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Gama

by TerryO :: Rate this Message:

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Hi,

I have noticed that the 1.9.05 version has an “envelope” algorithm. Can you please tell me about the algorithm, how it works and where it would be useful?

Thanks,

Terry

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Re: Gama

by Ales Cepek-3 :: Rate this Message:

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On Wed, Jun 25, 2008 at 03:41:21PM -0400, O'Shea, Terry (MTO) wrote:

> I have noticed that the 1.9.05 version has an "envelope" algorithm. Can
> you please tell me about the algorithm, how it works and where it would
> be useful?

Hi Terry,

"envelope" in gama-local algorithms is a shorthand for "Cholesky
decomposition using envelope method for reducing bandwidth". Envelope
(or skyline matrix/storage) is a simple sparse matrix scheme based on
reordering so that non-zeroes are near main diagonal. Gama uses a
simple RCM reordering algorithm for reducing the bandwidth (one of
open Gama tasks is to implement better strategies). See also

http://en.wikipedia.org/wiki/Sparse_matrix
http://en.wikipedia.org/wiki/Skyline_matrix
http://en.wikipedia.org/wiki/Reverse_Cuthill-McKee_algorithm

In case of small geodetic networks you cannot gain much but with
linear systems with O(10^3) unknowns envelope algorithm (sparse matrix
solution) reduces solution time from an hour or more to seconds on my
notebook (adjustment of gps vectors networks in gama-g3).

So if you have input data resulting in long computation time, try envelope.

Ales

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