4. Optional VLR Definitions¶
4.1. Classification Lookup¶
User ID |
LASF_Spec |
Record ID |
0 |
Record Length after Header |
256 records * 16 bytes per struct |
struct CLASSIFICATION {
uint8_t ClassNumber;
char Description[15];
}; //total of 16 bytes
4.2. Text Area Description¶
User ID |
LASF_Spec |
Record ID |
3 |
This VLR/EVLR is used for providing a textual description of the content of the LAS file. It is a null-terminated, free-form ASCII string.
4.3. Extra Bytes¶
User ID |
LASF_Spec |
Record ID |
4 |
Record Length after Header |
n descriptors * 192 bytes |
The Extra Bytes VLR provides a mechanism whereby additional information can be added to the end of a standard Point Record. This VLR has been added to LAS 1.4 to formalize a process that has been used in prior versions of LAS. It is envisioned that software that is not cognizant of the meaning of the extra bytes will simply copy these bytes when manipulating files.
This VLR is only required for LAS files where points contain user-defined “extra bytes”. This happens when the point record size is set to a larger value than required by the point type. For example, if a LAS file that contains point type 1 has a point record size of 32 instead of 28, there are 4 “extra bytes”. The Extra Bytes VLR contains a simple description of the type and the meaning of these “extra bytes” so they can be accessed in a consistent manner across applications. The extra bytes descriptor is defined as follows:
struct EXTRA_BYTES {
uint8_t reserved[2]; // 2 bytes
uint8_t data_type; // 1 byte
uint8_t options; // 1 byte
char name[32]; // 32 bytes
uint8_t unused[4]; // 4 bytes
anytype no_data; // 8 bytes
uint8_t deprecated1[16]; // 16 bytes
anytype min; // 8 bytes
uint8_t deprecated2[16]; // 16 bytes
anytype max; // 8 bytes
uint8_t deprecated3[16]; // 16 bytes
double scale; // 8 bytes
uint8_t deprecated4[16]; // 16 bytes
double offset; // 8 bytes
uint8_t deprecated5[16]; // 16 bytes
char description[32]; // 32 bytes
}; // total of 192 bytes
The 4 “extra bytes” could, for example, be of data_type 9 - a 4-byte floating point value - that specifies an “echo width” for each return. In this case there would be one EXTRA_BYTES struct in the payload of this VLR. In another example, four EXTRA_BYTES structs in the VLR payload could describe 14 “extra bytes” in each point record:
“laser pulse direction [0]” - data_type = 9 (float)
“laser pulse direction [1]” - data_type = 9 (float)
“laser pulse direction [2]” - data_type = 9 (float)
“pulse width” - data_type = 3 (uint16_t)
In this example, an array of three individual floats collectively specify a “laser pulse direction” for that point, and one uint16_t integer specifies a “pulse width” for that point.
The “extra bytes” are made accessible via a unique name. The “name” field distinguishes the additional point attributes that software may add to the points in a LAS file so they can be accessed later in a consistent manner by another software. Descriptive names such as “normalized reflectivity”, “echo width”, or “shading normal” are encouraged. The use of generic names such as “variable1” or “temp1” is discouraged.
Only one Extra Bytes VLR may exist in a given LAS file. If an Extra Bytes VLR already exists in a LAS file, then new Extra Byte definitions must be added to the existing Extra Bytes VLR.
Multiple sequential “extra byte” records can compose an array of associated values. It is recommended that each member’s name be consistent with other members, only varying by an index number wrapped in square brackets, as in the above example. Zero-indexed arrays are encouraged. Previous revisions of the LAS 1.4 specification utilized data_types 11-30 to define standard two- and three-member arrays, but this feature was never widely implemented and was deprecated in R14 to simplify implementation.
Any unused characters in the “name” or “description” fields must be set to zero.
