2048 x 1024 pixels and made up of 8 tiles. As the user zooms in further, they will hit 4096×2048, 8192×4096, 16384×8192 pixel zoom levels and so on. I’ve decided to provide terrain all the way up to 65536×32768 pixels.
Now that you have an idea of what tiles need to be provided, you need to actually create the images. This is the most time consuming part of the process. Things to consider include the image format and compression amounts to use on all the tiles, and these are dependent on the type of display you are trying to generate. Creating all the tiles manually would take forever, so it’s best to automate this process.
The Blue Marble imagery comes in 8 tiles of 21600×21600 each (the full set of images for every month of the year is around 25 GB). I start by creating the biggest tile zoom level and moving down from there. For my 65536×32768 zoom level, I’ll resize each of the 8 tiles into 16384×16384 pixel images. I use a simple Automator action in Mac OS X to do this. I created an action that takes the selected files in the Finder and creates copies of the images and resizes the copies to the specified resolution.
Now that I have 8 tiles at the correct resolution, I need to create the 512×512 tiles for the final product. For Seasonality, I also need to draw all the country/state borders at this point, because otherwise the maps are blank. I created a custom Cocoa app that will read in a map image with specified latitude/longitude ranges, draw the boundaries, and write out the tiled images to a folder. My app has the restriction of only handling a single image at a time, I’ll have to drag each of the 8 tiles in separately for each zoom level. It’s not ideal, but I don’t do this too often either. For the 65536×32768 zoom level, I end up with 8192 individual tile images. Smaller zoom levels result in far fewer tiles, but you can see why automation is helpful here.
It’s a lot of work, but in the end the results are great. For Seasonality, along with higher resolution terrain, I’m also bringing in the Blue Marble’s monthly images. If everything goes as planned, Seasonality will show the “average” terrain for every month of the year. Users will be able to see the foliage change as well as the snow line move throughout the seasons.