TopoScript .BIL File Format

TopoScript .BIL File Format

Q: What is the BIL file format read by TAP with TopoScript?

A: TAP 4.5 and later, which includes the TopoScript engine for reading elevation data files, can read the .BIL format using the script for USGS National Elevation Dataset (NED) data files.

(To index and read the .BIL files, be sure the NED script is indexed in your TAP system.)

The .BIL format TAP is looking for includes four files. The files must have identical file names with the specified file extensions. (This description is from the USGS BIL format documentation for Digital Elevation Model, or DEM, files.)

Data File (.BIL)

The elevation values in a simple binary raster. There are no header or trailer bytes imbedded in the image. The data are stored in row major (all the data for row 1, followed by all the data for row 2, etc.).

Header File (.HDR)

The DEM header file is an ASCII text file containing the size and coordinate information for the DEM. The following keywords are used in the header file:

BYTEORDER	byte order in which image pixel values are stored M = Motorola byte order (most significant byte first) I = Intel byte order (least significant byte first)
LAYOUT	organization of the bands in the DEM BIL = band interleaved by line (the DEM is a single band image)
NROWS	number of rows in the image
NCOLS	number of columns in the image
NBANDS	number of spectral bands for the image (must be 1)
NBITS	number of bits per pixel (32, 16, or 8)
BANDROWBYTES	number of bytes per band per row (number of columns times the number of bytes per pixel, determined from NBITS above; number of bytes = NBITS/8)
TOTALROWBYTES	total number of bytes of data per row (number of columns times the number of bytes per pixel. For the single band image, TOTALROWBYTES will always equal BANDROWBYTES
BANDGAPBYTES	number of bytes between bands; must be 0 for single band image

Example header file:

BYTEORDER M

LAYOUT BIL

NROWS 6000

NCOLS 4800

NBANDS 1

NBITS 32

BANDROWBYTES 9600

TOTALROWBYTES 9600

BANDGAPBYTES 0

World File (.BLW)

The world file is an ASCII text file containing coordinate information. It is used for georeferencing of image data. The world data file does not use keywords. Each entry must be included as shown in the example, in the order given.

The world file contains (in this order):

X dimension of a pixel in decimal degrees
Rotation term (always zero)
Rotation term (always zero)
Negative Y dimension of a pixel in decimal degrees
Longitude of the center of the upper-left pixel
Latitude of the center of the upper-left pixel

Example World File

0.0002777777778

0.0000000000000

-0.000277777778

-99.995833333333

39.9958333333333

Projection File (.PRJ)

The projection information file is an ASCII text file that describes the projection of the DEM and source map image.

The Projection value must be geographic, with units of DD (decimal degrees).
The Zunits (elevation values) can be meters, decimeters, feet, etc.
The Xshift and Yshift values must be zero.

Example Projection File

PROJECTION GEOGRAPHIC

DATUM NAD83

ZUNITS METERS

UNITS DD

SPHEROID GRS1980

XSHIFT 0.0000000000

YSHIFT 0.0000000000