Sometime this fall, after the leaves are off the trees, several small planes will fly back and forth over nine western Minnesota counties, bombarding the ground with hundreds of millions of laser beams.
If you were to spread out a beach towel anywhere in that area, one of the laser beams almost certainly would hit it.
Equipment on the planes will catch the beams as they rebound from the soil and other surfaces and calculate the distance they traveled. Then, combining the laser data with global positioning system coordinates, the equipment will produce highly accurate measurements of
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| LiDAR mapping helped plan a Stearns County pond. Photo: Nathan Hylla, Stearns County SWCD |
the location and the elevation of each spot where a beam hit the ground.
Eventually, those measurements will be used to create computerized models of the land’s surface – the hills, the valleys, the flats — that are much more detailed than any other currently available models.
The new topographic models, used in conjunction with existing aerial photos, will allow water planners and water managers to easily and relatively quickly make preliminary designs for flood-control projects or wetland restorations and target conservation practices and incentives to places where storm water runoff from farm fields is the worst.
“The real goal of this project is to provide a resource for managers, for decision-makers, to understand how water is flowing on the surface and how they can better manage it,” said Tim Loesch, a Minnesota Department of Natural Resources supervisor overseeing the laser-scanning project.
“We use it to tell where the flow is and how powerful it is,” said Beau Kennedy, a Goodhue County water planner who also is the Wetlands Conservation Act administrator for Goodhue’s soil and water conservation district. “It can find the smallest little gullies in
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| DNR supervisor Tim Loesch and LiDAR map |
the fields we target. We can find where the strongest potential for erosion is in fields and field edges, and then we concentrate our erosion efforts on those spots.”
The flights planned this fall are part of a $5.6 million effort, funded by the sales tax increase Minnesotans authorized in a 2008 constitutional amendment, to map land elevations in parts of the state that have not previously been mapped with the laser technique.
The scanning technique, known as Light Detection and Ranging, or LiDAR, has been used over about the last five years to collect new elevation data in much of northwestern and southeastern Minnesota and in some metro-area counties.
Last spring, the laser scanning was completed on 16 southwestern counties. But the enormous computer files filled with mathematical coordinates – latitude, longitude and elevation above sea level – for each spot where a laser beam hit the earth have yet to be fully processed and delivered to the state by the contractors who collected the data.
Northeastern and much of central Minnesota have not yet been mapped, and mapping of some Twin Cities counties will be completed or upgraded.
The massive files of new elevation data are being put to use by engineers and water planners as quickly as they become available. Some examples:
- In the Red River Valley, new elevation measurements – accurate to within 6 inches – are being used to plan options for diverting future floods around Fargo-Moorhead, to plan ring dikes around farmsteads vulnerable to overland flooding and to pinpoint low areas where big quantities of water could be temporarily stored on fields during flood seasons.
- In the Whitewater River Valley in southeastern Minnesota, the Minnesota Pollution Control Agency and the federal Natural Resources Conservation Service are experimenting with using LiDAR data to update surveys first made in the 1930s. The work is part of an effort to estimate how much the river bluffs have eroded onto the valley floor and how much sediment the Whitewater carries to the Mississippi River.
- In Stearns County in Central Minnesota, when Jaime and Cassie Gertken wanted to build a 12-acre wetland and pond on their farm as part of the federal Conservation Reserve Program, a technician from the county soil and water conservation district used LiDAR data to draft preliminary plans.
- On a statewide basis, the DNR is using LiDAR measurements to re-draw a map of Minnesota watersheds, parsing the land to show the drainage feeding lakes, streams and wetlands.
Nathan Hylla, the technician who helped the Gertkens plan their wetland, estimated that the availability of LiDAR elevation data saved 60 hours of preliminary surveying work on the site and the 70-acre watershed it drains.
“It would have taken three people to drive out to the farm, meet with the landowner, do the survey, come back and analyze the data to determine if a wetland restoration was even feasible,” Hylla said. “We determined that a wetland was feasible at his location. We said it’s going to be 5 feet deep, plus or minus. You’re going to have this much pool area. We were able to determine that without ever stepping into the field.”
After the preliminary assessment, surveyors visited the site to make final plans, Hylla said.
The benefit of the new LiDAR data over existing elevation mapping is the vastly increased number of elevations measurements LiDAR provides and the greater precision of those measurements.
The LiDAR scanning takes one elevation measurement, the height above sea level, every 1.5 meter across the land’s surface – that beach towel laid out on the ground.
But the “data cloud” containing all those coordinates is so vast and difficult to use that most of the agencies currently using LiDAR make somewhat less-precise elevation models with cells 3 meters on a side. That means they get one measurement for the elevation for any given patch of land about the size of two king-size mattresses.
By comparison, the best digital elevation models commonly available are derived from U.S. Geological Survey maps. When those maps are converted for use on a computer, they yield only one elevation measurement within a cell 10 meters on a side. That’s an area equal to 25 of those mattresses.
The difference in scale may not sound like a lot. But, especially in areas where the terrain is mostly flat, slight changes in elevation make big differences in how water flows on the land.
Topographical maps constructed with LiDAR have contour lines showing 2-foot changes in elevation. The USGS maps and the digital models derived from them show 10-foot differences.
On a computer screen, a three-dimensional map drawn with LiDAR is dramatically more detailed. “This is mind-boggling,” Sean Vaughn, a DNR technician mapping watersheds, said as he toggled between a conventional model and a LiDAR model. “This gives people who crave to map the Earth’s surface goose bumps.”
LiDAR measurements are not perfect. The laser beams shot from planes bounce back, not only from bare earth, but also from the tops of trees, weeds, hay bales, houses and other structures – everything that is on the land.
The vendors who fly the planes use mathematical algorithms and visual comparisons with aerial photos to sort out what is bare earth and what is not.
“Nobody should assume the data set is perfect,” said Chuck Fritz, the director of the International Water Institute at North Dakota State University. “There are errors associated with the data. We are trying to minimize the errors, but we are talking about over 60 billion data points.”
But Fritz, who over the last decade has led a state-federal-local effort to conduct LiDAR scanning on 50,000 square miles on both sides of the Red River, said the new, more accurate elevation measurements are changing the way water planning is done.
During flooding in 2009, officials in Climax, Minn., used LiDAR mapping to spot low spots in the city’s dikes, Fritz said. LiDAR also has been used in planning a massive project to divert future floods around Fargo-Moorhead.
“This effort will profoundly change how we make and defend water decisions in this part of the country,” Fritz said. “I believe that.”