Standard ExtraByte Definitions

!!!DRAFT!!!

(Use with caution. This document is still in draft format. Please reference Issue 37 and related for discussion.)

A point has "extra bytes" when the Point Data Record Length in the LAS header is set to a larger value than the minimum required by the Point Data Record Format. LAS 1.4 has introduced a new VLR that describes the meaning of these "extra bytes" with one descriptor per attribute. This structure has contained one Reserved field that had to be set to zero in the past, but as of Revision RXX can be assigned a number enumerated in this registry.

Adding Standard ExtraBytes to this registry makes it easier to write, read, and exploit these optional point attributes across different software packages and applications.

By not including this registry in the specification itself, we (the LAS Working Group) hope to keep this registry flexible and make it easier to add additional attributes. Users are permitted to deviate from this standard; however, any ExtraBytes not adhering to this standard must set the ID code to zero.

Have a standard ExtraByte you'd like to add? Create an issue on the GitHub interface and we'll talk about it!

DRAFT Standard ExtraByte Definitions

These definitions are currently draft and not yet finalized. See Issue 37 and Issue 60 to contribute your insight into the discussion.

The standard ExtraBytes are defined in the table below. Detailed descriptions are provided in the sections following.

Common Name	ID	Formal Name (required)	Size	data_type	Preferred Units	Description
Echo Width	200	echo width	2	3 (ushort)	Nanoseconds [ns]	Full width at half maximum [ns]
Height Above Ground	201	height above ground	2	4 (short)	Meters [m]	Vertical point to TIN distance
Group/Object ID	202	groupid	8	8 (ulonglong)	Unit-less	64-bit group identifier
Horizontal Uncertainty	112	sigma xy	1	1 (uchar)	Meters [m]	Horiz. coordinate uncertainty
Vertical Uncertainty	113	sigma z	1	1 (uchar)	Meters [m]	Vert. coordinate uncertainty
Bathymetric Flags	104	bathymetric flags	1	1 (uchar)	Bit field	Bathymetric bit flags
Refracted Range	114	refracted range	2	3 (ushort)	Meters [m]	Submerged vector length post-ref
Beam ID	1	beam id	1	1 (uchar)	Unit-less	Extended channel ID

• The ID field has not yet been implemented.
• The formal name is required and must be encoded exactly as defined, with unused space zero-filled.
• Size, data type, and units are strongly recommended defaults but not required. Deviations must be done judiciously.
• Description provided is a suggested default but may vary. Unused space must be zero-filled.
• Example scale, offset, and nodata values are provided below if applicable.

Echo Width

The echo width is defined as full width at half maximum of the received echo signal and is measured in nanoseconds (ns). The echo width can be regarded as a quality indicator of received echo signals. The width of a received echo signal depends on the geometry of the hit target.

• Valid range: 0.1 to 1000.0.
• Preferred units: Nanoseconds [ns]
• Recommended NODATA: 0 raw, 0.0 scaled
• Recommended scale: 0.1
• Recommended offset: 0.0
• Recommended min/max: true values
• Alternative name: pulse width

Height Above Ground

Stores the point’s height above a ground surface. Units will often be whatever vertical units are relevant to the LAS file, but meters are recommended for uniformity between implementations. Points below the ground surface will have negative values. This attribute can also be used for surfaces other than ground, such as a water or canopy surface.

There is a variant of this using 4-byte integers (signed long), but the standard version is 2 bytes.

• Valid range: -327.68 to 327.67
• Preferred units: Meters [m]
• Recommended NODATA: 32768 raw, 327.68 scaled
• Recommended scale: 0.01
• Recommended offset: 0.00
• Recommended min/max: true values
• Alternative name: height above surface

Group / Object ID

Provides an identifier for grouping points together as an object or group, such as an index value to a shapefile polygon, lookup identifier for an attribute table stored externally, or feature identifier for fine-grained segmentation.

• Valid range: 1 to max
• Preferred units: n/a
• Recommended NODATA: 0
• Recommended scale: n/a
• Recommended offset: n/a
• Recommended min/max: n/a

Horizontal & Vertical Uncertainty

Provides an alternative means to convey the total propagated uncertainty of a point’s coordinates, replacing the now-deprecated “XYZ Coordinate Uncertainty” previously included with the Topobathy LDP. Informally this could also be referred to as the accuracy or precision of the point coordinates.

The XY dimensions have been combined to make the horizontal uncertainty independent from the projection.

• Valid range: 0.00 to 2.54
• Preferred units: Meters [m]
• Recommended NODATA: 255 raw, 2.55 scaled
• Recommended scale: 0.01
• Recommended offset: 0.00
• Recommended min/max: true values

Bathymetric Flags

The bathymetry bit flags are intended as a means of indicating other topobathymetric point or processing attributes that have not yet been identified and/or do not merit a separate point class or separate Extra Bytes VLR.

Bit 0 is used to flag whether or not a refraction correction has been applied.

Currently, the other 7 bits are reserved for future use.

This attribute is defined in the Topobathy LDP.

• Valid range: 0 to 1
• Preferred units: n/a (bit field)
• Recommended NODATA: 0
• Recommended scale: n/a
• Recommended offset: n/a
• Recommended min/max: n/a

Refracted Range

The submerged distance along the slanted range vector from the origin to the measured point, defined as the post-refraction distance from the point of intersection with the water surface to the measured point itself. Useful for depth-based intensity corrections based on attenuation.

Also known as refracted vector length.

• Valid range: 0.01 to 655.35
• Preferred units: Meters [m]
• Recommended NODATA: 0 raw, 0.00 scaled
• Recommended scale: 0.01
• Recommended offset: 0.00
• Recommended min/max: true values
• Alternative names: refracted vector length, refracted depth

Beam ID

Conceptually identical to the Channel field in PDRF 6-10, but with a larger valid range. This field provides a method to uniquely identify the source pulse, beam, or pixel of the point.

• Valid range: 0 to 254
• Preferred units: n/a
• Recommended NODATA: 255 (max)
• Recommended scale: n/a
• Recommended offset: n/a
• Recommended min/max: n/a
• Alternative name: extended channel