tecznotes

Michal Migurski's notebook, listening post, and soapbox. Subscribe to this blog. Check out the rest of my site as well.

Dec 9, 2008 2:09am

making friends with hill shading

Living in a city that's quite hilly in places, street patterns make a lot more sense if you can see how they interact with the landscape. The inclusion of elevation data adds legibility to a map, and in the case of the Bay Area it's also interesting to see how overall urban development hugs the flatlands in most places. My goal here is still a beautiful map of Oakland for use with Oakland Crimespotting, with street-level details like schools, hospitals, and major buildings included.

I've just pushed a major update to the Bay Area cartography I've been working on. When I last posted about it in September, I had just added the Cascadenik CSS preprocessor to Dane's mapnik-utils repository. I was inspired to investigate elevation data by Andy Allan's addition of hill coloring to his award-winning OpenCycleMap project, and spurred on by finding the USGS BARD 10-meter elevation data for the San Francisco Bay Area.

Data

Turning a bag of digital elevation model (*.dem) files into shaded hills integrated with OSM map data is a multi-step process. Each file covers a rectangular area, and contains elevation in feet or meters for each included point. This is the northern part of San Francisco with Angel Island and a small bit of Marin showing. I exaggerated the colors somewhat to make it more obvious what the data contains:

Shading

OpenCycleMap doesn't actually use elevation data to simulate shadows; instead it's used to color the ground shades of green or brown, and to provide isolines. They look like this:

Andy told me that he used PerryGeo's DEM utilities to do his coloring, so I started there. It was a bit of a hassle to get hillshade.cpp compiled (see my comment on that page from Nov. 18), but eventually I was able to convert elevation files to GeoTIFFs with shading like this:

Now I had two problems. One was that the shading algorithm trims a single pixel off the edges of its input, because it can't correctly figure out the slope on the border of an area without data. The other was that the BARD *.dem files are published in a mix of meters and feet, so some sections appeared to have an exaggerated height compared to others. Happily, the heavy lifting of dealing with geographic raster data turns out to be mostly handled by the amazing GDAL library, so it was easy to write a Python script to stitch adjoining elevation files together into larger, overlapping, normalized panels and adjust for the feet-vs.-meters problem (stitch.py, 8K). It was also easy to port the C++ hillshading program to Python, which let me fine-tune some other annoying problems around the edges (hillshade.py, 4K).

Tiling

The library I use to generate map tiles, Mapnik, has a way to get raster images into a map, but it doesn't yet support niceties like warping or smooth interpolation. I still have a giant bag of multi-purpose tiling code sitting around from all my flea market mapping experimentation, so this turned out to be an easy step. I warped and tiled all the overlapping bits of shaded hill into a smooth, grayscale tile set that covers the entire SF Bay Area up to zoom level 15.

I've posted all of these hill shaded tiles to their own S3 bucket, so they can be used in slippy maps by anyone. The URL format for these is http://hills-bayarea.s3.amazonaws.com/{zoom}-r{row}-c{column}.png, e.g. Mt. Tamalpais and Mt. Diablo seen here:

I've also included a permissive crossdomain policy file, so these can be used in Flash unencumbered.

Compositing

The other thing lacking in Mapnik's RasterSymbolizer is a way to choose how a raster image visually combines with other cartography, so this ended up being a somewhat custom operation as well. I started with the OpenStreetMap style.mml style file I included as part of Cascadenik example data. I moved some roads up and down in the layering order, and made it split cleanly into two separate styles: ground.mml for ground cover, parks, and roads at very low zoom levels, and figure.mml for labels, buildings, bridges, symbols, and so on. The idea is that figure.mml and ground.mml together should look identical to style.mml, but that the split provides a convenient place to slip in a grayscale set of hills to lighten or darken the ground as necessary.

I implemented a version of Photoshop's Hard Light transfer mode because it seemed to look best in this situation. I also added a feature request to Mapnik in the hopes that this sort of thing will be a built-in feature of the library sometime.

Ta-Da

Check out the current version of the map for the results. OpenStreetMap and OpenCycleMap's own tiles are included on that page for comparison. If you see a mistake, you can correct it yourself or just mark it as a bug.

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