Format of the DEM index file

In workflows generating orthorectification, you can enter the name of a folder containing a set of DEM tiles in the DEM Source parameter.

When you specify a folder, the module reads it for a file named index.txt, which must match the DEM index format.

The DEM index file (index.txt) and each DEM tile must meet the following conditions. Each tile must:

Be in the same coordinate system
Be raster-aligned; that is, the geocoding for all tiles is defined so that all pixels fall on a common raster grid
Use the same data type
Be based on the same height datum
Use the same NoData value
Use the same ELFACTOR value

The following text shows an example of a DEM index file.

MAPUNITS       LONG/LAT D000
DATATYPE       16S
DBEC           1
BACKELEV       -32768.000
ELEVREF        MSL
ELEVUNIT       METER
ELFACTOR       0.000 1.000000000000
RES_XY         0.0008333333333333 0.0008333333333333
#Filename                ULX                    ULY                     LRX                   LRY
srtm_01_02.tif   -180.000416666676760    55.000416618227803   -174.999583333343420    49.999583284894470
srtm_01_07.tif   -180.000416690891140    30.000417247801863   -174.999583357557810    24.999583914468531
srtm_01_12.tif   -180.000416690891140     5.000416642442191   -174.999583357557810    -0.000416690891143
srtm_01_15.tif   -180.000416666676760    -9.999582994342006   -174.999583333343420   -15.000416327675339
srtm_01_16.tif   -180.000416666676760   -14.999583115413941   -174.999583333343420   -20.000416448747274
srtm_01_17.tif   -180.000416666676760   -19.999583236485876   -174.999583333343420   -25.000416569819208
srtm_01_18.tif   -180.000416666676760   -24.999583357557810   -174.999583333343420   -30.000416690891143

The first eight lines of the file describe common values that apply to each image; they must be in the order shown in the example with no comments or blank lines between them. The remaining lines describe the image boundary records for each image in the DEM tile data set. Comments (lines beginning with a #) are allowed in the list of DEM tiles in the file.

The format of the eight header lines is:

MAPUNITS: The EPSG projection code or PCI map-units string for the coordinate system.
DATATYPE: The data type of the files.
Acceptable values are:
- 8U
- 8S
- 16U
- 16S
- 32U
- 32S
- 32R
- 64U
- 64S
- 64R
DBEC: The channel number for the input digital elevation model (DEM) elevation channel to be processed.
BACKELEV: The background elevation (NoData) value in the input DEM elevation layer.
ELEVREF: The vertical reference for the elevation values contained in the source DEM, or for the constant ELFACTOR value, if it is used.
ELEVUNIT: The units of the elevation values that are stored as pixel values in the input DEM, as specified either by FILEDEM or by the first entry in ELFACTOR.
ELFACTOR: When an input digital elevation model (FILEDEM) is specified, the value of this parameter is used to shift and scale the DEM pixel values to values in the units indicated by the value of the ELEVUNIT parameter.
RES_XY: The x and y image-resolution values.

Each line following the header lines is treated as either a comment line or as a raster-tile description. Empty lines or those beginning with a number sign (#) are treated as comment lines. Each raster-tile description line contains four parameters separated by white space:

DEM raster-file name: path to the DEM raster file. This may be an absolute path or a relative path. A relative path is relative to the DEM index file itself.
Upper-left x-coordinate: the x-coordinate at raster-column location 0.0, raster-row location 0.0
Upper-left y-coordinate: the y-coordinate at raster-column location 0.0, raster-row location 0.0
Lower-right x-coordinate: the x-coordinate at raster-column location 'numPixels', raster-row location 'numLines', where numPixels and numLines are the dimensions of the raster.
Lower right y-coordinate: the y-coordinate at raster-column location 'numPixels', raster-row location 'numLines', where numPixels and numLines are the dimensions of the raster.

Note: This description of the raster boundary provides coordinates at the outer extents of the pixels; with butt-joined tiles, the tile records should indicate gap-free coverage.

As individual raster tiles are accessed, their properties are compared with the values listed in the DEM index file. If they do not conform to the indicated values, an error message appears in the log.