Lidar files preparation

For the preparation of the LiDAR files for their processing, the following steps have been followed:

1. LAS files conversion into LAZ (compressed format), for the processing and storage of data in order to reduce the storage space as much as possible, without losing data.

   las2las -i [directory][file-name].las -olaz
   
   The LAS files corresponding to the transverse passes are not processed, they are only considered in the quality control processes.
   
   

2. Generation of indexes.

   Access to data is optimized.

   lasindex -i [directory][file-name].laz -cores [core-number]
   
   

3. Information of the archives.

   LAZ files information is stored in a text file.

   lasinfo -i [directory][file-name].laz - otxt -cd -odir [directory] -cores [core-number]
   
   

4. Overlay of flight lines. The vertical and horizontal alignment between flight lines is checked.

   Vertical and horizontal alignment between lines.

   lasoverlap -i [directory][file-name].laz -step 2 -values -oasc
   
   

5. Transformation of ellipsoidal to orthometric coordinates.

   The conversion can be done using the LASTools commands or through a process designed with the FME Desktop tool (Safe software, British Columbia).
   
   

   [LASTools]
   
   

   • First of all, the IGN’s text file EGM08-REDNAP.asc, expressed in the official ASCII download format, in a BIL format raster file, using the GDAL library.

     The ASCII file header information contains a set of keywords (NCOLS, NROWS, XLLCENTER, YLLCENTER, CELLSIZE, and NODATA_VALUE), followed by the values of the cells in a higher order of rows.

     The BIL binary file is a storage scheme that allows you to store the pixel values of an image in a file.

     gdal_translate -of EHdr EGM08_REDNAP.asc EGM08_REDNAP.bil
     
     

   • The file is transformed from the BIL format to LAZ, to then apply the geoid model to the point cloud.

     las2las -v -i EGM08_REDNAP.bil -target_utm 30N -target_precision 0.001 -odix "_utm30N" -olaz -longlat

     lasheight -v -i [ruta][fichero].laz -ground_points EGM08_REDNAP_utm30N.laz -replace_z -olaz -odir [ruta_salida] -all_ground_points
     
     

   [FME]
   
   

   • First of all, an FME Convert_GRD_from Ngrid.fmw is executed, which allows transforming the official file EGM08_REDNAP.grd from the IGN, into a text GRD file.
     
     

     You can check the details and the process code in the section Workbenchs > Conversion EGM08_REDNAP
     
     

   • Second, a GDC file is configured with the path where the GRD file is located.

     The EGM08_REDNAP.grd file that is generated in GridData must be placed in C:\Program Files \ FME \ Reproject \ GridData \

     In C: \ Program Files \ FME \ Reproject \ GeoidHeightGrids \ Spain \ there is a file EGM08_REDNAP.gdc with the path of the previous file:

     # ftp://ftp.geodesia.ign.es/geoide/
     ..\..\GridData\EGM08_REDNAP.grd
     

     
     

   • Finally, the FME project LASelipsoide_LASgeoide.fmw is executed, transforming the LAZ files from ellipsoidal heights into orthometric heights.

     It is necessary to take into account that when the transformation to orthometric heights is performed with FME, the scale of the LAS files is modified. For this reason, the files must be scaled.

     Rescaling points in LiDAR files.

     las2las -i [directory][file-name].laz -rescale 0.01 0.01 0.01 -olaz -odir [directory]
     
     

     You can consult the details and the code of the process, in the section Workbenchs > Transformating from ellipsoidal to orthometric height
     
     

6. A transformation check is performed with external terrain control points, verifying the correct position of the information with respect to the real terrain.

   lascontrol -i [directory][file-name].laz -cp [directory][file-name].csv -parse xyz -cp_out [directory][file-name].txt