Ground Control Point Accuracy Check
2005 Lower Columbia River Projects (DOI, DOGAMI, COE and DNR)

May, 2006

Diana M. Martinez, PSRC,

This report examines the vertical accuracy of the LiDAR data for the projects in the Lower Columbia River in Washington and Oregon.  The projects were originated by Washington DNR, COE, DOI and DOGAMI. TerraPoint collected the data during winter 2005.  The LiDAR bare earth Digital Elevation Models (DEM) were evaluated against 180 ground control points (GCPs).


The accuracy specification in the contract between the Puget Sound LiDAR Consortium and Terrapoint is based on a required Root Mean Square Error (RMSE) ‘Bare Earth’ vertical accuracy of 30 cm for flat areas in the complete data set.  This is the required result if all data points in flat areas were evaluated.  Because only a small sample of points is evaluated, the required RMSE for the sample set is adjusted downward per the following equation from the FEMA LiDAR specification (adjusted from the 15 cm RMSE in the FEMA specification to 30 cm to accommodate the dense vegetation cover in the Pacific Northwest).

The vertical accuracy of the LiDAR DEMs was evaluated by finding the difference between the ground control points (GCP) and the LiDAR bare earth DEM.  Since the 30 cm vertical accuracy is required only for bare and flat areas, only GCPs that are in such areas could be used.  Three methods where used to identify these points:  (1)  calculate the surface curvature at that point, (2) calculate the slope and (3) visually check that the GCP is not in a forested area.  The maximum acceptable value for curvature is 3 (0 is flat land and +/-3 is moderate relief) and for slope is 10 (0 to 90 deg. scale).  The results of the curvature and slope operations indicated whether the point was suitable for the RMSE calculation or if the point needed to be visually inspected before being included.  Since most of these GCP are located on roads or near roads (as oppose to plain open areas),  the curvature and slope values may exceed the maximum acceptable values, but still qualify if they are not under tree cover or on rough steep terrain.  If available, control point physical description and photos are also used in determining the suitability of the individual points.

Point Selection Process and Preparation for RMSE Evaluation:

69 Points from the Washington DOT  (map)
69 points were extracted from WA DOT online monument database (  Some of these points were adjusted for being in a DOT case-and-cover, which puts them 23 cm below the surface unless otherwise noted.  Other points were adjusted for listed distances above or below the surface.  The number of points that fell inside the project areas was much greater than 69, but many had horizontal coordinates with poor accuracy of +/- 10 meters.  Some points could not be used because they were located on bridges, on steep slopes, in vegetated area and some had been destroyed by construction.

81 pts from the Corp of Engineers  (map)
COE provided 128 points from their survey monument database.  I used 81 of these points for our RMSE calculations.  The other points did not qualify because they were either outside our project area or they were in non-suitable locations.  Some points were in NAD27 horizontal datum and some points were is NGVD29 vertical datum.  I adjusted these points by reprojecting the ones in NAD27 to NAD83 HARN.  Then I used Vertcon from the National Geodetic Survey ( to adjust the elevation from NGVD29 to NAVD88.

30 pts from National Geodetic Survey (map)
I received 55 NGS control points for the project area from the Bonneville Power Administration.  BPA downloaded them from the NGS website.  Only 30 of them were suitable for evaluating the vertical accuracy.  The other points were located under heavy vegetation or on structures that were removed from the ground surface.

Finding enough suitable GCPs was not an obstacle for the DOI project area, which is mostly along the river corridor.  The other areas did not have as many existing control points, specially the DOGAMI project.  A better distribution of points in these areas is preferable but we couldn't find any more existing control points.

See below for details on each point.

Summary and RMSE calculation for 180 GCP:

After each point was evaluated for suitability and individual vertical error, they were aggregated to calculate the Root Mean Square (RMS).  The RMSE specification for 180 points is 27.192cm and the resulting RMS from the 180 points is 16.711cm   The RMS is by far less than the maximum RMSE in the spec, therefore the projects passes the RMSE test.  The mean error is calculated at 5.784cm, which is very close to zero and that is reassuring.

-  The evaluation results show that the points form a bell shaped curb with normal distribution centered close to 0, between 0 and 10.  Having an even number between -10 to 0 and 0 to 10 would be best, but the results are very good for this number of points.

-  The physical distribution of the points is good for the river corridor.  Points are not well distributed in the other project areas.

- Because there was several projects in close proximity in the Columbia River area, we decided to evaluate all of them as one project.  This is acceptable because they were all flown in the same time period, winter 2005, and also many of the flight lines cover adjacent projects..

GCP Table

Detailed results for each ground control point showing its location on the bare earth and the top surface.

                    GCP ID                         Unique identifier for each point.  It identifies the source of the point and the number
                    QQuad                          USGS quarter quad where the GCP
                    GCP Elev (ft)                 GCP Ortho Elevation in ft
                    DEM Elev (ft):                DEM Ortho Elevation in ft
                    GCP - BE DEM (ft)       Elevation differenced between the GCP and the BE DEM ( GCP minus BE DEM). feet
                    GCP - BE DEM (cm)     Elevation differenced between the GCP and the BE DEM ( GCP minus BE DEM). cm
                    Curvature                       Curvature value at the GCP
                    Slope                              Slope at the GCP
                    Included                          Was it was included in the RMSE calculations?

For some points I have included specific notes about why that point was disqualified.

GCP sources:

Points from WA DOT
Points from NGS
Points from COE