The Mapping Match: LiDAR v. Traditional Topo
Two projects on same dike system allow progressive Alabama company to compare LiDAR vs. traditional topographic survey
In 1999, a leading thinker, inventor and futurist named Ray Kurzweil introduced “The Law of Accelerating Returns.” Detailed in his book “The Age of Spiritual Machines,” Kurzweil’s concept is that the rate of change in areas like technology increase exponentially, and that one year of actual time may introduce up to 200 years of change. With that theory in mind, we have to wonder just how quickly the technology is evolving in the surveying and mapping profession.
- First, the Topographic Survey
- Next, the LiDAR Survey
- Comparing the Accuracy
- Comparing the Time Spent
- How About the Cost?
- Final Analysis
During the course of my career, I have seen the evolution of GPS surveying, digital photogrammetry and the widespread implementation of LiDAR. As a Licensed Surveyor, Certified Photogrammetrist and Certified GIS Professional, I have always operated within the context of how these technologies can best work together to satisfy client requirements. It is human nature to consider change as a threat and to take a defensive stance rather than embrace change. I remember how defensive the photogrammetric profession was when LiDAR was first introduced to our traditional mapping markets. Now, I sit and wonder if the same will hold true for the widespread adaptation of UAS for surveying and mapping.
Without question, I think technology has advanced to the point where we can safely ask the question: Can we replicate the accuracy of boots-on-the-ground topographic surveying with airborne LiDAR technology?
Last year, my firm Atlantic was tasked by the federal government to perform two separate projects over the same dike system — the largest dike system in the southeastern United States. The first task was a conventional topographic survey of 25 miles. Separately, a second task order was issued to perform a LiDAR survey of 153 miles.
The topographic survey was collected at 100-foot intervals, coincident with the stationing provided along the existing dike system. Collected points were required to be perpendicular to the provided dike alignment at a rate of 10Hz with a maximum of 12.5-foot spacing between sorted points.
Using three National Geodetic Survey (NGS) control monuments, Atlantic established temporary benchmarks throughout the project site consisting of #5 rebar capped and stamped for identification. Vertical and horizontal coordinates were established using First Order GPS static observations. The temporary benchmarks and NGS control monuments were used as check-in points to maintain accuracy during the project, as the GPS satellites change constellations throughout the day.
After creating a topographic profile using the combination of virtual reference station (VRS) and real time kinematic (RTK) data, Atlantic collected cross sections of the dike itself. Some areas of the project site were not suitable for running conventional profiles, so in order to maintain a continuous Digital Terrain Model (DTM), our team of surveyors collected individual topographic points within these areas.
The following table highlights the residual differences between GPS and known coordinates. It also highlights what we already know — conventional topographic survey is extremely accurate.
A few months later, we were tasked by the U.S. government to fly high-density LiDAR over the same dike system. The LiDAR mission was flown at a height of 3,168 feet with a point density of 17.59 points per square meter. We recovered 26 existing NGS benchmarks and validated the published coordinates to be used to calibrate the LiDAR data. We also collected an additional 11 First Order GPS observations to be used in conjunction with the 26 NGS benchmarks.
Those 37 points were used to validate the LiDAR data calibration results; however, the LiDAR data was controlled only with the airborne GPS solution and GPS base station data. Those 37 points were not used to adjust the LiDAR data to the earth. Flights were flown with a minimum of six satellites in view (12 degrees above the horizon) and with a Position Dilution of Precision (PDOP) of less than 3. Distances from base station to aircraft were kept to a maximum of 20 kilometers.
An independent set of survey control points were used to validate the accuracy of the LiDAR data. A total of 65 points located along the 153-mile corridor were used to measure the accuracy of the LiDAR and the subsequent processing/data classification steps.
The larger tables at bottom left highlight the accuracies achieved when comparing the LiDAR data to known coordinates.
The intended goal was not to create a comparative analysis between the two technologies, but to simply meet the individual needs of each group within this agency. The accuracy results of both surveys (conventional and airborne) did open our eyes to this question of whether or not we could replace some conventional topographic surveys with airborne technology.
The table below compares the accuracy of the conventional topographic survey and the airborne LiDAR survey.
The accuracies achieved through the LiDAR survey were unintended, but are indicative of what is now possible with the latest airborne LiDAR systems, such as Atlantic’s two Leica ALS70-HP high-pulse-rate systems.
Since the Area of Interest (AOI) size was not the same for the two task orders, I have computed the total time and then divided by the line mile to produce a time per line mile per technology. The table below contains those computations.
The conventional GPS survey produced a topographic dataset with approximately 1,320 topographic profiles of the dike system and each profile containing approximately 12-20 measurements. That means during that 1,921 hours we collected 26,421 measurements or roughly 13.75 measurements per hour spent.
In contrast, the 373 hours spent on collecting and producing the LiDAR dataset for the larger AOI yielded a little more than 2.9 billion individual measurements. That equates to a little more than 7.7 million measurements per hour spent.
With LiDAR providing so many more data points and far more expensive technology (i.e., airplanes and airborne LiDAR sensors), one might expect the cost to be higher than traditional survey, but putting survey professionals in the field also comes with significant expenses for trucks, ATVs, fuel, etc. The table below compares the cost for each.
So, can high-density survey from the air replace traditional survey? Not always. Some survey is necessary to support LiDAR accuracy, but overall LiDAR can produce similarly accurate measurements that have many advantages over traditional survey models.
Traditional surveying is accurate, but it is also costly and can be a public relations nightmare. When tasked to perform topographic survey on federal land, it is easy; it’s their/our land, with easy access, and nobody questions why you are there. For industries like petroleum engineering, you are likely surveying private land and sometimes questions arise. Landowners want to know why you are there and what you are planning to do, and lastly they may even decide to fight you on using their property. In addition, some areas are difficult to access, which increases the cost for traditional survey.
By contrast, LiDAR allows you to collect points from the air — upsetting no one. One of the downsides of LiDAR is it is more restrictive on when appropriate flying can be conducted. Also, heavy vegetation can prohibit a dense enough data collection to completely replace conventional surveying.
This comparative analysis does offer proof that LiDAR technology has progressed to the point to at least greatly supplement traditional topographic surveying for large projects. As Kurzweil predicted a little more than 25 years ago, technological evolution has an impact on our daily lives, our way of thinking and perhaps even our method of surveying.