| LIDAR for Flood Mapping
By Robert A. Fowler Floods are one of those major natural disasters you can watch on CNN, hopefully occurring somewhere other than where you live. They seem to be happening more often or, at least, being reported more often. Recent large floods such as those on the Mississippi and, in 1997, on the Red River have caused true hardship to the people who live in the affected areas, as well as having destroyed much property, livestock and wildlife.
In 1999, Lasermap was contracted to provide flood risk mapping data for 200 square miles in Manitoba, an area located south of Winnipeg in the flood-prone area of the Red River of the North. This wasn't the first time that Lasermap completed flood risk mapping. The company also did laser surveys in QuŽbec Province, specifically in the Haha River valley.
Much of the Red River valley is subject to repeated flood threats, and major flood events have occurred throughout recorded history. During 1997, a combination of natural events - heavy winter snowfall culminating with a major blizzard in April 1997, followed by a quick melt - resulted in huge areas being flooded both in the U.S. and in Canada. During these floods livestock was lost, numerous farms were inundated by floodwaters, and millions of dollars of personal property was destroyed or damaged. Most of the people living in North America have failed to realize the extent of this flood. For example, the primary north-south route, Interstate 29, was closed for an extended period of time. There were no open bridges across the Red River from Fargo to Winnipeg, a distance of more than 200 miles. Rail traffic needed to be re-routed along the entire corridor.
Toxaphene, a chemical pesticide that has been banned in Canada and the U.S since 1982, escaped from a flooded warehouse in Grand Forks, N.D., where it was stored. Trace contamination appeared over a huge area, reaching as far north as Lake Winnipeg. While this was declared a low health-risk hazard, it indicates some of the unpredictable side effects that can happen when floods occur.
Despite being the most recent flood on the Red River of the North, the International Joint Commission (the U.S./Canada trans-border committee overseeing water bodies that traverse the border) has cited a great deal of historical evidence that major floods of this size are far from unusual. Indeed, there is the potential for even larger floods in the future.
While people who live in flood-prone areas may be tempting fate, some basic data is required to determine various levels of flood risk, and to avert construction or habitation in areas where flooding is likely to be a relatively common occurrence. While this sounds eminently logical, there are huge areas in the Red River plain where the extent of this risk is not easily ascertained. Small differences in elevation may extend over several miles. The source of the risk can therefore appear to be remote for dwellings or businesses constructed in areas located well away from the river.
Therefore, the degree of accuracy of elevations in a digital-terrain format for flood risk surveys and mapping is important, and it can have significant ramifications if left unmet. Spot elevations are usually required to within six inches of accuracy, with contours plotted at one-foot intervals. In the past, detailed high-accuracy mapping has been an expensive proposition. But airborne LIDAR technology now allows large areas of precise data to be captured with relative economy.
LIDAR is an acronym for Light Detection and Ranging. LIDAR systems have been around for a long time, but it is only recently that these systems are usable with a high degree of accuracy from moving airborne platforms. This advance is due to the development of precise inertial measurement units and airborne GPS (Global Positioning System).
Laser systems, in and of themselves, are highly accurate in their ranging capabilities, providing distances with accuracy to within a couple of centimeters over a fairly wide range of distances. The package's accuracy limitations are due primarily to the GPS and IMU (Inertial Measurement Unit). These days, with a proper airborne GPS installation and carefully monitored GPS ground stations for solving translocation solutions, GPS results can be accurate to within five to seven centimeters. The IMUs will vary depending on the manufacturer, but they typically have an error of about 0.01 of a degree. One point of a degree is equal to 0.6 minutes. At 3000 feet above ground level, this translates to the same five to seven centimeters. The higher one flies, however, the larger the circle of error becomes.
When you add together all of these possible errors at a flying height of 3000 feet, the circle of possible error ranges from around six inches to one foot in horizontal accuracy, and up to six inches in vertical accuracy. And no amount of fiddling with the data will improve these figures very much!
LIDAR systems are typically used at fairly low-flying heights for detailed, accurate surveys. When these systems are flown higher, a couple of things happen. The beam of the LIDAR tends to distend further. In other words, the footprint on the ground gets bigger. And while the accuracy of the IMU stays the same - because accuracy is related to angular errors - the higher one flies, the greater is the effect of the angular error. For example, at 6000 feet, the possible IMU error is double the potential error of 3000 feet. However, this error is only in the horizontal coordinates. The elevation error computed from the laser range is not affected nearly as much.
However, there is one other consideration. As the LIDAR flies higher, and because of a larger footprint, there is more likelihood of some averaging of the return signal from ground undulations. Also, with this larger footprint, it is typically harder to penetrate vegetation with accuracy because, depending on the system, the receptor averages out some of the reflected signals it receives.
On the other hand, many new systems can record multiple returns from the same pulse. If the beam hits some leaves at the top of the tree canopy, but part of the beam travels further and hits a couple of branches, and then another part of the beam hits the ground, and if the beam has enough strength to reflect back, then the receiver will record the reflections from each return. The interpreter then must determine which return is which.
Because most LIDAR systems currently send out pulses of between 5000 and 25,000 times per second, the ground distance between reflected pulses is quite small. Usually this falls into the 2.5- to five-foot range. Even though some shots may not hit the ground (because of too much vegetation), enough of the shots will penetrate to provide a reliable digital terrain model. However, in heavily forested areas it should be remembered that, while there may be pulses hitting something every 2.5 feet, there are not necessarily ground elevations at that spacing. In heavily forested areas, many of the shots will be returned from the top of the tree canopy.
