The classification of a LIDAR data point consists of both a classification and a classification flag. The classification flag allows for a point to be simultaneously flagged for particular uses while maintaining the classification of the point. For example, a model key-point can be flagged as such while maintaining the ground classification. Classification flags are used to indicate special characteristics associated with the point.
There are currently four flags: Synthetic (0), Key-point (1), Withheld (2) and Overlap (3).
- Bit 0 – Synthetic: If set, this bit indicates that the point was created by a technique other than LIDAR collection, such as digitized from a photogrammetric stereo model or by traversing a waveform.
- Bit 1 – Key-point: If set, this bit indicates that this point is considered to be a model key-point and thus generally should not be withheld in a thinning algorithm.
- Bit 2 – Withheld: If set, this bit indicates that the point should not be included in normal processing. Points that are tagged with this flag are to be retained in the LAS file and included in exports of the LAS file. Withheld is synonymous with “Deleted”.
- Bit 3 – Overlap: If set, this bit (only available with LAS version 1.4) indicates that the point is within the overlap region of two or more flightline swaths. If this bit is used, then all points in the overlap region must have this bit set. Allows for the classification of overlap points to be preserved.
The LAS specification has included this ability to use classification flags since the release of the LAS v1.1 specification. However, the biggest drawback of the initial implementation was that it limited the initial 256 possible classifications available in the v1.0 specification to only thirty-two. With the release of LAS v1.4, the 256 classifications are once again possible using the new point data record formats, six through ten. LAS v1.4 also introduced the concept of the Overlap classification flag. Note that the class offsets discussed here do not apply to the Overlap classification flag. A user would also need to have LP360 in order to properly flag overlap points.
TerraScan has always supported an 8-bit classification code as per the original LAS specification. TerraScan users are free to define their own class map in any manner they feel is necessary for their work. To set the higher order bit flags, TerraScan users should add the appropriate value to the existing class of the point data. For example, if a point is “Ground” as class 2, then a “Withheld Ground” is class 130.
Table 1 – TerraScan Flag Settings
|0-31||Reserved for ASPRS Classification|
|31-65||ASPRS Classifications plus Synthetic Flag|
|66-98||ASPRS Classifications plus Key-point Flag|
|99-127||ASPRS Classifications plus Synthetic and Key-point Flags|
|128-159||ASPRS Classifications plus Withheld Flag|
|160-191||ASPRS Classifications plus Synthetic and Withheld Flags|
|192-223||ASPRS Classifications plus Key-point and Withheld Flags|
|224-255||ASPRS Classifications plus Synthetic, Key-point and Withheld Flags|
As TerraScan uses a single class code per point, which represents both the code and the flag, then the exclusion of certain flagged points, say those flagged as withheld, is done by ensuring that those classes, 128-255, as outlined in the table above are never used during processing. Hopefully, this quick reference table will assist users in determining the classification and flag to use when processing data in LAS v1.0 through 1.2.