The following is from the September issue of POB. The article "Intelligent Freeways" is about a Texas company using a Lynx mobile scanner to map a highway. Beginning at the bottom of page 46 it contains the following passage regarding collection of what we call "confidence points" in Oregon:
“To confirm the accuracy of Woolpert’s mobile LiDAR survey, TxDOT used a truck with two GPS units mounted on the cab. This quality control vehicle drove three passes at highway speeds along the 18-mile project area: eastbound, west-bound and the HOV lane. A driver and a technician collected data from two Trimble R8 GNSS receivers and two Trimble TSC3 data collectors, gathering data simultaneously at 500-foot intervals. Using two GPS units at the same time provides a check of one receiver against the other. This process produces higher accuracy for checking the surface.
Next, TxDOT imported the GPS points into CAICE software to model the surface from the vehicle to check the accuracy of the mobile mapper. The DTM created from the mobile mapper was overlaid on a DTM created from the vehicle. Then, the void area from the two DTM surfaces was extracted. TxDOT found a root mean square difference of no greater than .08 foot.
TxDOT then created 1-foot contour intervals from the vehicle GPS and overlaid these on the mobile contours to illustrate the accuracy of the data. GPS shots were also captured to check targets that Woolpert set. TxDOT tied these directly to the VRS system and found a maximum accuracy error of .03 foot.”
So that means that a DTM produced from GPS data collected at 500 foot intervals from a vehicle moving at freeway speed is basically as good as one created using a HD scanner.
