by Miguel Silva, SVP, RRC Companies
Inaccuracies in grading plans often lead to significant cost overruns and contentious disputes among parties involved in the Engineering, Procurement, and Construction (EPC) of large-scale solar PV power plants. Consider the ramifications of a 6-inch elevation discrepancy across a 1,800-acre site: this could result in up to 1.45 million cubic yards of additional earth being moved, drastically affecting both contracted cost and schedule.
This article aims to shed light on a critical problem that adversely impacts both the cost and construction schedule of solar projects. With costs of major solar project components such as PV modules, inverters, and trackers decreasing, the expense of grading or earthworks has become a significant fraction of total project cost. This issue is further exacerbated by three main factors:
- The scarcity of flat land available for developing and building these projects.
- The lack of adherence to proper specifications and best practices in the execution of lidar surveys, often exacerbated by the underutilization or improper use of UAV technology.
- Horizontal single axis tracker system limitations to adapt to uneven terrain and slopes.
- Remote Sensing, particularly in the context of UAVs and their capabilities in data collection.
- Spatial Reference System
- Derivative Products
Inaccurate grading plans manifest as discrepancies between civil IFC documents and field measurements, including elevation and earthwork quantities, during construction. Other associated risks relate to flooding and inadequate hydraulic/drainage analysis. These discrepancies arise from various sources, some of which could be organized in the following categories:
While each category could merit its own detailed analysis, we will briefly touch on each to foster open discussion, particularly focusing on the evolving role of UAVs in remote sensing and their impact on the accuracy and reliability of large-scale solar PV project planning..
Remote Sensing
Remote Sensing, as defined by the American Society for Photogrammetry and Remote Sensing (ASPRS), is ‘the art, science, and technology of obtaining reliable information about physical objects and the environment, through the process of recording, measuring, and interpreting imagery and digital representations of energy patterns derived from non-contact sensor systems.’
In the context of our discussion, we will focus on the collection of topographic data using lidar (light detection and ranging) technology. There are numerous methods for collecting ground elevation data using lidar. For large land areas, the most common approach involves using sophisticated sensor packages mounted on flying platforms such as fixed-wing aircraft, rotary-wing aircraft, and/or UAVs (Unmanned Aerial Vehicles).
With technological advances and the proliferation of UAVs (also known as drones), many individuals and companies have started offering mapping services based on lidar and aerial photography. However, often these services are provided without the necessary minimum training, appropriate equipment, or both.
To avoid errors which later will impact construction cost and schedule, it is crucial when requesting lidar services from vendors to ensure that they have a qualified lidar mapping scientist and/or photogrammetrist assigned to manage the project. In some states, like North Carolina, a professional license is required to perform these tasks; in other states like Texas, agencies such as the Department of Transportation require work to be supervised by ASPRS Certified Professional even though licensure is not required under state law.
Spatial Reference System
‘A spatial reference system (SRS) is the georeferencing and coordinate system assigned to geographic data. It defines how geographic data is mathematically transformed onto a flat map. The correct spatial reference settings and transformation allow geographic data that has different coordinate systems to line up in a Geospatial Information System (GIS).’ (Ref. University of Connecticut, CT ECO).
Many errors in spatial data arise from incomplete specification of the project coordinate system. This can result in small but significant discrepancies in horizontal and vertical coordinates and can lead to misunderstandings, particularly at project interfaces where information and deliverables are exchanged between various contributing parties.
As an example, a user might specify ‘NAD 83 State Plane Texas Central,’ expecting consistent interpretation by both users and software (e.g., ArcGIS vs. Civil 3D). However, within this state plane zone, numerous instantiations exist, each based on different NAD83 adjustments. For instance, there are ten different versions of NAD83 Texas State Plane Central, all sharing the same Federal Information Processing Standards (FIPS) code.
This situation can cause significant challenges in projects where precise geographic data alignment is crucial. It’s essential for users to not only specify the map projection but also be aware of the underlying datum and adjustment used to avoid potential errors and inconsistencies.
Derivative Products
Light Detection and Ranging (lidar) point cloud data enables us to create a wide array of spatial information products, often referred to as ‘derived or derivative products.’ These products are crucial for effectively understanding and addressing geospatial challenges. Examples include, but are not limited to, digital elevation models (DEM), digital surface models (DSM), digital terrain models (DTM), canopy height models (CHM), triangular irregular networks (TIN), contours, and land XML surfaces.
It is essential to understand project tolerances (e.g., pile reveal, slope constraints, etc.) and the accuracy and application of data (e.g., for post-development hydrology studies, grading plans, energy simulations, shadow analysis, etc.) to determine the appropriate type and format of derivative product required. For instance, various modeling techniques for topographic and hydrologic surfaces cater to different user applications in hydrology studies, such as:
- Photogrammetric DTM.
- Lidar DTM with no supplemental breaklines.
- Hydro-flattened lidar DTM.
- Lidar DTM with enhanced breaklines.
- Hydro enforced lidar DTM.
- Hydro conditioned lidar DTM.
Beyond these, there are numerous other elevation derivatives that should all be created from the same certified source, whether it is a lidar point cloud or a photogrammetric stereo model. These derivatives include:
- Hillshades
- Slope maps
- Aspect maps
- Curvature maps
- Profiles and cross-sections
- Height above ground maps
- Viewshed maps
- Watershed maps
Just because UAVs are readily accessible, and numerous online services offer a wide range of associated services from planning aerial missions to processing remotely sensed data, we must resist underestimating the complexities and challenges in specifying, collecting, and delivering lidar and imagery data, along with its many derivative products. In my home country of Venezuela, there’s a saying, “Zapatero a su zapato,” which translates to “let the cobbler stick to his last” in English, or in other words “stick to what you know. In the surveying and engineering professions, the commonly used phrase is “practice within your area of expertise.”
Embracing this popular wisdom, RRC has embarked on an ambitious journey to expand our team with individuals who bring the necessary knowledge and expertise. Our goal is to enhance our ability to provide authoritative advice, products, and services to the industry. So, please stay tuned for future updates!
RRC Companies, as a multidisciplinary engineering firm specializing in the renewable sector including wind, PV solar, and BESS, leverages Project Development Engineering team expertise supporting development and construction (approximately 95 GW), along with the resources of our Geotechnical Engineering, Land Surveying, and Civil Engineering groups. We are ideally positioned to address the challenges described above and to provide new, comprehensive, and reliable services to the industry.
Miguel Silva
Senior Vice President
RRC Companies
RRC is a Sustaining Member Company of American Society for Photogrammetry and Remote Sensing (ASPRS)