Closed ccrook closed 6 years ago
ccrook
commented
6 years ago @ndonnelly Am interested if you have any thoughts on proposed solution - does it meet the requirements (I think so), and is there a better way. This is not the result of long meditation - I just dashed down the proposed solution as I was writing up the issue. So feel free to offer alternatives!
ccrook
commented
6 years ago @ndonnelly Revised (slightly simplified) proposal - help documentation reads:
In order to apply this algorithm snapspec first identifies the stations as either control stations, test stations, or ignored stations. By default stations which have been observed and which are fixed in the adjustment are treated as control stations, and stations which do not have data are ignored marks. The remainder are tested. However there are some subtleties in a 3d adjustment where the horizontal or vertical ordinates may be fixed but the other ordinates not fixed.
If only horizontal tests are applied on a 3d adjustment then stations which are adjusted vertically but fixed horizontally are ignored. Conversely if only vertical tests are applied then stations which are adjusted horizontally but fixed vertically are ignored. The assumption here is that the constraint is applied due to lack of data, not because we believe the ordinate is correct. This assumes that the reason the ordinate is fixed is because of missing data, not because it is believed to be correct.
If horizontal and vertical tests are being applied then the default behaviour is to use a station at which only a horizontal or vertical coordinate is calculated. The uncalculated coordinate is treated as perfect (zero error) in the tests. This behaviour can be changed using the "ignore_constrained_stations" option, in which case stations that are fixed either horizontally or vertically in a 3d test are ignored.
For the moment this avoids explicitly selecting control stations (so can be implemented more rapidly!) In the future this may need additional options to fix select control stations (eg could imagine holding a levelled mark fixed horizontally in a 3d adjustment of levelling and trig levelling data, and wanting to use it as a control station).
ndonnelly
commented
6 years ago Thanks Chris,
Your proposed method looked good to me, I couldn’t think of any better approaches.
Nic
From: Chris Crook [mailto:notifications@github.com] Sent: Sunday, 7 October 2018 8:30 a.m. To: linz/snap Cc: Nic Donnelly; Mention Subject: Re: [linz/snap] snapspec handling of autofix stations (#76)
Closed #76https://github.com/linz/snap/issues/76 via e054dbehttps://github.com/linz/snap/commit/e054dbe22e1694d78226a8a5de378941de2aed09.
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snapspec can give misleading results for station ordinates not in the adjustment. Where a coordinate is fixed snapspec treats it as having an error of zero. This is appropriate where the coordinate is fixed to provide control, but not correct where the coordinate is fixed (or autofixed) where there is no information in the adjustment to define it.
This creates a number of problems for snapspec:
Proposed solution is: