ENVI LiDAR calculates the precision of the DEM with reference to a set of points measured with high precision. To test DEM precision, you first import the reference points into ENVI LiDAR. The Precision Test Points layer is added to the Layer Manager, and the reference points display in the Main window by two vertically mirrored triangles. The connection point between the triangles is located on the point's coordinates and elevation. To import the reference points, they must be in the same coordinate system as the opened project.

## Generate Precision Test Report

To calculate the precision and generate a test report, do the following:

- Import precision test points
- Switch to QA Mode
- Select
**Process > Generate Precision Report**from the menu bar.

For each point, ENVI LiDAR calculates the difference in elevation between the DEM and the reference elevation. Then ENVI LiDAR calculates the mean, standard deviation, and accuracy for the points. The Precision Report displays in the Operations Log and is saved in the \Products directory as PrecisionReport.csv. The calculation is done in the coordinate system in which the DEM was calculated.

The report contains a line for each point with the following columns:

- Point name/number
- X coordinate
- Y coordinate
- Z coordinate
- ENVI LiDAR-calculated Z coordinate
- Delta Z (in centimeters, ENVI LiDAR-calculated reference)

The report contains a computation of the following:

- Mean vertical error (= Σ(Zi_ENVI LiDAR-Zi_Reference)/(number of points))
- RMSE (=sqrt[Σ(Zi_ENVI LiDAR-Zi_Reference)2/n])
- Accuracy at 95 percent confidence level (=1.9600*RMSE)

## Remove Precision Test Points

To remove the precision test points from the Layer Manager and the Main window, right-click Precision Test Points in the Layer Manager and select **Delete All Precision Test Points**.