Hi Jim- > > What makes you think the IDV isn't using these values? > > A couple of reasons. It doesn't list LATITUDE or LONGITUDE among the > fields,which didn't seem like a good sign. The IDV does not display these as options since they are used as coordinates for the data variables. You can see a dump of the file using the Data Source properties menu under the MetaData tab. Displays are getting plotted > in rectangles with sides parallel to the window. The true grid is > probably curvilinear on the coordinates chosen for the map. Not in the file you sent me. If you look at the latitude values, they are the same across the x dimension (lines) and longitude is the same across the y dimension (samples). It is a rectilinear grid. > > The netCDF > > library converts the packed lat/lon values to unpacked versions. In > > the process, the lats go from -139.951 to -95.0994 and the lats > > from 25.0 to 49.851. For example, the latitude is declared as: > > > > short LATITUDE(lines, samples) ; > > LATITUDE:units = "" ; > > LATITUDE:missing_value = 51.f ; > > LATITUDE:valid_min = 24.99975f ; > > LATITUDE:valid_max = 49.90084f ; > > LATITUDE:scale = 0.001f ; > > LATITUDE:offset = 24.999f ; > > LATITUDE:scale_factor = 0.001f ; > > LATITUDE:add_offset = 24.999f ; > > > > The first value is 1, so that would translate to 1*.001+24.999 which > > would be 25. For the longitudes, the declaration is: > > > > int LONGITUDE(lines, samples) ; > > LONGITUDE:units = "" ; > > LONGITUDE:missing_value = -94.f ; > > LONGITUDE:valid_min = -140.0005f ; > > LONGITUDE:valid_max = -95.0994f ; > > LONGITUDE:scale = 1.e-007f ; > > LONGITUDE:offset = -140.0005f ; > > LONGITUDE:scale_factor = 1.e-007f ; > > LONGITUDE:add_offset = -140.0005f ; > > > > and the first value is 0, so that would make it 0*1E-7 + -140.0005 > > which would be -140.0005. > > Yes. > > > This is being set to missing. > > Is that what you are wondering about or is it something different? > > I wasn't aware that it is -- or should be -- set to missing. And > I have questions about the explanation given below. It looks like the roundoff is causing it to be set to missing. > > latitude is also set to NaN, but there, we have 24902*.001+24.999 = > > 49.901 which is beyond the valid_max of 49.90084. > > > > I got some clarification from the netCDF folks on the use of the > > missing and valid max/min values. Here's what they replied: > > > > <quote> > > Turns out the problem is missing_value, which must be in packed units: > > > > from > http://www.unidata.ucar.edu/software/netcdf-java/v4.1/javadoc/index.html > > > > *Implementation rules for missing data with scale/offset* > > > > 1. Always converted to a float or double type. > > 2. _FillValue and missing_value values are always in the units of the > > external (packed) data. > > Let me offer an example to see if I understand this. Say the scale factor > is > .001 and offset is 20. Then will a missing value (in packed units) of 15 > get > displayed by ncdump as 15*.001+20=20.015? ncdump shows only the raw values as far as I know. I don't see an option (at least in 3.6.1) to do the scale and offset. If you are using the netCDF-Java ToolsUI to dump the data, you can dump either the raw data or the transformed data. That's what I've been using to check out your file. The resulting scaled/offset missing value will be used to set data values to NaN after they have been unpacked if they are the same as the unpacked missing value. > > 3. If valid_range is the same type as scale_factor (actually the > > wider of scale_factor and add_offset) and this is wider than the > > external data, then it will be interpreted as being in the units > > of the internal (unpacked) data. Otherwise it is in the units of > > the external (packed) data. > > 4. The dataType is set to float if all attributes used are float > > (scale_factor, add_offset valid_min, valid_max, valid_range, > > missing_data and _FillValue) otherwise the dataType is set to double > > > > > > technically, valid_min and max should also be in packed units > > acording to the NUG, but we relaxed that in netcdf-java to > > accomodate some existing datasets that misread the manual. > > > > also, he is using floats, and is encountering roundoff error, > > looking in the debugger i see: > > > > valid_min = -140.00045776367188 > > > > which will also cause problems. > > Where does this roundoff error occur? It seems it must be just in the > output > of ncdump or the debugger would give the same value. In the netCDF-Java library. > > so: > > > > 1: dont use missing_value or valid min/max unless you need them, eg > > you actually have missing data. > > The lats and longitudes should always be there but there will be > missing values of some data. That is the best way. > > 2. missing_value must be in packed units. > > See my example above. Yes. > > 3. make valid_min and valid_max a little wider than you need to > > accomodate roundoff > > Or have them in packed units?? Yes, that would work. > > </quote> > > > > Also, take a look at the CF conventions about packed data: > > > > http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf- > > conventions.html#packed-data Don Ticket Details =================== Ticket ID: QAW-317402 Department: Support IDV Priority: Critical Status: Open
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