Wednesday, July 29, 2009

Accuracy, Plausibility, and OSM

I have been working on OpenStreetMap data for X-Plane this week. Use of OSM for global scenery is going to be a bit different from projects like X-VFR or other specific custom OSM-based scenery.

The issue at hand is accuracy vs. plausibility...
  • Accuracy: how much error is there between what exists in the real world and what exists in the scenery. Is that road in the right location? Is it the right type of road?
  • Plausibility: does the scenery as a whole look reasonable? Is that road on land or is it in the water? Is that river running up a mountain?
The global scenery needs to prefer plausibility over accuracy. Because we can't check and manually fix errors in the source data for the entire world, and because we don't cut the global scenery very often, it is important that the global scenery err on the side of reduced accuracy (remember, the global scenery isn't that accurate in the first place) rather than plausibility problems that will clearly be ugly and distracting.

The implication for OSM-based global scenery: not everything in OSM is going to show up in the global scenery. This would be true anyway simply due to the need to keep the global scenery compact. (I trust that OSM will grow to the point where it can source scenery larger than we can ship for the entire world.) But the global scenery generator may need to err on the side of not including data that might have plausibility problems.

Fortunately it is possible to build custom scenery from OSM as well. I don't see OSM-based global scenery as replacing efforts like X-VFR and others; rather custom scenery will always be able to use more OSM data , checking the data for accuracy, rather than reducing the data to maintain plausibility.

One technical note: I am working on an extention to the road .net file format that would allow road networks to be draped over terrain. This would allow overlay packages to add/replace road grids without having to know the shape of the base scenery mesh, and make it easier to both create custom road networks and to create the tools that manipulate them.

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