American Society for Photogrammetry & Remote Sensing Page 6 of 28
LAS SPECIFICATION
Version 1.4-R13
File Source ID: This field should be set to a value ranging from 1 to 65,535. If this file was
derived from an original flight line, this is often the flight line number. A value of zero (0) is
interpreted to mean that an ID has not been assigned. In this case, processing software is free to
assign a number. Note that this scheme allows a LIDAR project to contain up to 65,535 unique
sources. A source can be an original flight line or it can be result of merge and/or extract
operations.
Global Encoding: This is a bit field used to indicate certain global properties about the file. In
LAS 1.2 (the version in which this field was introduced), only the low bit is defined (this is the bit,
that if set, would have the unsigned integer yield a value of 1). This bit field is defined as:
Table 4: Global Encoding - Bit Field Encoding
Bits Field Name Description
0 GPS Time Type The meaning of GPS Time in the point records. If
this bit is not set, the GPS time in the point record
fields is GPS Week Time (the same as versions
1.0 through 1.2 of LAS). Otherwise, if this bit is
set, the GPS Time is standard GPS Time (satellite
GPS Time) minus 1 x 10
9
(Adjusted Standard
GPS Time). The offset moves the time back to
near zero to improve floating point resolution.
1 Waveform Data
Packets Internal
If this bit is set, the waveform data packets are
located within this file (note that this bit is mutually
exclusive with bit 2). This is deprecated now.
2 Waveform Data
Packets External
If this bit is set, the waveform data packets are
located externally in an auxiliary file with the same
base name as this file but the extension *.wdp.
(note that this bit is mutually exclusive with bit 1)
3 Return numbers
have been
synthetically
generated
If this bit is set, the point return numbers in the
point data records have been synthetically
generated. This could be the case, for example,
when a composite file is created by combining a
First Return File and a Last Return File. In this
case, first return data will be labeled “1 of 2 ” and
second return data will be labeled “2 of 2 ”
4 WKT If set, the Coordinate Reference System (CRS) is
WKT. If not set, the CRS is GeoTIFF. It should
not be set if the file writer wishes to ensure legacy
compatibility (which means the CRS must be
GeoTIFF)
5:15 Reserved Must be set to zero
Project ID (GUID data): The four fields that comprise a complete Globally Unique Identifier
(GUID) are now reserved for use as a Project Identifier (Project ID). The field remains optional.
The time of assignment of the Project ID is at the discretion of processing software. The Project
ID should be the same for all files that are associated with a unique project. By assigning a
Project ID and using a File Source ID (defined above) every file within a project and every point
within a file can be uniquely identified, globally.
Version Number: The version number consists of a major and minor field. The major and minor
fields combine to form the number that indicates the format number of the current specification
itself. For example, specification number 1.4 would contain 1 in the major field and 4 in the minor
field. It should be noted that the LAS Working Group does not associate any particular meaning
to major or minor version number.
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