*cs out EPSG:3042 and *declination auto - weird interaction

[Warren Family]

I'm getting a weird result depending on the order of *cs out EPSG:3042 
and *declination auto in the survex file, for my standard test dataset 
for Dow Cave / Providence Pot.  There is something about the interaction 
between these two commands which I don't understand.

I know that *declination auto should be preceded by a command defining 
the input co-ordinate system (in this case *cs OSGB:SD) -- this problem 
is about the interplay of *declination auto and the output co-ordinate 
system declaration.

Specifically, with this version of the top level .svx file the 
centreline is completely screwed up although the entrance *fix s are in 
the right places:
$ cat test_epsg3042_fail.svx
*cs OSGB:SD
*cs out EPSG:3042
*declination auto 97480 72608 225
*include DowCave/DowCave
*include DowbergillPassage/Dowbergill1
*include DowbergillPassage/Dowbergill2
*include ProvidencePot/ProvidencePot
*equate DowCave.dow2.31 Dowbergill1.dgp1.1
*equate Dowbergill1.dgp5.16 Dowbergill2.dgp6.46
*equate Dowbergill2.dgp7.1 ProvidencePot.PPot6.15

With this version it works though - the only difference is I swapped 
order of the *cs out and *declination auto lines:
$ cat test_epsg3042_succeed.svx
*cs OSGB:SD
*declination auto 97480 72608 225
*cs out EPSG:3042
*include DowCave/DowCave
*include DowbergillPassage/Dowbergill1
*include DowbergillPassage/Dowbergill2
*include ProvidencePot/ProvidencePot
*equate DowCave.dow2.31 Dowbergill1.dgp1.1
*equate Dowbergill1.dgp5.16 Dowbergill2.dgp6.46
*equate Dowbergill2.dgp7.1 ProvidencePot.PPot6.15

In both cases dump3d reports CS EPSG:3042 in the .3d file..

To reproduce this you'll need the rest of the data set - this is in the 
DowProv directory here:
https://github.com/patrickbwarren/qgis3-survex-import
This has a lot of *date commands spread over ca 30 years, so the mag 
declination is varying quite a bit.

Some other things I noticed:
  - so far as I checked this only appears to be a problem with 
EPSG:3042, which is the code for ETRS89 UTM zone 30N;
  - it works for *cs out UTM30N - which is however the WGS84 datum - 
either order of the two lines gives the same result;
  - it similarly works for *cs out EPSG:32630 which is the explicit code 
for WGS84 UTM zone 30N;
- it works for *cs out EPSG:7405 and *cs out:27700, in either order with 
the *declination auto line, (these are the codes for the Ordnance Survey 
British National Grid)

So to summarise - if *cs out EPSG:3042 precedes *declination auto in the 
survex file, the centreline (outside of *fix stations) is screwed up.  
But this co-ordinate system is quite a common one isn't it (?) since 
it's for the European datum equivalent for WGS84, with the western 
European UTM zone.

Version info:
os-release: Ubuntu 22.04.3 LTS (Jammy Jellyfish)
survex: 1.4.1
proj: release 8.2.1

Patrick