Value |
Meaning |
Size on Disk |
|---|---|---|
0 |
undocumented extra bytes |
specify value in
|
1 |
uint8_t |
1 byte |
2 |
int8_t |
1 byte |
3 |
uint16_t |
2 bytes |
4 |
int16_t |
2 bytes |
5 |
uint32_t |
4 bytes |
6 |
int32_t |
4 bytes |
7 |
uint64_t |
8 bytes |
8 |
int64_t |
8 bytes |
9 |
float |
4 bytes |
10 |
double |
8 bytes |
11-30 |
Deprecated |
deprecated |
31-255 |
Reserved |
not assigned |
Bits |
Field Name |
Description |
|---|---|---|
0 |
no_data_bit |
If set, the |
1 |
min_bit |
If set, the |
2 |
max_bit |
If set, the |
3 |
scale_bit |
If set, each value should be multiplied by the corresponding scale value (before applying the offset). |
4 |
offset_bit |
If set, each value should be translated by the corresponding offset value (after applying the scaling). |
The bit mask in the “options” field specifies whether the min and max range of the value has been set (i.e., is meaningful), whether the scale and/or offset values are set with which the “extra bytes” are to be multiplied and translated to compute their actual value, and whether there is a special value that should be interpreted as NO_DATA. By default all bits are zero which means that the values in the corresponding fields are to be disregarded. Any unused “no_data”, “min”, “max”, “scale”, or “offset” fields must be set to zero.
If the selected data_type is less than 8 bytes, the no_data, min, and max fields should be upcast into 8-byte storage. For any float these 8 bytes would be upcast to a double, for any uint8_t, uint16_t, or uint32_t they would be upcast to a uint64_t and for any int8_t, int16_t, or int32_t, they would be upcast to a int64_t.
If used, the min and max fields reflect the actual minimum and maximum values of the attribute in the LAS file, in its raw form, without any scale or offset values applied.
The “reserved” field, the “unused” field, and the “deprecated” fields must be set to zero and may be used in a future revision.
A LAS file contains “undocumented extra bytes” when there are “extra bytes” but when there is no Extra Bytes VLR that describes them or when there are more “extra bytes” than described in an existing Extra Bytes VLR.
When adding an “Extra Bytes” VLR to a LAS file that contains “undocumented extra bytes” they must be designated as data_type == 0 with the options bit field storing the number of undocumented bytes.
A LAS file has an “extra bytes mismatch” if the Extra Bytes VLR describes more “extra bytes” than each LAS point actually has. The occurrence of an “extra bytes mismatch” renders the Extra Bytes VLR invalid.
4.4. Superseded¶
User ID |
LASF_Spec |
Record ID |
7 |
This LASF Record ID is used to negate an existing VLR/EVLR when rewriting the file (to remove the undesired VLR/EVLR). It is used, for example, when updating a record such as projection information where a new EVLR is appended to the end of the LAS file. The existing VLR which has been superseded must be marked with the SUPERSEDED Record ID.
4.5. Waveform Packet Descriptor¶
User ID |
LASF_Spec |
Record ID |
n: where n > 99 and n < 355 |
Warning
This VLR is REQUIRED when using Point Data Record Formats 4, 5, 9, or 10.
These records contain information that describes the configuration of the waveform packets. Since system configuration may vary throughout a dataset, the LAS file supports up to 255 Waveform Packet Descriptors.
Item |
Format |
Size |
Required |
|---|---|---|---|
Bits per Sample |
uint8_t |
1 byte |
yes |
Waveform Compression Type |
uint8_t |
1 byte |
yes |
Number of Samples |
uint32_t |
4 bytes |
yes |
Temporal Sample Spacing |
uint32_t |
4 bytes |
yes |
Digitizer Gain |
double |
8 bytes |
yes |
Digitizer Offset |
double |
8 bytes |
yes |
Bits per Sample
2 through 32 bits are supported.
Waveform Compression Type
It is expected that in the future standard compression types will be adopted by the LAS committee. This field will indicate the compression algorithm used for the waveform packets associated with this descriptor. A value of 0 indicates no compression. Zero is the only value currently supported.
Number of Samples
The number of samples associated with this waveform packet type. This value always represents the fully decompressed waveform packet.
Temporal Sample Spacing
The temporal sample spacing in picoseconds. Example values might be 500, 1000, 2000, and so on, representing digitizer frequencies of 2 GHz, 1 GHz, and 500 MHz respectively.
Digitizer Gain and Offset
The digitizer gain and offset are used to convert the raw digitized value to an absolute digitizer voltage using the formula:
Note
Users seeking further clarity regarding LAS waveform encoding are encouraged to learn more on the Official LAS Wiki.