[gdal-dev] gdalwarp on a .vrt: Using internal nodata values (eg 0) for image (?)

[Ari Simmons]

I am experimenting with using 'gdalwarp' on a .vrt (first time), but I'm
not sure what I'm doing wrong. I've been running this:

>> gdalwarp -s_srs EPSG:4326 -t_srs EPSG:3857 -r bilinear -of VRT
merged.vrt srtm_merged_3857.vrt

and it processes fast (far far *too fast* for this global file) and returns

>> Creating output file that is 194835P X 479814L.
>> Processing input file merged.vrt
>> Using internal nodata values (eg. 0) for image merged.vrt

The return .vrt file definitely doesn't appear right...a quick look at the
returned file:

input:

Size is 432000, 208800
Coordinate System is:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433],
    AUTHORITY["EPSG","4326"]]
Origin = (-180.000416666666670,84.000416666666666)
Pixel Size = (0.000833333333323,-0.000833333333323)
Corner Coordinates:
Upper Left  (-180.0004167,  84.0004167) (180d 0' 1.50"W, 84d 0' 1.50"N)
Lower Left  (-180.0004167, -89.9995833) (180d 0' 1.50"W, 89d59'58.50"S)
Upper Right ( 179.9995833,  84.0004167) (179d59'58.50"E, 84d 0' 1.50"N)
Lower Right ( 179.9995833, -89.9995833) (179d59'58.50"E, 89d59'58.50"S)
Center      (  -0.0004167,  -2.9995833) (  0d 0' 1.50"W,  2d59'58.50"S)
Band 1 Block=128x128 Type=Int16, ColorInterp=Gray
  NoData Value=0
  Overviews: 216000x104400, 108000x52200, 54000x26100, 27000x13050,
13500x6525,
6750x3263, 3375x1632, 1688x816, 844x408, 422x204

and output file:

 Size is 193861,
479814                                                          Coordinate
System is:
PROJCS["WGS 84 /
Pseudo-Mercator",
GEOGCS["WGS
84",
DATUM["WGS_1984",
SPHEROID["WGS
84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]],
PROJECTION["Mercator_1SP"],
PARAMETER["central_meridian",0],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0],
UNIT["metre",1,
AUTHORITY["EPSG","9001"]],
AXIS["X",EAST],
AXIS["Y",NORTH],
EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0
+x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext
+no_defs"],
AUTHORITY["EPSG","3857"]]
Origin =
(-19837179.642485119000000,18807657.848855246000000)
Pixel Size =
(205.686440189378940,-205.686440189378940)                         Corner
Coordinates:
Upper Left  (-19837179.642,18807657.849) (178d12' 1.50"W, 84d 0'
1.50"N)        Lower Left  (-19837179.642,-79883575.764) (178d12' 1.50"W,
89d59'58.50"S)       Upper Right (20037399.339,18807657.849)
(179d59'56.47"E, 84d 0' 1.50"N)         Lower Right
(20037399.339,-79883575.764) (179d59'56.47"E, 89d59'58.50"S)
Center      (  100109.848,-30537958.958) (  0d53'57.49"E, 89d
2'43.78"S)        Band 1 Block=512x128 Type=Int16,
ColorInterp=Gray                                 Overviews: 96931x239907,
48466x119954, 24233x59977, 12117x29989, 6059x14995, 3030x7498, 1515x3749,
758x1875, 379x938, 191x471

One notably huge difference is that there is a huge jump in pixel size
(from 0.000833333333323 to 205.686440189378940)...
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