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

[Eli Adam]

On Thu, Jun 11, 2015 at 2:38 PM, Ari Simmons <ari.ucb.fire at gmail.com> wrote:
> One notably huge difference is that there is a huge jump in pixel size (from
> 0.000833333333323 to 205.686440189378940)...
>
> ah, duh. Unit change.
>
> On Thu, Jun 11, 2015 at 2:26 PM, Ari Simmons <ari.ucb.fire at gmail.com> wrote:
>>
>> 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

That is the right speed (~0 seconds) and disk space (~0MB).  That is
because it doesn't actually do anything except write down notes of
what it would have done (and will do in the future if you ask for it).
I like testing large VRT files by outputting a subwindow tif like
this:

gdal_translate -13813007 5569641 -13809113 5572598
srtm_merged_3857.vrt my_subwindow.tif --config  GDAL_CACHEMAX 800

If things look good on the subwindow, then it is time for all the I/O
waiting, processor cycles, and storage space.  Evaluating a few
places, especially where tiles come together can find things early
without all the wait.

HTH, Eli

>>
>> >> 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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