Evaluation of Lidar for Highway Planning, Location and Design Reginald R. Souleyrette, Shauna Hallmark, David A. Veneziano, and Sitansu Pattnaik Abstract : The location planning and design phase of highway construction can take longer than six years. The CANDO effort at the Iowa DOT (and similar efforts at other transportation agencies) is directed toward reducing the length of time required. In order to compress the time required, designers would like to have all terrain models and aerial photo products up front, so that they can evaluate details (e.g., earthwork, intersections, driveways, drainage, etc.) when choosing between alternatives. Once they determine a final alignment, they would like to begin detailed design as soon as possible rather than waiting for conventional photogrammetry. While LIDAR can theoretically provide terrain models in short time frames, it is not expected to provide accuracy and breaklines required for final detailed CAD drawings of construction plans. However, it may be good enough to make all the design decisions. (Breaklines can be drawn on high altitude photographs and superimposed on the LIDAR data to develop the TIN). Once location planners have narrowed the alternatives to a final alignment, low altitude photogrammetry can be conducted over a small area in a relatively short time to provide input to the final CAD drawing and engineer’s estimates process. In order for LIDAR to satisfy this need, vegetation and structures must be removed. We are investigating accuracies of the bare earth model required for LIDAR to satisfy the needs articulated above. LIDAR is compared to GPS control and conventional photogrammetry products for a bypass study area in eastern Iowa. Samples are taken on hard surfaces, ditches (where LIDAR picked up a ditch), tree covered areas (where the trees were removed from the LIDAR data), harvested crop area, and unharvested crop area (where crops were present in the field and filtered from LIDAR). We will report on areas where LIDAR does and does not seem to work well. Conclusions Results of accuracy evaluations suggest that LIDAR data may prove most useful in expediting the location process. With LIDAR, terrain information would be available to designers much sooner so preliminary analysis can commence. Initial terrain data collection would not be as dependent on environmental conditions (sun angle, cloud cover), since LIDAR is not affected by such conditions in the same manner as photogrammetry. This would allow data to be collected more days throughout the year. The increased availability of data would allow terrain to be analyzed earlier in the location process, allowing issues to be identified and addressed at an earlier time. In this manner, the utilization of LIDAR data collection could produce time and cost savings by allowing expedient data collection to occur on a large corridor scale, which only limited areas being mapped by more time consuming and costly means. Study Area: Iowa 1 Corridor Accuracy Results Several comparisons were made to determine the accuracy of LIDAR as it compared to both photogrammetry, as well as GPS readings collected in the study area. The most commonly used statistic to describe accuracy is RMSE. Additional statistics presented include the mean (the average difference of points), as well as the NSSDA statistic (the value which 5% of all points may exceed). Accuracy evaluations performed indicate that LIDAR data cannot replace photogrammetric data in the final design stages of the highway location and design process. This does not entirely limit the applicability of LIDAR data to the location process. The potential for LIDAR in the process appears to be as a supplement to photogrammetry. Accordingly, the use of LIDAR data could expedite the location and design process through making terrain information available to designers at earlier stages of the location process. PhotogrammetryLIDAR TIN Grid Surface Overlay ++ = Cells of Interest Elevation Differences Methodology A Grid Comparison technique was selected to compare elevations between the test dataset (LIDAR) and the reference datasets (photogrammetry and GPS). Grids of 1 meter resolution were created through two techniques: Inverse Distance Weighted interpolation, and the conversion of Triangulated Irregular Networks into grids. ArcView GIS and its Spatial Analyst and 3D Analyst extensions were used for grid interpolation and TIN creation, respectively. These extensions allowed for the specification of output grid cell size to be made, as well as how many neighboring points could be used to influence the calculation of a grid cell elevation. Hoel and Garber: Traffic and Highway Engineering Photogrammetry RTK GPS TOTAL STATION SURVEY Existing DOT Data Collection Methods and Limitations