LAS文件格式
LAS文件是二进制格式，其标准格式由美国摄影测量与遥感学会（ASPRS）的 LiDAR 专业委员会进行发布，最新的版本为 2019 年 7 月发布的 LAS 1.4 - R15。LAS1.4 版本的文件结构可分为 4 部分，包括公共头文件区、变长记录区、点数据记录区和扩展变长记录区。下面简要介绍LAS文件各部分编码格式。
公共头文件区
公共头文件区记载数据点的数量、格式、数据范围、变长记录区数量、点云数据真实坐标的 X、Y、Z方向的比例因子和偏移量等 LAS 文件的基本信息，所有项目需填写数据，若无数据则置零。下表为 LAS 1.0 - 1.4 R15公共头文件区的对比。
| Item | LAS 1.0 | LAS 1.1 | LAS 1.2 | LAS 1.3 R11 | LAS 1.4 R15 | Format | Size |
|---|---|---|---|---|---|---|---|
| File Signature (“LASF”) | √ | √ | √ | √ | √ | char[4] | 4 bytes |
| File Source ID | 保留字节，置零 | √ | √ | √ | √ | unsigned short | 2 bytes |
| Global Encoding | 保留字节，置零 | 保留字节，置零 | √ | √ | √ | unsigned short | 2 bytes |
| Project ID - GUID Data 1 | √ | √ | √ | √ | √ | unsigned long | 4 bytes |
| Project ID - GUID Data 2 | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| Project ID - GUID Data 3 | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| Project ID - GUID Data 4 | √ | √ | √ | √ | √ | unsigned char[8] | 8 bytes |
| Version Major | √ | √ | √ | √ | √ | unsigned char | 1 byte |
| Version Minor | √ | √ | √ | √ | √ | unsigned char | 1 byte |
| System Identifier | √ | √ | √ | √ | √ | char[32] | 32 bytes |
| Generating Software | √ | √ | √ | √ | √ | char[32] | 32 bytes |
| File Creation Day of Year | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| File Creation Year | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| Header Size | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| Offset to Point Data | √ | √ | √ | √ | √ | unsigned long | 4 bytes |
| Number of Variable Length Records | √ | √ | √ | √ | √ | unsigned long | 4 bytes |
| Point Data Record Format | √ | √ | √ | √ | √ | unsigned char | 1 byte |
| Point Data Record Length | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| Legacy Number of Point Records | √ | √ | √ | √ | √ | unsigned long | 4 bytes |
| Legacy Number of Point by Return | √ | √ | √ | √ | √ | unsigned long[5] | 20 bytes |
| X Scale Factor | √ | √ | √ | √ | √ | double | 8 bytes |
| Y Scale Factor | √ | √ | √ | √ | √ | double | 8 bytes |
| Z Scale Factor | √ | √ | √ | √ | √ | double | 8 bytes |
| X Offset | √ | √ | √ | √ | √ | double | 8 bytes |
| Y Offset | √ | √ | √ | √ | √ | double | 8 bytes |
| Z Offset | √ | √ | √ | √ | √ | double | 8 bytes |
| Max X | √ | √ | √ | √ | √ | double | 8 bytes |
| Min X | √ | √ | √ | √ | √ | double | 8 bytes |
| Max Y | √ | √ | √ | √ | √ | double | 8 bytes |
| Min Y | √ | √ | √ | √ | √ | double | 8 bytes |
| Max Z | √ | √ | √ | √ | √ | double | 8 bytes |
| Min Z | √ | √ | √ | √ | √ | double | 8 bytes |
| Start of Waveform Data Packet Record | √ | √ | unsigned long long | 8 bytes | |||
| Start of First Extended Variable Length Record | √ | unsigned long long | 8 bytes | ||||
| Number of Extended Variable Length Records | √ | unsigned long | 4 bytes | ||||
| Number of Point Records | √ | unsigned long long | 8 bytes | ||||
| Number of Points by Return | √ | unsigned long long[15] | 120 bytes |
点数据记录区
