Are point clouds from external photogrammetry programs compatible?

Issue

Are point clouds from external photogrammetry programs compatible?

Answer

Please refer to the explanations below.

Explanation

A lot of our customers are using external photogrammetry programs such as Pix4D, Agisoft Metashape and Context Capture, etc. So let’s see whether point clouds generated from these programs are also compatible with Smart Construction Dashboard.

The short answer

The following formats are supported by Smart Construction Dashboard:

• txt (recommended)
• csv (recommended)
• las
• laz

It means that no matter which photogrammetry program you are using, as long as the file is exported in the type of formats mentioned above, your point cloud will be compatible with Smart Construction Dashboard.

Please note that we do recommend using txt and csv formats when exported from an external photogrammetry program. Because due to background processes, some las and laz point clouds can be rendered at a wrong height.

 

The long answer

Warning

Technical explanation

The following formats are supported by Smart Construction Dashboard:

• txt (recommended)
• csv (recommended)
• las
• laz

las & laz "spatial reference" metadata

It is possible to add metadata to las and laz files, one of the metadata fields is specially dedicated to "spatial reference". This "spatial reference" allows different programs to place the point cloud correctly within other datasets. 

This is also what allows Smart Construction Dashboard to accept las files in WGS84, La, Lo, H coordinates and reproject them into the project coordinate system. 

Using CloudCompare we can clearly see this metadata added to the las file.

The issue with "spatial reference" metadata and external photogrammetry programs

However, most photogrammetry programs currently don't support geoid or any vertical reference systems. In most cases, GCPs (Ground Control Points) are used to correct the height of a flight.

When creating a las file, a spatial reference will be automatically attached to the point cloud. However, this spatial reference will only contain a horizontal reference system but not a vertical one. When you upload this point cloud to the Smart Construction Dashboard, the Dashboard will see this spatial reference and assume that the point cloud is rendered at the ellipsoid height. That’s where it gets problematic. If the project has been set to include a geoid, the geoid offset will be then applied to the point cloud. But remember the geoid offset is already applied by the application of the GCPs, so this will cause the point cloud to be too high or too low in comparison to the rest of the project. In other words, when uploading a las or laz file, the geoid height above the ellipsoid will have been applied twice: once by the application of the GCPs and once by the Smart Construction Dashboard.

The solutions

1. Render the point cloud in csv or txt and export it like this from your photogrammetry program.
2. Convert the las/laz to csv/txt. This can be done easily by using the free tool CloudCompare . Simply load the las point cloud into CloudCompare and save it as txt or csv.
3. When flying an RTK flight without GCP's you could also export the flight with "EPSG 4326 - WGS 84". Smart Construction Dashboard will then project the point cloud to the correct position. It's quite handy when you're using a custom localization    ( Remark: Only Agisoft Metashape has been confirmed to support this workflow).