Even
Thanks for pointing into this that I was not aware of as it would be the
main point of my answer to Kyle's question.
But still, as it is referred in the ticket (sorry for lousy formatting
but I never learn how to do it better) the main issue occurred in the
conversion to another format (geotiff for that matter). So though an
option exists ('unscale') to account for offset/scale the natural
expectancy is that the conversion does not change the data values.
Redoing the tickets example we can see (not shown than because I
thought it of lesser interest)
C:\SVN\mironeWC\src_C\t>gdalinfo lixo.tiff -mm
Driver: GTiff/GeoTIFF
Files: lixo.tiff
Size is 21, 21
Coordinate System is `'
Origin = (-10.500000000000000,10.500000000000000)
Pixel Size = (1.000000000000000,-1.000000000000000)
Metadata:
NC_GLOBAL#Conventions=COARDS/CF-1.0
NC_GLOBAL#title=lixo.grd
NC_GLOBAL#history=grdmath -R-10/10/-10/10 -I1 X Y MUL = lixo.grd
NC_GLOBAL#GMT_version=4.5.4
z#long_name=z
z#_FillValue=1.#QNAN0e+000
z#actual_range=-1, 1
z#scale_factor=0.01
x#long_name=x
x#actual_range=-10, 10
y#long_name=y
y#actual_range=-10, 10
Image Structure Metadata:
INTERLEAVE=BAND
Corner Coordinates:
Upper Left ( -10.5000000, 10.5000000)
Lower Left ( -10.5000000, -10.5000000)
Upper Right ( 10.5000000, 10.5000000)
Lower Right ( 10.5000000, -10.5000000)
Center ( 0.0000000, 0.0000000)
Band 1 Block=21x21 Type=Float32, ColorInterp=Gray
Computed Min/Max=-100.000,100.000
It's true that we can still see a trace of the previous info about the scale
z#actual_range=-1, 1
z#scale_factor=0.01
but a user would need to be very very attentive to realize that and to
know that the data values were off by an 0.01 factor.
I think that in these situations the scale factor would better be
applied by default (like with the gdal_info example)
Joaquim
Kyle,
Frank added in GDAL 1.7 a '-unscale' option to gdal_translate, so people can
always use gdal_translate -unscale to apply the offset and scale (they can even
do that do a VRT file to save disk space), and then use gdalinfo on the result.
Extract from the gdal_translate man page:
"-unscale : Apply the scale/offset metadata for the bands to convert scaled
values to unscaled values. It is also often necessary to reset the output
datatype with the -ot switch."
Is there a need for such an option in gdalinfo ? I have no strong opinion
about this. An issue I see is that usually -stats record the computed stats in
a .aux.xml file for later retrieval. The interaction with a -unscale option
would be tricky... What happens if the user computes with -unscale and then do
gdalinfo without it... Or the other way round.
Le jeudi 28 octobre 2010 19:58:55, Kyle Shannon a écrit :
Hello,
I have been working on ticket #3797. In the example given, gdalinfo is
the calling the netcdf driver. I agree with Frank's opinion on this in
ticket #1660:
Note that GDALRasterBand has methods to get the offset and scale. The
normal
GDAL practice would be to return them via those methods - not to apply
them on the fly. Then it is up to the caller to do so if they wish.
gdal shouldn't be in charge of scaling the data to what may be a different
data type, or unpacking it. But in this case, gdalinfo is the caller.
Should the functionality of the scaling be added to gdalinfo? Maybe
optionally reporting the stats as scaled data? Any thoughts?
# ============================
Kyle Shannon
Physical Science Technician
RMRS Fire Sciences Lab
Fire, Fuels& Smoke - RWU 4405
5775 Highway 10 W.
Missoula, MT 59808
(406)829-6954
kshan...@fs.fed.us
# ============================
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