What’s the deal with LiDAR? May 24th, 2019

Posted in: General

In the watershed management world LiDAR has become a hot topic. The provincial government is in the process of collecting new (or in many cases first time) LiDAR data for many locations in municipal Manitoba. These data are made freely available online and have the potential to completely change how management decisions are made on the prairies. Organizations including Manitoba’s Conservation Districts, the Red River Basin Commission, local Municipalities and others have long awaited the arrival of these data so that they can better answer one simple question: where does the water go?

DEM – Digital Elevation Model: Elevations derived from LIDAR data can be combined to create a model of the landscape we can use to identify potential locations to temporarily retain water to reduce downstream flows. This change from on-the-ground surveying to using LiDAR can now advance a project in days, where we used to wait months and years for access to survey a single potential retention area.
 

 

What is LiDAR?

LiDAR (Light Detection and Ranging) is a remote sensing method that uses light pulses from a plane-mounted laser to determine the distance from the plane to the surface of the Earth and to every object sitting on the Earth. These data are in turn used to determine ground level elevations and can be used to create a detailed elevation map with accuracies of within a few centimetres. As an end product, LiDAR allows us to create a model of the Earth’s surface using coloring to represent a 3D surface in two dimensions.

LiDAR being Flown: An airplane flies over the surface of the land using a laser scanner to determine relative distances to the ground. Connection to GPS provides the exact location of the plane/scanner. http://www.qpeak.com/scientific-enterprises

What’s it good for?

At the SRRCD we rely heavily on LiDAR when we’re planning projects; especially those associated with the movement of water across the land. Staff can manipulate the LiDAR data to create contour maps, determine elevations and estimate flood areas. We also use LiDAR to determine water catchment areas, and model “what-if” scenarios. For example, staff can overlay theoretical dams across water channels and use the data provided by the LiDAR to determine how many acres of land would be flooded by the dam and to what depth. When paired with information from local flow stations (visit https://wateroffice.ec.gc.ca for more information) staff can also determine the effects of a 1 in 100 year rain fall event or the dam dimensions required to ensure downstream property is protected during high water events.

Depth: Looking closer the Lidar can reveal the amount and depth of water that can be detained at a location. This is an important step in deciding if a project will be cost effective.

 

Getting the most out of LiDAR data

LiDAR data must always be interpreted with caution and MUST be properly conditioned before use. In order to ensure quality data, SRRCD resource technicians and municipal staff across the district have been working tirelessly for the past 3 summers to collect data on the exact location and elevation of all culverts and bridges. When LiDAR data are initially collected, the difference between solid earth barriers and roadways transected by culverts or bridges cannot be discerned. Technicians must manually enter in “burn lines” for all culverts, bridges etc in order to prevent structures such as roadways from being seen by computer models as impermeable barriers to the flow of water. Without this “conditioning” of the data, when water flow models are used, every roadway would act as a dam and the results would not be representative of the actual movement of water across the landscape.

Watershed: By following the flow paths back upstream we can determine the area contributing to the flow reaching a potential storage project. One square kilometre of land can contribute up to 25,000 cubic metres (10 swimming pools) of water from just 2.5cm of rain.

 

Limitations of LiDAR

The collection of LiDAR data relies heavily on laser beams bouncing back from to Earth to the airplane. This allows for many interesting applications of the data including using them to identify and classify land cover types such as forest, prairie and ice field. However, limitations do exist especially around areas of standing water, extremely thick vegetation or built up organic matter. In many cases the laser will simply bounce off the surface of the water or the top of the dense cattail overgrowth thereby returning a false elevation value (i.e., the height of the cattails instead of the bottom of the wetland or the elevation of the surface of the lake rather than the bottom of the lake). These limitations must be recognized and accounted for by all technicians who use the data. For every project, SRRCD staff go out and manually survey areas in and around wetlands and other potentially tricky locations to ensure that LiDAR data correctly represents the real world landscape or are corrected accordingly. Even with the time it takes to ground truth the data, LiDAR saves huge amounts of time. Projects questions that use to take up to 3 years to figure out on the ground can now be answered in a matter of days using LiDAR. Project conception to completion with LiDAR has been overwhelmingly progressive. Projects are modeled, designed, licensed, tendered and constructed within months while exploring new potential projects at the same time for next year. This work is also now all done in-house. The LiDAR has helped us achieve:
37 sites analyzed initially
12 sites modeled for implementation
6 projects completed this year with Provincial LiDAR
Without LiDAR, we complete one retention project per year with no other projects analyzed or modeled.

 

Flow: The landscape model from the Lidar also reveals the pathways water follows as it moves across the landscape. This helps us target projects to benefit areas with the greatest problems. The flowpaths allow our team to begin looking for solutions almost instantly.

​In today’s world, landscape management is a hot topic on most decision makers’ agendas. The way water moves across the landscape and the obstructions we put in its path can have large consequences (flooding, drought, drainage) for both local and downstream residents. The information acquired through LiDAR is a modern tool that allows decisions to be made in an objective manner and without fear of the unknown. When elevations across an entire landscape can be discerned and water flow paths mapped, decisions regarding every aspect of responsible water management become much easier. The Seine-Rat River Conservation District continues to use LiDAR based data in our efforts to manage water at a watershed level and with the best interest of all in mind. Sound decisions are rooted in sound information; to which LiDAR contributes greatly.