[SUP-205] grib_compare: what is relative difference? Is this expressed in %,
fraction, ... ? Created: 12/Jun/12 Updated: 28/Jul/14 Resolved: 27/Jun/12
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Daniel Varela Santoalla
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Description
Hi,
I am trying to use the GRIB PI tool grib_compare.
I am comparing statistically 2 different files and I would like to understand what
absolute/relative difference mean. I take that absolute difference is in the units of the variable
found different, but what is relative difference? Is this expressed in %, fraction, ... ?
Thanks for your help, I have gone through the manual in your web site but can't find a
description.
Comments
Comment by Paul Dando [ 27/Jun/12 ]
Let's look at an example.
% grib_compare -c data:n 2t_128_nearest.grib 2t_128_nearest_env.grib
-- GRIB #1 -- shortName=2t paramId=167 stepRange=0 levelType=sfc level=0
packingType=grid_simple gridType=regular_gg -double [packedValues]: 131072 out of 131072 different
max absolute diff. = 8.7258148193359375e+00, relative diff. = 0.0303396
max diff. element 34426: 2.87605209350585937500e+02
2.78879394531250000000e+02
tolerance=0.0000000000000000e+00 packingError: [0.000984192]
[0.000984192]
values max= [317.515] [317.528]
min= [213.217] [213.172]
In this case:
max absolute diff = 287.605209350585937500 - 278.879394531250000000
= 8.7258148193359375e+00
relative diff.
= (287.605209350585937500 278.879394531250000000)/287.605209350585937500
= 0.0303396
In other words:


The maximum absolute difference is computed by comparing the difference for each
value between the two fields and is in units of the field being compared.
The relative difference is then the maximum absolute difference divided by the larger of
the two values. It is dimensionless.
Further examples can be found in the grib_api manual:
http://www.ecmwf.int/publications/manuals/grib_api/grib_compare.html
I hope that helps
Paul
Comment by Daniel Varela Santoalla [ 27/Jun/12 ]
Hello Julia
Please note Paul's answer on this issue. Do come back to us if you have any further question or
any problem to access this link.
http://software.ecmwf.int/issues/browse/SUP205?focusedCommentId=14489&page=com.atlassian.jira.plugin.system.issuetabpanels:commenttabpanel#comment-14489
Regards
Daniel Varela
Software Support
Comment by Daniel Varela Santoalla [ 27/Jun/12 ]
Answered
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