Re: ElevationGrid II

[Martin Reddy <reddy>]

Gavin Bell wrote:
>   -- Double-precision coordinates (DoubleCoordinate maybe?) to support CAD
> applications that require very-high-precision models.  I'm suprised a
> VRML-For-CAD working group hasn't started to talk about this, and about
> adding trimmed NURBS and other features common to CAD applications.

This is one of the issues that the GeoVRML Working Group is interested in
looking at. Single-precision floats don't provide enough accuracy to model
the Earth down to anywhere near meter accuracy (diameter of Earth ~= 12,000 
km = 12,000,000 m = 8 digits of precision. Single precision values only
store around 6 digits of precision).

At SRI we have gotten round this limitation for our VRML terrain
visualisation application by implementing a hierarchy of local coordinate
systems, each of which are then transformed into the global coordinate
system. But support for SFDouble/MFDouble types in VRML would make life way
easier! 

> Since you posted to the geovrml newsgroup, I assume you're interested in
> spherical coordinate systems that would be more appropriate for
> representing geographical features.  I haven't heard of anybody proposing
> or implementing such extensions (SphericCoordinate maybe?).

For those who are interested, the GeoVRML Working Group page has this to say
about coordinate systems:

"In VRML, a world has a simple, local, Cartesian coordinate system with the 
 Y axis defined as "up". The most basic problem in georeferencing a VRML
 world is thus to be able to transform from a standard georeferencing system
 to the VRML local system. While it would be extremely difficult, and
 undesireable, to standardize on a single coordinate system, we can
 certainly hope to define a standard way of specifying where a VRML world's
 zero is, and recommend accurate tools for managing the absolute to relative
 coordinate transforms."

Cheers,

Martin.


---------------------------------------------------------------------------
Martin Reddy                                   SRI International, AI Center
                                               Menlo Park, CA 94025-3493
reddy@ai.sri.com                               Tel : (650) 859-6468
http://www.ai.sri.com/~reddy                   Fax : (650) 859-3735