Survey Control

Three forms of mapping control support airborne LiDAR mapping: airborne GPS control, ground control, and quality control checkpoints. Contract specifications should reference the following standards and guidelines: FGDC-STD-007.4-2002, Geospatial Positioning Accuracy Standards PART 4: Standards for Architecture, Engineering, Construction (A/E/C) and Facility Management, as well as CHAPTER 3 of this manual, Applications and Accuracy Standards, specifically Table 3-6, Elevation Data Vertical Accuracy Standards. Other references relevant to mapping control include EM 1110-1-1002, Survey Markers and Monumentation; EM 1110-1-1003, Navstar Global Positioning System Surveying; and NOAA Technical Memorandum NOS NGS-58, Guidelines for Establishing GPS-Derived Ellipsoid Heights (Standards: 2 cm and 5 cm), version 4.3.

(1) Airborne GPS Control. Airborne LiDAR is acquired with the use of ABGPS for recording the 3D (X/Y/Z) coordinates of each pulse, plus an inertial measurement unit (IMU) for recording the roll, pitch and yaw of the sensor, when each pulse is transmitted and received (see Figure 1-8). When six exterior orientation parameters of each pulse (X/Y/Z and roll/pitch/yaw) are known, requirements for surveyed ground control are greatly reduced. ABGPS receivers must be capable of tracking both coarse acquisition (C/A) and pseudorange (P-code) data. They must provide dual frequency (L1 and L2) and multi-channel capability with on-the-fly ambiguity resolution and be able to log GPS data at 1-second epochs or better. GLONASS receivers capable of receiving satellite information from GPS and GLONASS constellation are preferred over GPS-only receivers.

(2) Ground Control. These are the well-established points that GPS ground base stations will be placed on to facilitate accurate positioning of the aircraft In the U.S., this typically involves the use of a Continuously Operating Reference Station (CORS) or the identification and recovery of well-documented permanent control monuments or benchmarks from the National Geodetic Survey’s National Spatial Reference System (NSRS) − (go to http://www.ngs.noaa.gov, then click on Survey Mark Datasheets and/or CORS). If a control network of horizontal control monuments and/or vertical control benchmarks does not exist, a control network will first need to be established per references cited above. Figure 1-9 shows an example NGS Data Sheet with the red arrow point at the horizontal and vertical network accuracy at the 95% confidence level. In addition to data shown here, Data Sheets typically also include additional information such as: State Plane and UTM coordinates; U.S. National Grid spatial address; explanations of how horizontal coordinates, ellipsoid heights and orthometric heights were determined; station description and instructions for finding the monument; station recovery history, etc.

(3) Quality Control Check Points. The quality control checkpoints are typically collected bya survey team independent of the LiDAR vendor so that these checkpoints remain “blind” during the LiDAR acquisition and calibration processing. This can be another contracted party or district personnel. The ASPRS Positional Accuracy Standards for Digital Geospatial Data, at Appendix C, provides detailed guidelines on the number and location of check points. Google Earth or other open source imagery can be used for point selection unless alternative orthophotography is available.

(4) Check Point Distribution. When possible, dispersed surveys (Figure 1-10), which provide a more legitimate assessment of data accuracy throughout the project area for different flight lines, are recommended. For dispersed surveys, no two survey checkpoints should be closer than 5,000 feet from the next closest point. If cost and accessibility are an issue, then cluster surveys can be performed (Figure 1-11). Cluster surveys are typically five points when five land cover categories are being tested, one per category, with a minimum spacing of about 1000 feet between points. Clusters should be dispersed following the ASPRS guideline that at least 20% of the points must be in each quadrant. These types of surveys work best with real-time kinematic (RTK) surveys where a base station can be established and five points (all at least 1000 feet apart from each other) can be surveyed. RTK is also ideal for establishing inter-visible pairs for conventional surveys to establish forest points. Please note inter-visible pairs cannot “count” as check points as they typically do not conform to the minimum distance rule. Furthermore, no two checkpoints in a single cluster should be for the same land cover class, and it is often difficult to identify all five land cover classes within the area of a single cluster.

(5) Check Point Location. In addition to land cover classes, location and distribution, the surveyor also needs to avoid known pitfalls in selection of checkpoint locations. It is important for the surveyor to understand that the horizontal coordinates of QA/QC checkpoints do not normally match the horizontal coordinates of individual LiDAR pulses. Instead, LiDAR elevations are interpolated from surrounding points to determine the most probable elevation of the LiDAR data at the horizontal coordinates of each QA/QC checkpoint. Interpolation assumptions are reasonably valid only when the following guidelines are followed with checkpoint selection:

1. Each checkpoint should be on terrain that is flat or uniformly sloping within 5 meters in all directions from the checkpoint coordinates. Interpolation procedures can fail if the terrain undulates up and down surrounding the checkpoint, or if the slope is curved (concave or convex). Steep slopes should also be avoided for location of checkpoints.
2. There should be no breakline within 5 meters of a checkpoint. Breaklines define the edge between two intersecting surfaces with different slopes. This rule can best be explained by using a breakline on a bridge abutment as an example of where checkpoints should not be located. Interpolation of LiDAR elevations around a bridge abutment would normally include a point on top of the bridge deck and another point over the side of the bridge, perhaps near water level 10 feet lower; interpolating between these two elevations (even if both LiDAR elevations were perfect) would erroneously show that the LiDAR data had an elevation error of 5 feet.
3. Similarly, checkpoints, even on flat terrain, should avoid logs, tree stumps, rock piles, or other elevated features that could be mapped by LiDAR pulses within 5 meter of a QA/QC checkpoint.
4. For survey of checkpoints to be used for horizontal accuracy assessments, surveyors should avoid selecting checkpoints with a high probability of being obscured when mapped with LiDAR (or imagery). Because clearly defined point features are required, horizontal checkpoints are commonly surveyed on corners of paint stripes on asphalt. Such points should not be located under trees (in parking lots) for example, because the black/white intensity variations will not be visible. Similarly, such points should not be selected in actual parking spaces where vehicles are liable to be parked at the time the LiDAR data are acquired.
5. For these reasons, in spite of check point pre-selection, final checkpoint locations cannot be determined in the office but must be left up to the field surveyor. Flexibility must be given to the surveyor as field conditions, including accessibility, are unknown. The surveyor must use the guidance above to plan where checkpoints are likely to be located, but then must make the final decisions in the field, ensuring points are well spaced, have the correct number of land cover categories, and avoid the pitfalls identified above.