It should be noted that a direct test of the accuracy of the elevation data may be performed by conducting field measurements after the locations of the unique lidar returns are known. By surveying checkpoints on the known xy coordinates of ground surface lidar point data, a direct comparison can be used as a means of verifying vertical accuracy, removing the introduction of errors caused by interpolating the elevation value from a TIN, a DEM or contour interpolation. Given that most elevation data will be used to generate additional mapping products through TINing, DEM generation, or contour maps, ASPRS does not recommend this procedure as a final QC procedure for reporting the vertical accuracy of datasets. When direct measurements are used to calculate the vertical accuracy of the dataset, this fact must be clearly reported along with a caution about the potential of introducing further interpolation error in any derived mapping products.