gdalwarp cambia el valor de los píxeles

Question

Tengo un problema con la reproyección de un raster (geotiff): después de realizar la reproyección de EPSG:31468 a EPSG:32632, los valores de los píxeles han cambiado.

En QGIS utilicé Raster->Projection->warp para reproyectar. Es un raster de velocidad del viento y en el raster original los datos tienen valores de 0 - 13, en el archivo reproyectado son de -32766 a -17211

Cuando miro el metafichero de la trama original dice STATISTICS_MAXIMUM=-17211 STATISTICS_MEAN=-30685.561753801 STATISTICS_MINIMUM=-32766

¿Por qué muestra sin embargo un rango de datos de 0-13 (que es correcto)? Cuando hago clic en un píxel me da la misma información (0-13).

¿Qué puedo hacer para obtener el mismo rango de datos en el archivo de reproyección?

    Driver: GTiff/GeoTIFF
    Files: .....20160613T0100.tif
           .....20160613T0100.tif.aux.xml
    Size is 701, 401
    Coordinate System is:

PROJCS["MGI / Austria Lambert",
    GEOGCS["MGI",
        DATUM["Militar_Geographische_Institute",
            SPHEROID["Bessel 1841",6377397.155,299.1528128,
                AUTHORITY["EPSG","7004"]],
            TOWGS84[577.326,90.129,463.919,5.137,1.474,5.297,2.4232],
            AUTHORITY["EPSG","6312"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4312"]],
    PROJECTION["Lambert_Conformal_Conic_2SP"],
    PARAMETER["standard_parallel_1",49],
    PARAMETER["standard_parallel_2",46],
    PARAMETER["latitude_of_origin",47.5],
    PARAMETER["central_meridian",13.33333333333333],
    PARAMETER["false_easting",400000],
    PARAMETER["false_northing",400000],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","31287"]]
Origin = (19500.000000000000000,620500.000000000000000)
Pixel Size = (1000.000000000000000,-1000.000000000000000)
Metadata:
  AREA_OR_POINT=Area
  NC_GLOBAL#Conventions=CF-1.6
  NETCDF_DIM_EXTRA={time}
  NETCDF_DIM_time_DEF={720,4}
  NETCDF_DIM_time_VALUES={0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719}
  projection#false_easting=400000
  projection#false_northing=400000
  projection#grid_mapping_name=lambert_conformal_conic
  projection#inverse_flattening=299.152812800001
  projection#latitude_of_projection_origin=47.5
  projection#longitude_of_central_meridian=13.3333333333333
  projection#longitude_of_prime_meridian=0
  projection#semi_major_axis=6377397.155
  projection#spatial_ref=PROJCS["MGI / Austria Lambert",GEOGCS["MGI",DATUM["Militar_Geographische_Institute",SPHEROID["Bessel 1841",6377397.155,299.1528128,AUTHORITY["EPSG","7004"]],TOWGS84[577.326,90.129,463.919,5.137,1.474,5.297,2.4232],AUTHORITY["EPSG","6312"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4312"]],PROJECTION["Lambert_Conformal_Conic_2SP"],PARAMETER["standard_parallel_1",49],PARAMETER["standard_parallel_2",46],PARAMETER["latitude_of_origin",47.5],PARAMETER["central_meridian",13.33333333333333],PARAMETER["false_easting",400000],PARAMETER["false_northing",400000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","31287"]]
  projection#standard_parallel={49,46}
  time#standard_name=time
  time#units=hours since 2016-06-01 00:00:00
  WS#add_offset=28.7745609082447
  WS#grid_mapping=projection
  WS#scale_factor=0.0008781835105977141
  WS#valid_range={-32766,32767}
  WS#_FillValue=-32767
  x#standard_name=projection_x_coordinate
  x#units=m
  y#standard_name=projection_y_coordinate
  y#units=m
Image Structure Metadata:
  COMPRESSION=DEFLATE
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (   19500.000,  620500.000) (  8d 5'27.33"E, 49d22' 4.74"N)
Lower Left  (   19500.000,  219500.000) (  8d26'18.47"E, 45d46' 4.44"N)
Upper Right (  720500.000,  620500.000) ( 17d45' 3.70"E, 49d24' 5.38"N)
Lower Right (  720500.000,  219500.000) ( 17d27'28.54"E, 45d47'57.17"N)
Center      (  370000.000,  420000.000) ( 12d56' 1.00"E, 47d40'45.38"N)
Band 1 Block=701x5 Type=Int16, ColorInterp=Gray
  Min=-32766.000 Max=-17211.000 
  Minimum=-32766.000, Maximum=-17211.000, Mean=-30685.562, StdDev=1700.194
  NoData Value=-32767
  Offset: 28.7745609082447,   Scale:0.000878183510597714
  Metadata:
    add_offset=28.7745609082447
    grid_mapping=projection
    NETCDF_DIM_time=289
    NETCDF_VARNAME=WS
    scale_factor=0.0008781835105977141
    STATISTICS_MAXIMUM=-17211
    STATISTICS_MEAN=-30685.561753801
    STATISTICS_MINIMUM=-32766
    STATISTICS_STDDEV=1700.1935729576
    valid_range={-32766,32767}
    _FillValue=-32767

Answer 1

Su conjunto de datos es de tipo INT16, con valores permitidos entre -32766 y +32766. Para obtener los valores correctos de los datos, hay que aplicar la escala y el desplazamiento, como se indica en los metadatos del archivo:

Offset: 28.7745609082447,   Scale:0.000878183510597714

Así que con un valor almacenado de -32254, el valor real será:

-32254 * 0.000878183510597714 + 28.7745609082447 = 0.44963

En algunos casos, gdalwarp no puede manejar la escala y el desplazamiento correctamente, por lo que hay que hacerlo manualmente:

 gdal_translate -ot FLOAT64 -unscale src_dataset tmp.tif
 gdal_calc.py -A tmp.tif --type='Float64' --outfile=result.tif --calc="A*0.000878183510597714+28.7745609082447"
 gdalwarp -s_srs EPSG:31287 -t_srs EPSG:32632 result.tif warped.tif