Mar 12 2010

Sharing Terrain With the World – Google Earth style

It’s not fully 3D immersive, but hey, 2-1/2D ain’t half bad. The “dsm40cm” model of Marin County has been published as the county’s default terrain on Google Earth. It’s a great pleasure to work with folks who are not troubled by a county representing its surface on a 40cm single-precision float grid that weighs in at 77 GB. In terms of data bulk, that is about the same as the entire 30-meter version of the US National Elevation Dataset.

What one gets when piling that much detail into a single county of around 520 square miles of land area is every building pad, driveway, and crown of road paving that were resolved. The dsm40cm model was derived from an ESRI Terrain Dataset that incorporates our best available topographic contours (1:4800 scale 10-foot; 1:2400 scale 2-foot,) photogrammetric break and water lines, FEMA LiDAR and NCALM (GeoEarthScope) LiDAR data sets. The Terrain Dataset currently comprises 40 GB of vector GIS data.

When the finely detailed surface grids were first developed, we broke the county up into 20 work areas to maintain ArcGIS 9.3.1 in a stable and productive state, and 30cm posting interval grids were generated that covered the entire county–at least during development. When necessary, these grid tiles were mosaicked with ERDAS Imagine into a single seamless grid. The 40cm version was produced directly as a single seamless grid using ArcGIS 9.4 beta 1, on a workstation imaged with Windows Server 2003. The WGS84 UTM, NAVD88-Geoid 2003 result was provided to the Google Earth team earlier this year.

As with all GIS data sets, it seems, the more detailed it is, the more rapidly it may need updating. In the works for the next year or so are several improvements to the dsm40cm model. First: the photogrammetric break lines will be segregated into steeper sets that tend to run along ridges, and shallower slopes that tend to delineate road cuts and building pads. The ridge set will be used as soft constraints to resolve some artifacts where they rise above some contours.
Second: incorporate new LiDAR data as it becomes available. Some data has already been provided for the lowest part of Lagunitas creek, and it appears that Prof. Ellen Hines of San Francisco State University’s Department of Geography and Human Environmental Studies has been funded by USGS to gather LiDAR county-wide this year.

So there will be revisions, but an exciting aspect is to see data flows being brought into existence that support different levels of mirror world development.
Publishing the dsm40cm model in Google Earth is an important (and beautiful) threshold to cross. Making use of the dsm40cm model in county operations such as creek and watershed delineation will be the practical benefit that drives the work in the first place. And before too many more weeks, there may be entirely new approaches to publishing the data in an immersive environment (neither Second Life nor Opensim) to share.

Building pad in Kent Woodlands shows driveway-level detail

Kent Woodlands building pad and driveway, in the shadow of Mt. Tam

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