Road Repair Analysis and Planning
Transportation and public works departments have a responsibility to survey, maintain, and repair roads as needed. Therefore, these departments need to collect data on road conditions, analyze the current situation to identify needs for maintenance, and plan road repairs. The tools and functions available in Global Mapper Pro assist teams with all aspects of this workflow.
The ability to collect 3D data is becoming more commonplace as its use is more prominent in GIS and other industries. Transportation infrastructure engineers are now able to capture up-to-date information and model an area of interest in 3D in order to methodically assess the situation and simulate the changes that need to be made.
Lidar data, an active form of data collection using laser pulses, creates a high-density 3D point cloud that is then loaded into Global Mapper for further analysis. Lidar scanners can be attached to fixed-wing or rotary aircraft, drones, terrestrial vehicles, and can also be handheld. With recent developments, some newer smartphones even have the capabilities to create 3D data.
Loading and Visualizing Data
In this example, a terrestrial lidar point cloud for a small downtown area is loaded into Global Mapper Pro to be explored and analyzed. The default data view in Global Mapper is a 2D top-down view, but the 3D Viewer, opened from the Viewer toolbar, and Path Profile-cross section tool, enabled from the Analysis toolbar, can be used to better visualize the data in three dimensions.
Multiple views in Global Mapper help to better understand the 3D point cloud.
The high-resolution data for a portion of the street is terrestrial collected lidar, meaning the lidar scanner was set up at a static location on the ground. This method of data collection works well for capturing a higher level of detail for the ground and near ground features, but data for high vegetation canopy and building roofs will not be captured.
Further exploring the characteristics of this data, the metadata for the layer is viewed and the attributes for individual points are displayed with the Feature Information tool. Viewing the attributes for this point cloud provides a look at what values are available to be used for visualization and analysis. In this example, the dataset contains true color (RGB) values and intensity.
The layer metadata and feature information can provide valuable insight into the data.
Colored by RGB and intensity values, the detail captured in this point cloud data can be seen.
To understand the point cloud better and to start analysis of the roadway, the data needs to be classified. Classification designates a class for each point to better describe what is represented in the data. Common classifications are ground, building, and vegetation. For a road scenario like this, the most important classification is ground. Accurately classifying ground representing points allows the ground in the data to be separated from the other, unclassified returns.
Global Mapper offers manual classification options, but a far more efficient method involves the automatic classification tools. Using the Automatic Ground Classification tool with user-specified input parameters to guide the algorithms, Global Mapper applies ground classification to the appropriate points.
Accurately classifying ground points allows the data to be filtered to only the ground representing points for further exploration and analysis.
Creating and Exploring Terrain
In Global Mapper, the process for converting a point cloud into an initial terrain surface is simple. When working with point cloud data, the Create Elevation Grid tool includes several binning methods to create a smoothed grid that will use the minimum, maximum, or other value from each user-defined samplearea. Using the binning minimum method along with a ground classification filter generates an accurate Digital Terrain Model (DTM) that best shows the shape of the road.
The generated elevation grid is a solid raster surface modeling the road. The Path Profile works well to show the differences in elevation and shape of the road surface.
The elevation surface generated in Global Mapper can be colored with different shaders in order to better visualize elevation, slope, and aspect. Clearly visualizing changes in elevation and slope helps to more fully understand the current condition of the road, curb, and sidewalk.
Roads need to be relatively flat, with a slight slope to help handle rainwater runoff and mitigate flooding. On this particular section of road, there are sidewalks with a step-up curb that should be square and consistent along the road edge. The slope direction or aspect shader can be applied to the gridded data to show the consistency of slope and slope direction for either side of the road.
Applying different shaders, such as slope and slope direction, enhance different characteristics of the data in the 2D view.
Slope Direction Arrows
Looking at the digital elevation model shaded by slope, general trends across the roadway can be gleaned, but to show the slope and direction more discreetly, vector features can be added and shown as quiver plot arrows. By creating a regular grid of point features covering the elevation model and sampling the slope and elevation at each point, local measurements are generated. To better visualize the slope and direction with the generated points, change the symbology to a quiver plot style, with the slope and slope direction for each location determining the direction and magnitude for the arrows.
Using a quiver plot symbol based on slope and slope direction, inconsistencies in the roadway can be seen.
In an urban area, like this downtown setting, water runoff is of great concern. Using the DTM and the Watershed Creation tool in Global Mapper, simulated outflow channels are created to show the predicted flow of water based on the available elevation model. Ideally, the water will flow to the storm drains positioned along the road to avoid flooding.
The Watershed Creation tool creates stream features modeling water flow over a terrain grid.
Examining the generated streams, a well-defined watershed appears to follow the centerline of the road and the outflow accumulates along the curbs as expected. While it is good that the curb feature will catch the runoff as it approaches the buildings and businesses downtown, the flow on the west side of the road does not channel to an adequate number of drains, which will likely cause some unwanted pooling. This current drainage issue should be considered when drafting a plan for the repair of this section of the road.
Extracting Key Features
As planning for the road repair begins, vector features representing any hard edges, such as curbs, need to be created. Once in vector format, these features can be used to generate a new terrain model representing the repair plan for the road.
The curbs of this road can be extracted as breaklines in Global Mapper Pro. 3D line features can be created manually or with an Automatic Breakline Generation tool found in the Analysis menu. The Automatic Breakline Generation applies user-defined input parameters to create lines along edges where there is a significant difference in elevation or slope. With this tool, lines at the apex of the curb along either side of the road are created. Some of these lines are broken into smaller sections but, as vector features, they are easily combined and cleaned up to create a set of 3D breaklines defining the current curb structure.
Automatic breakline creation extracts 3D vector lines that can then be manually edited and cleaned up.
The manual method for 3D line extraction requires additional user input and is done with a series of perpendicular Path Profile views. An individual vertex is placed and adjusted in each cross-section view in order to form the shape of the 3D line. This method is used to more accurately extract the current high point, or crown line, for this roadway.
Using a series of perpendicular Path Profile views, the current crown of the road is extracted as a 3D line.
Drafting the Proposed Road Repairs
Viewing additional path profiles of the generated road surface and extracted features, it is clear that the road is not level. Not only are the slopes of the pavement on either side of the highpoint inconsistent, but the left and right curbs do not sit at the same elevation. This slope pattern is consistent for the extent of the road as it is perpendicular to the slope aspect of a hill.
A path profile across the width of the road shows the current shape and slope.
Planning the Solution
When considering options to repair this road, creating a new surface with the crown line in the center is not an ideal solution. Since the edges of the road are at different elevations, the slope to either side of the crown would be different. The upslope side would be within an acceptable slope range, but the downslope side would exceed the acceptable cross slope for an urban area.
Types of allowed crown points and cross slopes for urban streets. (Department of Transportation. DelDOT Road Design Manual, 2011.)
Keeping in mind the location of this road and the drainage infrastructure already in place, the best solution is to create a straight slope street. To better manage the slope that will need to be applied, the curb height will also be offset. By using a shorter curb on the upslope side, and a taller one on the downslope side, the overall cross slope for the roadway is reduced.
Modeling the Repaired Sidewalks
Before modeling the straight slope roadway surface, the extracted breaklines are used to flatten the sidewalk to the curb height on either side of the paved road. By combining the curb breaklines into longer features for each side of the road, and creating single sided buffer areas from each line, polygons representing the sidewalks are generated. Since the created buffer areas are derived from the 3D breaklines, these polygons inherit the elevations of the original breaklines.
3D lines are used with the Create Buffer Features tool to generate 3D polygons modeling the sidewalks.
The first section of a new elevation grid can now be generated from the 3D sidewalk areas. Using these 3D polygons to generate a new elevation grid layer yields sections of a digital elevation layer modeling the flattened sidewalks.
The Path Profile tool is used to compare the original roadway sidewalks to the new, ideal sidewalks.
Modeling the Repaired Road Surface
To best create a model representing the entire roadway, a vector feature describing the area should be generated. To generate 3D lines for the edges of the roadway, the extracted breaklines representing the tops of the curbs are shifted in the Z (elevation) direction to keep the horizontal position of the lines but apply the vertical change from the sidewalk height to the roadway surface. Since the west, upslope side of the road will need a shorter curb, and the downslope side a taller one, the breaklines for either side of the road are selected and shifted individually.
Shown as black dots in the Path Profile view, the shifted breaklines appear at the curb position, but at the correct elevation in relation to the flattened sidewalk grid.
Creating an area from the shifted 3D lines provides a single feature that covers the roadway. The per-vertex elevations from the shifted lines will be retained, making the roadway polygon sloped to connect the bottom of the curbs, or road surface on either side. Generating a digital elevation model from this 3D polygon creates a model of the ideal straight slope road surface.
A cross sectional view of the original road surface and repaired model show the difference in shape and slope.
Combining the Sidewalks and Road Surface
With two sections of an elevation grid generated in the Global Mapper workspace, one for the repaired and flattened sidewalks, and one for the straight slope road surface, these sections can be exported and further analyzed by separate departments if necessary. To combine the sidewalks and roadway models into a single layer, they are displayed together in the Global Mapper workspace and exported to a single elevation file.
Exporting multiple elevation models from Global Mapper will combine them into a single surface.
As a final assessment, a water flow analysis is performed on the planned road surface model to check that the runoff on the road will be improved by the straight slope solution. Using the Watershed Creation tool once again and comparing the streams derived from the modeled road surface to those created from the original road surface, the difference in predicted runoff to the storm drains can be seen. With more storm drains present on the east, or downslope side of the street, the repaired road model shows improved water flow to these locations.
As a final comparison, the Watershed Creation tool is used to predict flow patterns across the straight slope street model.
WORK MADE EASY
WITH GLOBAL MAPPER
Department of Transportation. DelDOT Road Design Manual, 2011. https://deldot.gov/Publications/manuals/road_design/pdfs/04_cross_section_elem.pdf.
Department of Transportation. NJDOT Design Manual – Roadway, 5-3 Major Cross Section Elements. state.nj.us/transportation/eng/documents/BDC/pdf/RDMSec5-20150117.pdf.