点数据记录区位于变长记录区之后，主要用于存储点云数据，包括每个激光点的三维坐标信息、回波强度信息等相关属性，是 LAS 文件的核心。在 LAS 1.4 版本中，有 11 种点数据类型。点数据类型在公共头文件区中已经指定，每个 LAS 文件只有 1 种点数据类型。LAS 1.0 - 1.3 仅包含 0 - 5 共 6 种点数据类型，LAS 1.4 R15 包含 0 - 10 共11 种点数据类型。下表为 0 - 10 点数据类型的对比。
| Item | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Format | Size |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| X | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | long | 4 bytes |
| Y | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | long | 4 bytes |
| Z | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | long | 4 bytes |
| Intensity | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| Return Number | √ | √ | √ | √ | √ | √ | 3 bits (bits 0-2) | 3 bits | |||||
| Return Number | √ | √ | √ | √ | √ | 4 bits (bits 0-3) | 4 bits | ||||||
| Number of Returns (Given Pulse) | √ | √ | √ | √ | √ | √ | 3 bits (bits 3-5) | 3 bits | |||||
| Number of Returns (Given Pulse) | √ | √ | √ | √ | √ | 4 bits (bits 4-7) | 4 bits | ||||||
| Classification Flags | √ | √ | √ | √ | √ | 4 bits (bits 0-3) | 4 bits | ||||||
| Scanner Channel | √ | √ | √ | √ | √ | 2 bits (bits 4-5) | 2 bits | ||||||
| Scan Direction Flag | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | 1 bit (bit 6) | 1 bit |
| Edge of Flight Line | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | 1 bit (bit 7) | 1 bit |
| Classification | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | unsigned char | 1 byte |
| Scan Angle Rank (-90 to +90) – Left Side | √ | √ | √ | √ | √ | √ | signed char | 1 byte | |||||
| User Data | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | unsigned char | 1 byte |
| Scan Angle | √ | √ | √ | √ | √ | short | 2 bytes | ||||||
| Point Source ID | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | √ | unsigned short | 2 bytes |
| GPS Time | √ | √ | √ | √ | √ | √ | √ | √ | √ | double | 8 bytes | ||
| Red | √ | √ | √ | √ | √ | √ | unsigned short | 2 bytes | |||||
| Green | √ | √ | √ | √ | √ | √ | unsigned short | 2 bytes | |||||
| Blue | √ | √ | √ | √ | √ | √ | unsigned short | 2 bytes | |||||
| Wave Packet Descriptor Index | √ | √ | √ | √ | unsigned char | 1 byte | |||||||
| Byte Offset to Waveform Data | √ | √ | √ | √ | unsigned long long | 8 bytes | |||||||
| Waveform Packet Size in Bytes | √ | √ | √ | √ | unsigned long | 4 bytes | |||||||
| Return Point Waveform Location | √ | √ | √ | √ | float | 4 bytes | |||||||
| Parametric dx | √ | √ | √ | √ | float | 4 bytes | |||||||
| Parametric dy | √ | √ | √ | √ | float | 4 bytes | |||||||
| Parametric dz | √ | √ | √ | √ | float | 4 bytes | |||||||
| NIR | √ | √ | unsigned short | 2 bytes |
下表为 LAS 1.0 - 1.4 R15 可以使用的点数据类型。
| 版本 | 点数据类型 |
|---|---|
| LAS 1.0、1.1、1.2、1.3 R11 | 0、1、2、3、4、5 |
| LAS 1.4 R15 | 0、1、2、3、4、5、6、7、8、9、10 |
下表为 0 - 10 点数据类型的最小尺寸。
| 类型 | 最小尺寸 | 类型 | 最小尺寸 | 类型 | 最小尺寸 |
|---|---|---|---|---|---|
| 0 | 20 bytes | 4 | 57 bytes | 8 | 38 bytes |
| 1 | 28 bytes | 5 | 63 bytes | 9 | 59 bytes |
| 2 | 26 bytes | 6 | 30 bytes | 10 | 67 bytes |
| 3 | 34 bytes | 7 | 36 bytes |