LAS Global Encoding Bytes Explained

The Global Encoding Bytes in an LAS file 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 in the LAS v1.4 specification as:

Global Encoding – Bit Field Encoding

Bits Field Name Description
0 GPS Time Type The meaning of GPS Time in the Point Records
0 (not set) -> GPS time in the point record fields is GPS Week Time (the same as previous versions of LAS)
1 (set) -> GPS Time is standard GPS Time (satellite GPS Time) minus 1 x 10^9. 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

 

So what does this all mean to someone who is not a programmer?

If one uses a program such as LP360 or LDDump to see the Global Encoding value, one will see the resulting value of the two bytes and not the individual bit values. So what we have are 16 bits that may have a value of either 0 or 1 and each bit is an increasing power of 2 when used to determine the value. There is a really nice explanation found here. More to the point, some common examples of Global Encoding values seen in LAS file headers and their meanings can be found in the chart below.

 

Global Encoding Value Global Encoding Bytes Description
0 0000000000000000 The meaning of GPS time in the point record fields is GPS Week Time, and the Coordinate Reference System (CRS) is GeoTIFF.
1 0000000000000001 The meaning of GPS Time in the Point Records is standard GPS Time, and the CRS is GeoTIFF.
5 0000000000000101 The meaning of GPS Time in the Point Records is standard GPS Time (satellite GPS Time) minus 1 x 10^9, the waveform data packets are located externally (wpd), and the CRS is GeoTIFF.
16 0000000000010000 The meaning of GPS time in the point record fields is GPS Week Time, and the CRS is WKT
17 0000000000010001 The meaning of GPS Time in the Point Records is standard GPS Time (satellite GPS Time) minus 1 x 10^9, and the CRS is WKT
21 0000000000010101 The meaning of GPS Time in the Point Records is standard GPS Time (satellite GPS Time) minus 1 x 10^9, the waveform data packets are located externally (wpd), and the CRS is WKT

 

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