NGP Standards and Specifications

Lidar Base Specification Appendix 1: Maximum Surface Height Rasters

Lidar Base Specification 2021 rev. A

Requirements for the creation of Maximum Surface Height Rasters (MSHRs), the preferred proof of performance for withheld bit flag usage. 

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Maximum Surface Height Rasters (MSHRs) provide a method for quickly and conveniently assessing withheld-flagging of the Lidar Point Cloud (LPC). MSHRs are created from the highest non-withheld point in the LPC in each pixel. Properly flagged points will produce a raster with uniform appearance. Improperly flagged non-valid returns will produce areas of non-uniformity (see images for examples).

The purpose of this metadata deliverable is to ensure proper use of the withheld bit flag for points that cannot be reasonably interpreted as valid surface returns.  These are points that a contractor has chosen to leave in the classified point cloud deliverable.  The use of the withheld bit flag is required for all points that are not valid surface returns starting with Lidar Base Specification 2020 rev. A. The withheld bit flag supersedes any classification applied to these points.  Phrased differently, provided the withheld bit flag is appropriately applied to the point data records, any classification, including class 0, is acceptable. 

Proper use of the withheld bit flag is very significant in point cloud processing and the creation of derivative products such as a digital elevation model. Proof of performance that the withheld bit flag was used properly is required. The preferred proof of performance is to create and deliver MSHRs as described below.

Maximum Surface Height Raster

  • Raster Creation:
    • Spatial resolution (pixel dimension) of the images shall be identical to the ground sample distance of the DEM deliverable in the project’s linear unit (meters or feet). 
    • The MSHR shall be representative of the associated data delivery.
    • The raster shall be in the same coordinate reference system (CRS) as the point cloud data to ensure alignment with the point cloud.
    • All returns shall be used to create the raster.
    • Points flagged as withheld shall be excluded.
    • The raster shall be built with the highest elevation point value from each pixel.
    • The raster shall be 32-bit, floating point.
  • Raster file formats and version control: 
    • The Maximum Surface Height Raster shall be delivered as a GeoTIFF file by tile.
    • The tile size and naming convention shall match the point cloud and DEM deliverables.
    • Any change in the withheld flags in the point data record shall require re-creation of the MSHR for that tile.
Example of Maximum Surface Height Raster (MSHR)

Figure A-1. Example of Maximum Surface Height Raster (MSHR) created from hightest lidar point cloud value in each pixel. Areas in boxes illustrate non-valid returns that were not properly flagged as withheld. 

(Public domain.)

Example of Maximum Surface Height Raster (MSHR)

Figure A-2. Example of Maximum Surface Height Raster (MSHR) created from hightest lidar point cloud value in each pixel. Areas in boxes illustrate non-valid returns that were not properly flagged as withheld. 

(Public domain.)

 

Example of Maximum Surface Height Raster (MSHR)

Figure A-3. Example of Maximum Surface Height Raster (MSHR) created from hightest lidar point cloud value in each pixel. Areas in boxes illustrate non-valid returns that were not properly flagged as withheld. 

(Public domain.)