.las format is the standard data format for lidar data. And a .las dataset can be divided into 3 part: 1.PUBLIC HEADER BLOCK,2.VARIABLE LENGTH RECORDS and 3.POINT DATA RECORD, in PUBLIC HEADER BLOCK part the descriptions of the dataset is recorded. The descriptions included number of points, length of each point, las dataset type and so on. So this part should be processed carefully to get the informations of the dataset; and in VARIABLE LENGTH RECORDS this part do not needed necessarily and in most dataset this part does not exist; the final part is POINT DATA RECORD. This part recorded all points informations.
By the way I will introduce the struct of the program along with the dataset struct in detail:
#pragma pack(1)
class LASHeader
{
public:
enum { HEADER_SIZE = 227 };
public:
char file_signature[4];
unsigned int reserved;
unsigned int project_ID_GUID_data_1; // - 9-12
unsigned short project_ID_GUID_data_2; // - 13-14
unsigned short project_ID_GUID_data_3; // - 15-16
unsigned char project_ID_GUID_data_4[8]; // - 17-24
unsigned char version_major; // * 25
unsigned char version_minor; // * 26
unsigned char system_id[32]; // * 27-58
char generating_software[32]; // * 59-90
unsigned short file_creation_day; // - 91-92
unsigned short file_creation_year; // - 93-94
unsigned short header_size; // * 95-96
unsigned int offset_to_point_data; // * 97-100
unsigned int number_of_variable_length_records;// * 101-104
unsigned char point_data_format; // * 105
unsigned short point_data_record_length; // * 106-107
unsigned int number_of_point_records; // * 108-111
unsigned int number_of_points_by_return[5]; // * 112-131
/*
* Xcoordinate = (Xrecord * x_scale_factor) + x_offset
* Ycoordinate = (Yrecord * y_scale_factor) + y_offset
* Zcoordinate = (Zrecord * z_scale_factor) + z_offset
*/
double x_scale_factor; // * 132-139
double y_scale_factor; // * 140-147
double z_scale_factor; // * 148-155
double x_offset; // * 156-163
double y_offset; // * 170-177
double z_offset; // * 164-169
double max_x; // * 178-195
double min_x; // * 186-193
double max_y; // * 194-201
double min_y; // * 202-209
double max_z; // * 210-217
double min_z; // * 218-225
public:
LASHeader();
LASHeader(const LASHeader& header);
LASHeader& operator=(const LASHeader& header);
inline unsigned short GetFile_Source_ID() { return (version_minor == 1 && version_minor == 1) ? reserved >> 16 : 0; }
inline unsigned int Getreserved() { return (version_minor == 1 && version_minor == 1) ? reserved &= 0x0000FFFF : reserved; }
inline bool HasPoint() const { return point_data_record_length >= 12 ? true : false; }
inline bool HasIntensity() const { return point_data_record_length >= 14 ? true : false; }
inline bool HasReturnNumber() const { return point_data_record_length >= 15 ? true : false; }
inline bool HasNumberofReturn()const { return point_data_record_length >= 15 ? true : false; }
inline bool HasScanDirectionFlag() const { return point_data_record_length >= 15 ? true : false; }
inline bool HasEdgeofFlightLine()const { return point_data_record_length >= 15 ? true : false; }
inline bool HasScanAgnleRank() const { return point_data_record_length >= 17 ? true : false; }
inline bool HasFlightLine()const { return point_data_record_length >= 17 ? true : false; }
void Setreserved(unsigned int reservedi);
void SetFile_Source_ID(unsigned short id);
bool HasGPSTime()const;
bool HasLASColorExt4() const;//
bool HasLASColorExt6() const;
int HasLASPointExt() const; //
bool HasPointDataStartfile_signature() const { return version_minor == 0 && version_minor == 1; }
void ReadHeader(FILE *fs);
void WriteHeader(FILE *fs) const;
const static unsigned short Data_Record_Length_of_Format0; //
const static unsigned short Data_Record_Length_of_Format1; //
const static unsigned short Data_Record_Length_of_Format2; //
const static unsigned short Data_Record_Length_of_Format3; //
const static unsigned short HeaderSize_Def; //
const static char ErrorPointCnt[]; // "number of point is more than LasFileHead.Number_of_point_records";
const static char ErrorVarLenRecordCnt[]; // "number of variable Length Record is more than LasFileHead.Number_of_variable_length_records";
const static char ErrorOffsettoData[]; // "number of bytes of Variable Length Record is more than LasFileHead.Offset_to_data";
};
#pragma pack()
header struct is as the code above: as usual the size of the header is 277 bytes and the means of each byte is in the code, and be careful that usful data only occupy 225 bytes, so for jump to the data record directly you should seek 227 bytes.
enum eLASEchos
{
eLidarEchoOnly = 0,
eLidarEchoFirst = 1,
eLidarEchoMidian = 2,
eLidarEchoLast = 3
};
#pragma pack(1)
enum eLASClassification
{
elcCreated = 0,
elcUnclassified = 1,
elcGround = 2,
elcLowVegetation = 3,
elcMediumVegetation = 4,
elcHighVegetation = 5,
elcBuilding = 6,
elcLowPoint = 7,
elcModelKeyPoint = 8,
elcWater = 9,
elcOverlapPoint = 12,
elcDanger = 13,
elcTowerRange = 14,
elcDeletedPoint = -1
};
#pragma pack()
static eLASClassification GetLidarClassification(unsigned char clsType)
{
return (eLASClassification)clsType;
}
/*Las1.2��ɫ��չ*/
#pragma pack(1)
struct LASColorExt
{
unsigned short Red;
unsigned short Green;
unsigned short Blue;
};
#pragma pack()
//las Point def
#pragma pack(1)
class LASPoint
{
public:
/*
* constructor
*/
void Write(FILE *fs, const LASHeader& info) const;
void Read(FILE *fs, const LASHeader& info);
/**
* extract from binnary data
* @param data
* @param info
*/
void ExtractFromBuffer(const unsigned char* data, const LASHeader& info);
void ExportToBuffer(unsigned char* data, const LASHeader& info) const;
public:
Point3D m_vec3d;
unsigned short m_intensity;
unsigned char m_rnseByte;
char m_classify;
char m_scanAngle;
unsigned char m_userdata;
unsigned short m_flightID;
double m_gpsTime;
LASColorExt m_colorExt;
};
#pragma pack()from the code we can know the each point data struct:
point(x,y,z);intensity;rnse;classify;scanAngle;userdata;flightID;gpstime(not necessary);color(not necessary).
there are some import things we should take care: 1.the point is saved as type int to save memory,the scale and offset is recorded in header; 2.rnse refers to return number(3 byte), number of returns(3 byte), scan angle direction(1 byte), edge of flight line(1 byte); the four part is recorded together to save memory; 3. the color part is not recorded by 3 shorts data in format 1.1 in format 1.1 the color part is recorded into a int.
