Archive for April 11th, 2008

Apr 11 2008

OpenBerkurodam and the well-tempered scale

Published by under BART Station,OpenSim,Scale Issues

Enough carefree hours in the main SL Agni grid, already! Back to matters of creation.

Next big thing should be a terrain prototype for civic application. No special business process in mind here, just a demo of the draped imagery on real-life terrain in a way that could scale up city-wide. For starters, there must be a better correspondence between the US National Grid and the dimensions of the simulated region. Sure Neal Stephenson may have suggested binary 2^n dimensionality, and there may be plenty of reasons in the simulator code to make use of the full range of 256 meters. But after more than a handful of regions, the starting corners get downright ugly.

So I won’t do it that way. By scaling up, larger even than real-life, the regions can be built sixteen-to-a square kilometer. In a worldwide sense, except for the matter of 62 or 64 matchlines, the US National Grid (a.k.a. Military Grid Reference System or MGRS) has the whole world in its hands, so harmonizing region design with that grid plan covers a whole lot of ground. To minimize my effort at constructing regions, while planning for worldwide sim grid extensibility, I have chosen to configure the overall sim to represent 250-meter square patches of real earth using each of its 256-meter square regions.

This scales the real-world up a shade in the sim, to (1.024 : 1) but allows every fourth region in X and in Y to start on an exact grid kilometer. That scale produces 16.0000 regions per square kilometer, rather than 15.2588 regions/square km. From the geography side of things, this harmony is attractive since every fourth region will snap to a grid kilometer instead of every 1000th region. Even at that, the grid kilometer that 1000 of those 256 meter regions snap to is 256 kilometers, which is much clumsier to locate by name.

Thus the “well-tempered” moniker for this scale is well deserved, as any real-world USNG/MGRS grid coordinate could then be used to search for the relevant simulator region from a moderately simple bit of string manipulation. For Berkeley, and the western part of California, the zone is “10S” and the 100-kilometer grid within that is “EG” for San Francisco and Berkeley area. Put together, the US National Grid designator for the 100-km square is sometimes called “10SEG”, depending on where folks do or don’t put spaces.

If we always have exactly sixteen (16) regions per square kilometer, then we can use the shorthand version of the USNG grid names that only detail down to 10-meter increments. In this way, a region with its southwesterly corner at WGS84 UTM zone 10 north, 564000 meters Easting, 4191250 meters Northing, can have the US National Grid 10-meter designation of “10SEG 6400 9125”, which could be mashed together without spaces, or used to name a simulator region such as “10SEG_6400_9125” in a slightly more readable form. For those of us who no longer have youthful eyes, the tiny little display on the Second Life client for the region name motivates the use of spaces.

So here’s a graphic of the plan: a 40-region prototype (5 x 8 regions), which will be configured with only Basic Physics, but real-life LiDAR-based terrain, and four megaprim sculpties per region to drape imagery (10 x 16 terrain sculpties) such as 10cm natural color orthophotography. Here’s where I hope to take this:

Charter Design: US National Grid standardized OpenSim regions for Berkeley Downtown / Main UC Campus

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