Labor Intensive Time-consuming Costly Dictated by conditions (time of year, sun angle, weather, etc.) May require data collectors to locate in-field Frequent equipment movement required (Total Station and RTK GPS) Approval required to work on private property (Total Station and RTK GPS) Photogrammetric Process Schematic Existing Highway Location Process Source: Iowa Department of Transportation Proposed LIDAR Schematic Define wide area corridor for data collectionDefine wide area corridor for data collection Collect LIDAR data and supporting GPS control pointsCollect LIDAR data and supporting GPS control points Collect aerial photography (either digital or hard copy) of sufficient resolution for high accuracy photogrammetry products (either as part of the LIDAR flight or separately, if environmental conditions dictate).Collect aerial photography (either digital or hard copy) of sufficient resolution for high accuracy photogrammetry products (either as part of the LIDAR flight or separately, if environmental conditions dictate). Process LIDAR data for input into aerial triangulationProcess LIDAR data for input into aerial triangulation Produce breaklines from triangulated imageryProduce breaklines from triangulated imagery Filter and refine LIDAR to produce a bare earth DEM.Filter and refine LIDAR to produce a bare earth DEM. Combine LIDAR DEM with the breaklines to form planning level DTM.Combine LIDAR DEM with the breaklines to form planning level DTM. Use to produce orthophotos, contours and TINsUse to produce orthophotos, contours and TINs Produce and evaluate alignment alternatives, select final alignment.Produce and evaluate alignment alternatives, select final alignment. Photogrammetric mapping for high accuracy design level terrain model.Photogrammetric mapping for high accuracy design level terrain model. Aerial triangulation of imagery only within narrow corridor limitsAerial triangulation of imagery only within narrow corridor limits Production of additional breaklines and densified DTM for the localized area of the alignmentProduction of additional breaklines and densified DTM for the localized area of the alignment Produce final construction plan, including geometric designs and earthwork quantitiesProduce final construction plan, including geometric designs and earthwork quantities Proposed LIDAR Integration The existing photogrammetry process requires early collection and processing of data to support final design in order to avoid delays. However, only the final design stages of project development require the accuracies provided by conventional photogrammetric processing. This presents the opportunity for integrating less accurate LIDAR terrain data into the early phases of the location process, with more accurate photogrammetric data being produced only for final alignments during later phases. With the use of LIDAR for preliminary analysis terrain data are available earlier in the process, allowing alignments to be identified sooner and, subsequently, photogrammetric data to be produced for a limited area in a shorter timeframe than would be the case for a large-scale corridor. The integration of LIDAR data collection into the location process would consist of the following steps: Estimated Time and Cost Savings Placement of photo control. Fly the corridor and collect aerial photography of the required resolution.Fly the corridor and collect aerial photography of the required resolution. Develop and scan (if hard copy photographs were taken as opposed to digital aerials) imagery and convert to digital format.Develop and scan (if hard copy photographs were taken as opposed to digital aerials) imagery and convert to digital format. Aerial triangulation.Aerial triangulation. Produce breaklines and masspoints to create a Digital Terrain Model (DTM).Produce breaklines and masspoints to create a Digital Terrain Model (DTM). Use DTM to produce additional products (orthophotos, contours, and Triangulated Irregular Networks (TINs)).Use DTM to produce additional products (orthophotos, contours, and Triangulated Irregular Networks (TINs)). Identify a final, preferred alignment.Identify a final, preferred alignment. Densify the existing networkDensify the existing network Create detailed design plans, and cut and fill quantity estimatesCreate detailed design plans, and cut and fill quantity estimates Existing Photogrammetric Data Collection Process Currently, in Iowa, the collection and production of photogrammetric data occurs during the Project/Engineering Information phase of the location process (refer to the Can-Do chart). Once a corridor has been defined, photogrammetric data are ordered and a series of steps spanning months, or even years, is initiated. Photogrammetric mapping consists of seven steps, which include: