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Small and mid-sized communities in the Chicago region are working together to cooperatively acquire and apply geospatial data to serve a wide range of municipal functions. This organization of municipalities formed to leverage collective buying power and resources to gain access to high resolution and high accuracy LiDAR and other geospatial tools. Importantly, they are also driving the development of new applications for LiDAR beyond traditional stormwater and floodplain management. Urban forestry is emerging as one such facet of municipal services that can also benefit greatly from the geospatial data they are collecting.
The GIS Consortium (GISC) is a formal organization of 32 municipalities in six counties in the Chicago suburbs. Formed in 1999 as a group of four communities, the GISC has since experienced steady growth while also maintaining a strong focus on the theme of cooperation. The group represents communities ranging from 2 square miles up to nearly 20 square miles, with populations of 7,000 up to 60,000. The core objective of the organization is to acquire and collectively apply GIS and base mapping resources for all members. Since geospatial services such as LiDAR and multispectral imagery are available to every member of the GISC, even the smallest communities can affordably acquire this data and learn how to apply it from the collective knowledge base of the GISC.
The organization has developed a common GIS platform supported by base mapping derived from aerial surveys conducted by Ayres Associates’ Geospatial Services division based in Madison, Wisconsin. The mapping primarily comprises orthoimagery and impervious surface mapping derived from multispectral aerial imagery and 1-foot interval contours and high resolution digital elevation models derived from LiDAR acquired at 20 points per square meter using a Riegl 680i system.
LiDAR and aerial imagery missions are conducted each spring for the GISC to maintain up-to-date base mapping. While most of the communities are still planning for their first LiDAR missions, some have already completed two LiDAR projects since 2010. The group has developed base level standards for their LiDAR data, but the product specifications remain flexible enough to support customization to meet localized needs and conditions.
The collective nature of the group also brings a wide range of disciplines from a myriad of municipal departments. With this, there is an increased awareness for the rich data that remains locked inside the LiDAR point clouds. This is encouraging the development of new LiDAR derivatives to support diverse municipal functions. The communities are now exploring how to extract 3D buildings, utilities, vegetation, and tree canopy information.
LiDAR for Urban Forestry
Urban forest ecology and management is emerging as an important issue where communities may find additional value in their LiDAR investments. Society is recognizing that urban forests are a valuable asset for growth and prosperity. Not only do city trees positively impact property value and help to conserve energy by shading and cooling buildings, they also contribute to a number of health benefits by improving the quality of our air and water. A diverse population of trees reduces stormwater runoff and its accompanying pollutants from entering fresh water sources, thereby helping to maintain the clean, uncontaminated water that is essential to our daily life.
While stormwater and floodplain management will remain the primary driver for investment in high resolution, high accuracy LiDAR missions, the data can also be used to create 3D canopy models and high definition digital surface models to provide both qualitative and quantitative attributes for urban forestry. The specific output of LiDAR derivatives can be designed to address specific local needs.
LiDAR specialists at Ayres Associates are taking a two-pronged approach to mapping and measuring tree canopy extent from LiDAR. A more robust solution utilizes watershed or objectbased segmentation to extract individual tree canopies, and this data is then merged with multispectral imagery for detailed analysis. The second approach is through a process of consolidation of canopies from vegetation point classes (figure 5). This approach provides a more generalized canopy for large-area analysis, such as an urban forest preserve or an entire community.
Consolidated Tree Canopy Extents
Focusing on the urban canopy as a whole is an economical approach to large area forestry analysis. In this case, the ability to measure in overall extent allows a community to track trends and infill canopy voids. The methodology that we employ is a process of aggregation of classified vegetation points. We have developed an approach for the City of Des Plaines, GISC member since 2002, using their second LiDAR mission completed in 2015, which was acquired at an average density of 20 points per square meter.
This process of tree canopy extraction in Des Plaines began with a classified point cloud, specifically ground, high vegetation, and building classes. Objects like power lines, light poles, basketball hoops, and even rooftop points often get lumped into the high vegetation class due to the automated nature of typical point cloud processing. We perform enhanced classification editing to eliminate anomalies and achieve better representation of actual vegetation and buildings (the better the classification, the more accurate the resulting delineation). We then class the vegetation by arbitrary but logical height classes (low, medium, and high). The height classifications are selected according to local conditions. With mature urban forests like in the City of Des Plaines, we define high vegetation as 10 feet or higher. In newer, developing communities we may use 6 feet as the high vegetation threshold since many of the trees may have been planted in the last decade.
After classification is complete, modeling and GIS editing take over. These high vegetation classified points are extracted from LiDAR processing software and brought into a GIS, where it is aggregated to create polygons, filtered to eliminate most of the anomalies in the canopy–such as buildings, distribution wires, and poles–and smoothed to create representations of the tree canopy. The results are then further refined using minimum area thresholds. The final canopy extents are output as polygons, which are then ingested into the GISC’s base mapping geodatabase and available to be displayed within the group’s webbased GIS platform for use by municipal departments and City residents.
Segmentation-based Canopy Extraction
An example of a more targeted application for LiDAR in an urban forest is how it can be applied to mitigation efforts directed at invasive species. The Emerald Ash Borer (EAB), an invasive beetle, is killing ash trees at a staggering rate in the Midwest, and the range of this infestation is expanding.
The Village of Tinley Park, IL, member of the GISC since 2011, provides a useful case study in how LiDAR can aid in the identification and response to this issue. A recent survey of tree species by the Village, with a total area of just 16 square miles, has revealed that most of its 11,000 ash trees within the public right-of-way had been infested by EAB. The results are devastating; much of the urban forest canopy was already dying when the Village made the difficult decision to remove nearly 10,000 susceptible trees within those public areas.
Ayres Associates has been working to develop efficient and effective methodologies for applying LiDAR and multispectral imagery to aid in the combat of EAB infestations. We are building workflows based on segmentation and classification rules to accomplish three primary goals of identifying: 1) change of tree canopy extent, 2) ash trees, and 3) dead/dying trees. Tinley Park has been an effective proving grounds for this product development, not only because of the severity of the EAB issue, but also because of the abundance of high resolution LiDAR and multispectral imagery that we’ve collected for the GISC over the years.
The first step in the process involves tree canopy delineation based on characteristics of color variation (orthos) and feature heights and geometry (LiDAR). Canopy delineation is the result of a number of customized object-based segmentation or watershed segmentation rules. These are designed to identify and represent individual tree canopies rather than groups of trees. By identifying individual trees, we are able to better characterize attributes of a forest stand, such as numbers of trees and distribution of canopy extents.
After segmentation, we use multispectral imagery and groundtruthing to train our software to identify and classify the predominant tree species within test areas and to specifically single out the ash trees. The customized algorithms we employ for the species identification are built in close cooperation with trained ecologists that can detect and understand the radiometric and geometric clues hidden in the LiDAR and imagery. These rules are established within the test areas and are then extrapolated across a larger project area. Through our quality control analysis of the resulting classification in Tinley Park, we have demonstrated successful identification of ash trees up to 80% when compared to field data.
Although Tinley Park is taking an aggressive approach to combat EAB, their current efforts are focused on public spaces. However, many susceptible or dead ash trees remain on private lands, where they are no less a problem, posing potential safety hazards for homeowners. A systematic approach to ash tree identification using LiDAR and multispectral imagery can support analysis and response throughout the community.
High definition LiDAR point clouds are key to the successful outcome of both the segmentation-based and consolidation approaches to tree canopy extraction. Acquiring LiDAR at a minimum of 20 points per square meter yields sufficient returns on canopy, even when flights occur in the spring, leaf-off state. We have also found that 3D buildings and impervious surface features also extracted from high definition LiDAR can be effectively paired with the vegetation data for a more holistic view.
Ayres Associates works closely with the GIS Consortium and other communities around the region to identify new methods for extracting secondary information from these rich point cloud data sets. This not only brings added value to these significant investments for these communities, but also builds broader support for these projects in an era where municipal budgets can be tight and unpredictable. Extraction of tree canopy information is one such application that is economical and brings immediate benefits to urban communities dealing with big and costly issues.
To learn more about the GIS Consortium, please visit www.gisconsortium.org.
Jason Krueger, CP, GISP, is manager of the Aerial Mapping Group, Ayres Associates Inc. Mike Seidel is a geospatial technician with the Aerial Mapping Group, Ayres Associates Inc.
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