From: Ralph Hartley (hartley_at_aic.nrl.navy.mil)
Date: Mon Feb 12 2001 - 20:57:55 CET
Received: (from mdom_at_localhost) by karto.ethz.ch (8.9.3/8.9.3/SuSE Linux 8.9.3-0.1) id UAA09909 for cavexml-outgoing; Mon, 12 Feb 2001 20:54:45 +0100 Received: from sun0.aic.nrl.navy.mil (sun0.aic.nrl.navy.mil [132.250.84.10]) by karto.ethz.ch (8.9.3/8.9.3/SuSE Linux 8.9.3-0.1) with ESMTP id UAA09905 for <cavexml_at_cartography.ch>; Mon, 12 Feb 2001 20:54:43 +0100 Received: from aic.nrl.navy.mil (pc31.aic.nrl.navy.mil [132.250.84.181]) by sun0.aic.nrl.navy.mil (8.9.3+Sun/8.9.3) with ESMTP id OAA07037 for <cavexml_at_cartography.ch>; Mon, 12 Feb 2001 14:54:43 -0500 (EST) Message-ID: <3A884043.8060502@aic.nrl.navy.mil> Date: Mon, 12 Feb 2001 14:57:55 -0500 From: Ralph Hartley <hartley_at_aic.nrl.navy.mil> User-Agent: Mozilla/5.0 (X11; U; Linux 2.2.16-22 i686; en-US; m18) Gecko/20010124 X-Accept-Language: en To: cavexml_at_cartography.ch Subject: Re: Other areas that haven't been discussed. References: <Pine.GSO.4.05.10102121042070.19817-100000_at_cor.oz.cc.utah.edu> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-cavexml_at_karto.baug.ethz.ch Precedence: bulk Reply-To: cavexml_at_cartography.ch
John Halleck wrote:
> "THE" spanning tree? Spanning trees are not unique.
> (I usually go for minimal spanning trees myself... which
> for the cave surveys I have are unique, and not order dependent.)
> [I will admit that there could be cases where the minimal
> tree not unique, but I've never seen them actually occur in
> real life.)
And even that is not unique unless there is agreement on the metric with
respect to which the tree is minimal. The length of the shots in the
tree is just one possibility. Another useful tree would be one where the
non included shots are chosen to be the ones believed to have the most
error. That is, the spanning tree that gives the best coordinates for
the stations. Of course that tree depends critically on assumptions
about the error distribution, and it could be very difficult to compute
(and thus important to save).
> Another thing I forgot to mention is that the spanning tree
> usually depends on the order in which the survey shots are
> presented to the program. I have not seen a program where
> the unadjusted coordinates do not depend on the input order,
> though a reordering routine is possible.
>
> Bob Thrun
This is another reason it has to be output. Some things will not be
compatible (e. g. anything depending on unclosed lineplots) if the same
spanning tree is not used in each program.
John Halleck again:
> Note that CaveXML support for these don't have to be anything
> extensive. For Unique compact point numbers it is only
> one more (optional) attribute on a point. For spanning trees it is
> just one more (optional) attribute on a shot.
Except that mixing raw data and processed data is evil. The problem is
when you have a single set of survey shots which has been processed by
more than one program. Keeping the processed data separate from the raw
data (as the proposed processeddata element does) lets each program
report which (if any) tree it used, without mangling each other's
results, even if they cannot agree on a common tree.
But a format that includes unclosed lineplots already includes a way to
output a tree.
Ralph Hartley
This archive was generated by hypermail 2b30 : Thu Mar 01 2001 - 18:00:01 CET