[ncl-talk] reading PRISM .bil files?

[David Brown]

Following up on Rick's comments, he is exactly right about the format.
I downloaded a sample precipitation file from the web site, and tried
to read and plot it using the suggestions at the web page Rick
referred to. The function 'cbinread' works well and the data is little
endian, so no byte swapping is needed on my Intel-processor-based
machine. The data is written from top to bottom, so the Y Axis does
need to be reversed to plot in NCL using the native projection which
is basic lat/lon (cylindrical equidistant).

Here is the code and attached is the PNG file output.

dat = cbinread("PRISM_ppt_early_4kmD1_MTD_20141016_bil.bil",(/621,1405/),"float")
dat at _FillValue=-9999
wks = gsn_open_wks("png","bil-test")
res = True
res at cnFillOn = True
res at cnLinesOn = False
res at cnLineLabelsOn = False
res at cnFillMode = "rasterfill"
res at mpMaxLatF=49.916666666669
res at mpMinLatF= res at mpMaxLatF -  .041666666667 * 621
res at mpMinLonF= -125.000000000000
res at mpMaxLonF = res at mpMinLonF + .041666666667 * 1405
res at mpLimitMode = "latlon"
res at tfDoNDCOverlay = True
res at tiMainString = "PRISM_ppt_early_4kmD1_MTD_20141016_bil.bil"
plot = gsn_csm_contour_map(wks,dat(::-1,:),res)

 -dave








On Fri, Oct 17, 2014 at 11:08 AM, Rick Brownrigg <brownrig at ucar.edu> wrote:
> Hi Daniel
>
> "Band-Interleaved Format"  My understanding of the format is that the .bil
> file itself is just a "brick of floats" (or integers, bytes, etc.) that is
> MxNxP, where P is the number of bands that make up this image. It is
> organized as all of the floats (ints) of the first row for band-1, followed
> by the floats for first row's 2nd ban, ..., the floats of first row's Pth
> band, then likewise for subsequent rows, ...
>
> In your example, there's only 1 band. We see that it is 621x1405, comprised
> of 32-bit floats. Hence the first row is 5620 bytes in length:  1405 *
> sizeof(float) = 1405 * 4 = 5620.  This should be relatively straightforward
> to read:
>
> http://ncl.ucar.edu/Applications/r-binary.shtml
>
> Its not clear to me from the header whether the first row is the
> bottom/southern-most, or the top/northern-most.
>
> Does that help?
> Rick
>
>
> On Fri, Oct 17, 2014 at 10:37 AM, Daniel Barandiaran
> <dbarandiaran at gmail.com> wrote:
>>
>> Has anyone dealt with these files?  I've done some work with binary files
>> before, but I am no pro by any means.  I've read a little about the data
>> format of these .bil files, but I'm still not sure about exactly how these
>> files are formatted.  They have a file header, which contains the following:
>>
>> BYTEORDER      I
>> LAYOUT         BIL
>> NROWS          621
>> NCOLS          1405
>> NBANDS         1
>> NBITS          32
>> BANDROWBYTES   5620
>> TOTALROWBYTES  5620
>> PIXELTYPE      FLOAT
>> ULXMAP         -125
>> ULYMAP         49.9166666666687
>> XDIM           0.04166666666667
>> YDIM           0.04166666666667
>> NODATA         -9999
>>
>> any help would be appreciated!
>>
>> --
>> Danny Barandiaran
>> Department of Plants, Soils, and Climate
>> Utah State University
>>
>>
>>
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>
>
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