[gdal-dev] NetCDF driver in gdal_translate ignores xllcorner and yllcorner?

Thu May 20 22:37:54 EDT 2010

Dear GDAL developers

I have been using GDAL for a while with great success and really appreciate
the toolkit.
As of recently, I now have the need to read NetCDF files and convert them
into AAIGRIDs. However, the cellsize is wrong and I don't think xllcorner
and yllcorner are correct either.

I have searched this forum and I think this may be related to the thread
named "[gdal-dev] netcdf to geotiff and aaigrid" from 2008. However, I
didn't find a resolution in that thread - hence this posting.

The content of the NetCDF file is obtained by running ncdump merapi.res.nc:

netcdf merapi.res {
dimensions:
        x = 59 ;
        y = 64 ;
        alt = 6 ;
        time = UNLIMITED ; // (4 currently)
variables:
        float x(x) ;
                x:units = "m" ;
                x:description = "UTM. West-East distance" ;
        float y(y) ;
                y:units = "m" ;
                y:description = "UTM. South-North distance" ;
.... (many lines removed here)
data:

 x = 412432.3, 413449.5, 414466.8, 415484, 416501.2, 417518.5, 418535.7,
    419553, 420570.2, 421587.4, 422604.7, 423621.9, 424639.1, 425656.4,
    426673.6, 427690.8, 428708.1, 429725.3, 430742.6, 431759.8, 432777,
    433794.2, 434811.5, 435828.7, 436846, 437863.2, 438880.4, 439897.7,
    440914.9, 441932.2, 442949.4, 443966.6, 444983.8, 446001.1, 447018.3,
    448035.6, 449052.8, 450070, 451087.3, 452104.5, 453121.8, 454139,
    455156.2, 456173.5, 457190.7, 458207.9, 459225.2, 460242.4, 461259.6,
    462276.9, 463294.1, 464311.3, 465328.6, 466345.8, 467363.1, 468380.3,
    469397.5, 470414.8, 471432 ;

 y = 9106709, 9107724, 9108740, 9109756, 9110772, 9111788, 9112804, 9113820,
    9114836, 9115851, 9116867, 9117883, 9118899, 9119915, 9120931, 9121947,
    9122962, 9123978, 9124994, 9126010, 9127026, 9128042, 9129058, 9130073,
    9131089, 9132105, 9133121, 9134137, 9135153, 9136169, 9137185, 9138200,
    9139216, 9140232, 9141248, 9142264, 9143280, 9144296, 9145311, 9146327,
    9147343, 9148359, 9149375, 9150391, 9151407, 9152422, 9153438, 9154454,
    9155470, 9156486, 9157502, 9158518, 9159534, 9160549, 9161565, 9162581,
    9163597, 9164613, 9165629, 9166645, 9167660, 9168676, 9169692, 9170708 ;

...... (lines removed)

 TOPOGRAPHY =
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 51.99445,
    124.9918, 103.0026, 170.9917, 191.9973, 183.0012, 214.9956, 215,
    204.0016, 246.9934, 231.0025, 221.0016, 226.999, 224.0005, 249.9954,
    211.0071, 232.9959, 242.9981, 224.0038, 231.9984, 222.0021, 240.9959,
    271.9932, 265.0016, 284.9954, 286.9995, 289.9993, 293.999, 311.9955,
    274.0097, 294.9946, 282.0034, 302.9944, 317.9959, 311.002, 314.9989,
    330.9954, 338.9977,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 105.9893, 183.9907,
    183.9983, 189.9981, 199.9982, 200.999, 215.9977, 224.9986, 217.0007,
    264.993, 260.0003, 273.9973, 246.0043, 249.9992, 291.9925, 299.998,
    300.9991, 277.004, 256.0034, 272.9964, 252.004, 246.001, 243.0005,
    256.9968, 242.0032, 268.9941, 253.0037, 249.0013, 272.9945, 271.0006,
    277.9984, 263.0042, 266.9991, 277.9973, 312.9904, 322.9971, 295.0079,
    295.0002, 273.0067, 336.9814, 334.0009,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 96.98979, 189.9899,
    180.001, 205.9969, 205.9997, 213.9988, 236.9966, 240.999, 255.9975,
...... etc etc

========================

The command I use to extract this topography variable is
gdal_translate -of AAIGrid  NETCDF:"merapi.res.nc":TOPOGRAPHY
merapi.topography.asc

and the header of the resulting file is
ncols        59
nrows        64
xllcorner    0.000000000000
yllcorner    -64.000000000000
cellsize     1.000000000000
NODATA_value 9.9692099683868690468e+36
      0      0      0      0      0      0      0      0      0      0
0      0      0      0      0      0      0      0      0      0      0
51.99444
9615478515625 124.9918365478515625 103.00257110595703125 170.99169921875
191.9973297119140625 183.001190185546875 214.99560546875    215
204.001617431640625
 246.993438720703125 231.0025177001953125 ........

---------------------------------------------------------

I do not understand why the cellsize is 1.000000 when it should be in the
order of 1000.
Also, I do not understand why xllcorner and yllcorner aren't obtained as
e.g. the minimum values of the x and y vectors.

The version of gdal-bin is 1.5.2-3ubuntu1 and my platform is Ubuntu 9.04 (64
bit).

By the way, trying to contour directly from the NetCDF puts all the contours at the south pole. I suspect this is another manifestation of the origins being ignored.

Can anyone help please?

Cheers and thanks
Ole Nielsen

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