While there seem to be too many things to take into consideration when determining the technical requirements of a LIDAR survey, there are really only two things a client must specify: the accuracy desired in terms of elevation and position of spot elevations; and the point spacing that is required. If you require a point every five feet with accuracy to within six inches, or a point every 30 feet with accuracy to within two feet, or any other combination, these become the technical requirements - remembering, of course, that not every shot will hit the ground. It is only fair to state that these specifications are valid in areas clear of vegetation.
LIDAR data is usually acquired within an angular scan of a number of degrees. Toward the apogee of the scan, errors do tend to increase. This is simply a function of geometry. Within a framework of twenty degrees either side, however, there is a reasonable chance that the beam will penetrate to the ground. Directly beneath the aircraft, any hole in the canopy will definitely be penetrated. LIDAR technology is the only survey method that will do this. While aerial photography can also see through the canopy directly below the camera, in order to measure ground elevation, the photogrammetrist has to use two photographs from different positions. In vegetated areas, one photo may see the ground, but the next photo likely will not.
A different problem occurs with radar survey systems. These systems are practically all side-looking, which means that they shoot the radar beam at an angle off to one side, rather than vertically straight down. This technique is best used in flood plain mapping where there is very little vegetation, since it has problems penetrating densely vegetated areas and does not survey all the "shadow" areas behind objects. There are also problems with layover, a phenomenon caused by the wavelength of the radar and its interpretation.
LIDAR is, therefore, particularly well suited to flood mapping. The large number of accurately generated points provides a wealth of data for flood plain modeling. And, as mentioned previously, accuracy is paramount. In many flat areas, floodwaters that rise by an additional foot can inundate thousands of acres. Such knowledge can mean the difference between flood remediation efforts being effective or not.
Figure One shows a small portion of the Haha River in Quebec, surveyed by LIDAR and rendered as a three-dimensional image of the site. The relief is color-coded, with the darkest blue depicting the river and then upwards through shades of green to yellow to red, with red being the highest ground. Vertical exaggeration is used in this rendition to accentuate the differing levels of elevation. The contours generated from the DTM are listed below. This project covered an area 13 kilometers along the river by two kilometers wide.
This survey was a pilot project for the environment department of the government of Quebec. It was conducted in early spring when traces of snow were still on the ground. Despite the snow, Lasermap was able to achieve an accuracy to within a range of 15cm (6 inches), much to the surprise and delight of the client.
A much more extensive project was conducted for the IJC in 1999. The U.S./Canada International Joint Commission is responsible for water bodies and river basins that cross the border between the two countries. The Red River valley and the watersheds of its tributaries wander back and forth across the various borders between North Dakota, Minnesota, and Manitoba. The Pembina River, which flows into the Red River, crosses the border several times. This entire area was a victim of massive flooding during 1997.
The number of people affected by flooding in these low-lying areas is huge. For example, one estimate has close to 70 percent of the total population of Manitoba living in a flood risk zone! Because the area is so flat, determining the areas likely to flood at any given level is extremely difficult. Existing digital terrain models were generally accurate only to within a rate of plus-or-minus three to 10 feet at best and, in some areas, only within plus-or-minus 30 feet.
Lasermap was contracted to fly a 200-square-mile area south of Winnipeg. The flying height was 2000 feet above ground level, and line spacing was such that each line overlapped the next by 50 percent. In effect, this provided 100 percent overlap and significantly increased the number of data points. The data was collected using Lasermap's upgraded Optech 1020, a system ideally suited to the low-level flying that was required to meet the stringent specifications of this project. Data points were basically acquired at approximately two-foot intervals, which resulted in the creation of a huge data file. This file was thinned out to produce the required deliverables, for it otherwise would have been too large for the client to handle.
The ground crew monitored the various GPS ground stations and, as a quality control measure, also surveyed some GPS profiles. This aspect is important in LIDAR surveying. To ensure that the accuracy for flood mapping is met, one needs a contractor with experience in quality control for monitoring the various systems. The GPS data should have consistency, with a view of a minimum of six satellites. Inertial data should resemble the flight trajectories. The laser should have consistent returns in the 95 to 98 percent range, except over water. Ground stations are needed to compute the proper translocations for airborne data, and to ensure that one is on the same coordinate reference frame as the client. Many of these quality control checks must be done in the field on a daily basis. Therefore, if the satellite lock is lost or some other malfunction occurs, the contractor has the opportunity to re-fly the affected portion of the mission before demobilizing.
Figures Two through Six show examples of how the differences in water levels can affect flooded areas. These models were created with one-meter increments in water levels. The background image is a shaded relief produced from the LIDAR data, with the main watercourse being the Red River south of Winnipeg. These illustrations cover an area nearly 4.5 miles wide in an east-to-west direction.
Interestingly enough, one of the quirks that occurred on this project - fortunately discovered after only a couple of tiles were processed - was that the Optech software (which automatically removes vegetation) assumed that the berms surrounding many outlying farmhouses and buildings were dense hedges, and thus removed them from the view. Once the technicians noted what had happened, they more carefully monitored the process by using the video tracking record to quality-control the software results. The berms, of course, were put back in as an indication of the existing remediation infrastructure.
The result was a textbook LIDAR project, about which the client's independent consultant said, "The Manitoba Geomatics people have completed a number of fairly rigorous comparisons of the LIDAR data to other data sets in their possession, and found the LIDAR data is well within specified accuracy. In one case, the average difference between the LIDAR and other kinematic GPS surveys was 6cm. I think everyone agrees...that we have a textbook product."
These projects proved conclusively that LIDAR, properly used and flown, is an ideal tool for flood risk mapping. Not only that, it is the least expensive method of covering a large area with detailed data. This is the second article in an irregular EOM series about LIDAR, written by Robert Fowler, vice president of sales and marketing for Lasermap Image Plus. He can be reached via e-mail at [email protected] Back |
