8.0 CURRENTLY AVAILABLE GUIDELINES
This section briefly discusses currently available mobile LIDAR guidelines and reports. Relevant portions of those documents have been incorporated into these Guidelines. Many other agencies (FAA, 2011; FGDC, 1998; NDEP, 2004; NOAA, 2008; USGS, 2012) have provided recommendations, guidelines, or standards for acquiring and delivering geospatial data. Some of these (FGDC, 1998 and NDEP, 2004) are broad specifications that pertain to all remotely sensed geospatial data while others pertain more directly to LIDAR data (FAA, 2011; NOAA, 2008; USGS, 2010).
Common trends can be seen in the various LIDAR specifications, including:
- Standard accuracy reporting methods,
- Requirements for ground point density,
- Requirements for scan overlap,
- Number and distribution of control/check points for accuracy verification, and
- Types of deliverables.
Although most of these guidelines are currently focused on aspects of airborne LIDAR Systems, some of their fundamental principles can be adapted to produce guidelines more relevant to mobile LIDAR. However, most of these documents do not directly or adequately address the needs of many transportation agency applications. For example, the accuracy, resolution, coverage, and look angle of mobile LIDAR data varies significantly from that achieved with airborne LIDAR. Particularly, true 3D error vectors are important for many applications and current airborne LIDAR guidelines focus on vertical error only.
8.1 Geospatial data accuracy
The Federal Geographic Data Committee (1998) developed the National Standard for Spatial Data Accuracy (NSSDA), which provides guidance on reporting spatial data accuracies. This document provides the foundation for the reporting found in most available standards and guidelines. The NSSDA uses a root mean square error (RMSE) to estimate positional accuracy reported in ground distances at 95% confidence. Datasets should be tested with a minimum of 20 control points and reported as:
Tested ____ (meters, feet) vertical (or horizontal) accuracy at 95% confidence level
In cases where the data were not tested and accuracy is merely estimated, the following statement is used:
Compiled to meet ____ (meters, feet) vertical (or horizontal) accuracy at 95% confidence level
Recommendation: Follow FGDC accuracy reporting standards.
The National Digital Elevation Plan (NDEP) guidelines further developed the NSSDA to include three types of accuracy reporting: fundamental vertical accuracy (FVA, open terrain, optimal conditions), consolidated vertical accuracy (CVA, combined accuracies obtained in all land covers), and supplemental vertical accuracy (SVA, accuracies reported for individual land covers). For example, accuracies in dense forests will be much lower than in open terrain.
Table 5: Existing geospatial guidelines relevant to mobile LIDAR
8.2 ASPRS guidelines
The American Society of Photogrammetry and Remote Sensing (ASPRS) is striving to be the go-to source for LIDAR technology in the US. Several efforts are underway, including:
- The ASPRS Mobile Mapping Committee is developing guidelines for mobile mapping. However, this is currently a work in progress at the outline stage.
- ASPRS Vertical accuracy guidelines for airborne LIDAR. This document reinforces the NSSDA and NDEP guidelines and provides guidance for establishing control specific to airborne LIDAR.
- ASPRS horizontal accuracy guidelines for airborne LIDAR. This document provides background on the difficulties in determining horizontal accuracies from airborne LIDAR.
- ASPRS Geospatial Procurements (DRAFT). This document is intended to aide entities with the best approach to commercial geospatial products, defined with a COTS specification. The document distinguishes between professional\technical services and commercial geospatial products. It also recognizes state and federal laws. A proposed procurement methodology of license data terms and conditions, cost/value, service provider defined technical specification, services to support geospatial products and deliverables are addressed.
8.3 Transportation agency LIDAR standards
Chapter 15 of the California Department of Transportation (Caltrans, 2011) Surveys Manual is one of the first developed sets of specifications that explicitly addresses the required information and data quality that should be provided with a static or mobile LIDAR survey. These specifications contain a two-part classification system for mobile LIDAR surveys. Type ‘A’ is a higher accuracy hard surface survey used for engineering applications and forensic surveys. Type ‘B’ is used for lower accuracy applications (e.g., asset inventory, erosion, environmental and earthwork surveys). These specifications are broad enough to not limit service provider equipment and technology but provide details regarding data acquisition and processing procedures, including the minimum overlap between scans, maximum PDOP, minimum number of satellites, maximum baseline, validation point accuracy requirement, IMU drift errors, and other factors pertaining to the geo-referencing accuracy of the point cloud. One needs to have a relatively high level of understanding of mobile LIDAR technology in order to utilize the Caltrans standards effectively.
Other transportation agencies have begun developing standards and guidelines for MLS. These Guidelines are meant to provide the agency with a reference document that can be tailored to their specific needs. For example, Florida DOT recently released guidelines which are very similar to the Caltrans guidelines. However, the Florida DOT guidelines add a Type C, Lower Accuracy Mapping category for planning, transportation statistics, and general asset inventory surveys.
8.4 FAA advisory circular
The Federal Aviation Administration (FAA) has produced a draft Advisory Circular related to remote sensing technologies. The FAA document includes a section that discusses considerations for use of several forms of LIDAR (static, mobile, and airborne) for airport surveys and anticipated accuracies and resolutions for each method. The document also discusses calibration procedures for LIDAR systems and provides guidance when such calibrations are necessary. Specific requirements for mobile LIDAR workflows include: redundancy, monitoring acquisition, local transformation and validation points, data processing, data filtering and clean up, geo-referencing, and data integration.