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Acquiring Better Topographic Information

Dec 21, 2009

image of newslettersIn a previous post we described the flood tolerances of different tree species. The variance between flood-tolerant and intolerant species can occur within 1 foot of elevation change. In another post ("Ducks, Groceries and Water Depth") we discussed the optimal depth of foraging waterfowl. Once again the ideal range is less than 2 feet deep. In another post ("Pies, Ducks and Acorns") we discussed how habitats historically flooded seasonally across these subtle elevation changes. So it makes sense that having a good understanding of the topography will help us manage Duck Creek’s wetland resources.

image of surveyorBut how do you get elevation data? USGS has 10-foot contours for the whole United States, but that doesn’t do us any good when we are talking about less than 3-foot elevation change across 500 acres. Well, there are a couple different ways, and most of them aren’t quick and easy. You can do a traditional survey, but this a slow process. The timber doesn’t make it any easier because your line of sight is limited by all of the trees. You can do some stuff with aerial photography (photogrammetry is the technical term), but once again the timber causes problems with estimating the elevational change of your ground (bare terrain, some more jargon). In the open ground you can use traditional survey methods and laser levels, but that still leaves you with data gaps in the timber.

A few years ago we tried a more basic technique. We called it "red-neck lidar." We flooded Pool 3 to full pool. Next we assembled 20 willing persons and equipped them with waders, GPS units and measuring sticks. We tromped across 500 acres of flooded timber over two days. Every 20 meters or so we stopped, measured the water depth and marked the location with a GPS waypoint. We covered Pool 3 up with thousands of waypoints linked to water depths. This dataset (red-neck as it might seem) worked reasonably well. We used a mapping software program to create a topographic surface from all of our water depths and GPS points. It turns out that this was better than the 1-foot contours that had been collected years ago with traditional survey methods. However, that was a week’s worth of work and included 20 different people to map just 500 acres. Mingo and Duck Creek encompass almost 30,000 acres, so this methodology has its limitations as well.

image of pool being surveyedLidar, which we referred to with our primitive method, is a recent technology often used along the coast for floodplain mapping. It uses a laser scanner as a range finder to estimate the height or elevation of the ground. This sensor is on the belly of a plane and can map the topography of large areas in a short amount of time. If this is done during the winter (when the trees aren’t leafed-out) it can even map the topography of forested areas. Last year we partnered with USGS, the U.S. Army Corps of Engineers and USFWS to map the topography of the Mingo Basin using lidar. By having this data we would be able to take some steps in better understanding where water wants to go, how deep it is and how it might affect the trees and critters that use Duck Creek and Mingo. It is a fundamental piece of data that we will be using in the upcoming renovations.

image of LIDARWe recently received the dataset and… well, to put it bluntly…I think it is pretty awesome…however, I am a biologist and am in this field because I think data is cool…especially topography data…but I digress.

If you’ve been in the draw room this duck season you may have seen some of the lidar data without even knowing it. There are maps up on the walls that show the topography of Unit A and pools 2, 3 and 8. Over the next few years, I’ll try to illustrate how this information is being used to guide our management actions and renovation plans. Hopefully, with the information we’ve provided so far you can start to see some of the applications.

The image below is a sample of the lidar data collected within the Mingo/Duck Creek Basin.

image of LIDAR for Duck Creek

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