In continuation of our series on “Best Practices” for solar development professionals when procuring professional surveying and mapping services, this post will take a look at topographic data collection for solar development projects. Topographic mapping is required to facilitate site/civil design for solar developments and thus is a vital component to consider.
We are frequently asked if publicly available LiDAR can be utilized as a source for topographic information. Our typical response is “it depends”. If a two-foot contour interval is sufficient for site/civil design and the site topography is relatively uniform, then yes, publicly available LiDAR may be suitable. However, our experience is that LiDAR with a point density of ≤ 2 points per square meter (ppsm) in wooded terrain is not sufficient to meet the accuracy requirements for two-foot contours. Public LiDAR data sets that have point densities greater than 4 ppsm have proven to meet the accuracy standards for two-foot contours.
Public LiDAR data sets acquired prior to 2012, typically don’t meet the accuracy standards, especially in wooded or brushy areas.
Frequently, the public LiDAR data sets are not dense enough to identify and extract breakline information, which can significantly impact the accuracy of the digital terrain model (DTM). Creating a DTM surface from publicly available LiDAR on sites with irregular terrain and numerous breaks in grade may not provide an accurate representation of the ground surface.
Ground truthing and check shots are required to confirm the accuracy of LiDAR data sets to ensure the data meets the required ASPRS (American Society of Photogrammetry and Remote Sensing) standards for vertical mapping. The vertical accuracy is determined by taking the root mean square error (RMSE) between the LiDAR derived elevations and the ground survey check shot elevations. Topographic mapping sufficient for two-foot contours must have an RMSE of ≤20cm in non-vegetated terrain (ASPRS Vertical Data Accuracy Class VI).
Every Developer and Design Professional should be aware that accurate breaklines and planimetric information (e.g. buildings and structures, headwalls, railroads, paved and unpaved roads, utilities, signs, etc.) cannot be extracted from a public LiDAR data set due to the sparsity of the point cloud.
Ground Surveys/UAV/Manned Aircraft
In some instances, the site/civil engineer requires topography with a one-foot contour interval for storm water and access road design, which will require employing conventional surveying techniques (i.e. ground survey), UAV-mounted LiDAR and aerial photography, or manned aircraft LiDAR and aerial photography.
The size of the project and land cover type (i.e. woodlands, abandoned agricultural lands, or active agricultural lands) will dictate the most efficient means of collecting topographic and planimetric information.
Ground surveys may be more economical to conduct if a site is relatively small (i.e. less than 30 acres) and is open agricultural lands, while UAV LiDAR data and aerial photography acquisition is most economical for open or wooded sites ranging from 10 to 500 acres. Acquisition of LiDAR and aerial photography from a manned aircraft is typically more economical for large projects ranging from 300 to several thousand acres. The disadvantage of utilizing manned aircraft is that data must be collected during periods of “leaf-off” without snow cover. UAV LiDAR can be acquired in “leaf-on” conditions because of the greater point density due to the low altitude and slow speeds the UAV’s operate at.
For solar developments, we would generally recommend acquiring topographic information suitable for 50 scale (1” = 50’) topographic mapping with a one-foot contour interval utilizing any of the three methods cited above.
The following table summarizes the advantages and disadvantages for topographic data collection options available for solar development projects.
It is our experience that allocating financial resources for the collection of accurate topographic and spatial information on the front end of project development pays dividends throughout the life cycle (civil/site design and construction) of the project.