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I've got a 3 arc second digital terrain model represented as a 32-bit GeoTIFF that looks like this:

$ gdal_info gom03_v1_0.tif

Driver: GTiff/GeoTIFF
Files: gom03_v1_0.tif
Size is 10200, 7800
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 = (-71.499580383300781,45.999583333330001)
Pixel Size = (0.000833333333333,-0.000833333333332)
Metadata:
  TIFFTAG_SOFTWARE=nc2geotiff
  AREA_OR_POINT=Area
Image Structure Metadata:
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  ( -71.4995804,  45.9995833) ( 71d29'58.49"W, 45d59'58.50"N)
Lower Left  ( -71.4995804,  39.4995833) ( 71d29'58.49"W, 39d29'58.50"N)
Upper Right ( -62.9995804,  45.9995833) ( 62d59'58.49"W, 45d59'58.50"N)
Lower Right ( -62.9995804,  39.4995833) ( 62d59'58.49"W, 39d29'58.50"N)
Center      ( -67.2495804,  42.7495833) ( 67d14'58.49"W, 42d44'58.50"N)
Band 1 Block=10200x1 Type=Float32, ColorInterp=Gray

Note that the AREA_OR_POINT attribute is set to AREA.

When I convert this to ARC ASCII Grid format using GDAL:

gdal_translate gom03_v1_0.tif -of AAIGrid test.asc

and look at the top of the resulting ASCII file, I see:

$ more test.asc

ncols        10200
nrows        7800
xllcorner    -71.499580383301
yllcorner    39.499583333335
cellsize     0.000833333333
 315.814208984375 314.712371826171875 ...

These xllcorner,yllcorner values are the same as the GeoTIFF Lower Left values above. From the official description of the ESRI ASCII Raster Format, however, either xllcorner,yllcorner or xllcenter,yllcenter can be specified. Since the GeoTIFF values represented AREA (not POINT), isn't the specification of xllcorner and yllcorner incorrect?

Shouldn't the ASCII file in this case really read as below?

$ more test.asc

ncols        10200
nrows        7800
xllcenter    -71.499580383301
yllcenter    39.499583333335
cellsize     0.000833333333
 315.814208984375 314.712371826171875 ...
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1 Answer 1

up vote 2 down vote accepted

No, the xllcorner and yllcorner specification is correct, because the GeoTIFF specifies Corner Coordinates (not center coordinates).

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Uggh. Should have realized that. Thanks! –  Rich Signell Mar 14 '13 at 19:06

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