id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
136	Precision of thinned Impact Heights	Dave Offiler	Dave Offiler	"The decision was taken for ROPP-1 v1.1 that the set of 247 Impact Heights (specified in the thinner control files 
'ropp_io/data/ropp_thin_*-247.dat'
should be given only to the nearest 1m. 

The original values devised by Chris/Sean were to a precision of 1mm which seemed unjustified given the Level 1b specification of 6m for IP accuracy (see RTS-1030 in the URD), a 10cm limited precision in BUFR and much lower accuracy requirement for Level 2 heights (20m) for NWP applications (see RTS-1070 in URD).

It is accepted that 1m precision is quite adequate for NWP assimilation purposes and meets user requirements for this type of application.

However, Axel & Stig have pointed out that using this 1m lower precision could (theoretically) result in an additional, if small, source of error when comparing BA quality statistics prepared using the higher 1mm precision thinning control IHs, though this claim has apparently not been tested to establish the level of error induced by using different precisions. If all use the same precision, there is no problem, but for unknown reasons, Axel seem unwilling to change from the 1mm version, and DMI are using the ROPP v1.1 versions...
 
Given that the BUFR representation is limited to 10cm precision in IP, the 1mm precision will in any case be lost to users performing comparisons using BUFR data as the source.

In view of the 6m nominal IP accuracy, BUFR limitation and potential for induced errors, we suggest a compromise precision of 10cm for the IHs in the thinning control files, which is the best that BUFR can provide (unless the template is upgraded; not a short-term option)

The impact of using 10cm and 1m vs the reference 1mm precision on the (O-B) statistics using a common set of RO & NWP profiles should be established before the 10cm control files are included in the ROPP release (nominally v2.0, possibly as a post-v1.2 optional update). A user can point the ROPP tools (ropp2ropp, ropp2bufr)
at any control file they wish from the command line. If 10cm files are prepared, Stig will perform such a test (TBC)

Whether or not 10cm precision is agreed for ROPP releases, a common 
precision needs to be agreed and adopted by EUM and the SAF for (non-BUFR) validation. Note that end users (including MetO) will
be using BUFR products, so will be limited to a 10cm precision 
even if 1mm (or 10mm) were in the thinner control files).
"	defect	closed	normal	2.0	ropp_io	1.1	fixed	thinner, precision, impact heights	Huw Lewis