I have finally gotten around to generating a contour map of Clary Lake using data collected by the Department of Environmental Protection during their September 2012 bathymetric survey. In addition to the high resolution image pictured at left there’s a smaller version available, and a 24″x36″ PDF which is quite large but which can be viewed or printed out at about any scale you wish. Feel free to download them, you’ll find them and the other files pertaining to the original bathymetric survey over on the Maps, Charts, and Graphs gallery. Each image has a link in the description field to the full-sized image.

The data set consists of 3,265 points with each point defining a 3-dimensional location on the face of the earth (i.e., the latitude, longitude, and water depth below the top of the dam). The depth was measured with a depth meter and each depth measurement was paired with the latitude and longitude of the point obtained via GPS receiver. The data in the form of a spread sheet was supplied to us back in April 2013 along with a colorized bathymetric map of the lake generated from the data. This map was a marked improvement over the original depth map of Clary Lake, prepared by the Department of Inland Fisheries & Wildlife back in late 1940s, but it lacks some real utility because it doesn’t show you what the bottom of the lake actually looks like- where the hills are, where the holes are, what the depth is at any given spot. According to the original depth map, the deepest spot in Clary Lake was only 23′ whereas we now know that the deepest spot is actually 29′ below the top of the dam.

I had planned on making a contour map of the lake using the bathymetric data ever since it became available, but I lacked the right software for the job. I was going to use the surveying/mapping software I use at work since making a depth map of a lake from XYZ data is really no different from making a contour map of land using XYZ data. I could have made it work but for various reasons that solution had limitations. And it wasn’t a high priority. Then last spring I became aware of a free and open source Geographic Information System called QGIS. It turned out this was the ideal software for the task, the only problem being that I had no idea how to use it. So it’s taken me a while to come up to speed. I must say, I’m impressed very with QGIS and I’m very impressed with the map!

The map requires a little explanation. OK, a lot of explanation:

1. It is first and foremost a work in progress, it is not without errors, and it is subject to change. Contour interpolation software isn’t perfect and the TIN (triangulated irregular network) generated from the State data isn’t perfect either. For one thing, while the State collected sufficient data for their purposes, there were still very large areas without any point coverage. You can look at the data point plot  and you’ll see what I mean.

    

2. The water depths are in feet below the top of the dam and are based upon the North American Datum of 1983 (NAD83). Based upon my GPS elevation survey the top of the dam has aNAD83 elevation of 150.64′. This figure is in complete agreement withthe the elevation of the top of the dam derived from the data the State collected during theirbathymetric survey. Latitude and longitudeare represented in the WGS 84coordinate reference system. This is likely the same system your handheld GPS is using.

    

3. For all the data the State collected, they actually managed to miss the deepest part of the lake. Fortunately, DavidHodsdon over the years has managed to figure out exactly where the deepest part of the lake is, and he and Jack Hollandhave been recording the Latitude and Longitude (inWGS 84coordinates) when they go out to conduct their water quality monitoring activities, as well as the depth of the water at thatlocation so I was able to fill in the hole.

    

4. When the State collected their data, they couldn’t drive their boat over the shallowest spots in the lake so the ledges and rocks aren’t necessarily fully represented on this map. Yet. My plan is to supplement the map withadditional elevation data on the known rocks and ledges, and to slowly fill in the gaps in their coverage.

    

5. The State also wasn’t able to collect data right up to the shoreline since the lake level at the time of the survey was over 3′ below the top of the dam, so I went looking for a way to fill in the gap between their collected data and the actual high water mark. It turned out to be rather easy, and sufficiently accurate for the purpose: the Maine Office of GIS offers 2′ contours covering most towns in the State (as well as a whole raft of other information). I downloaded these contours and added them to the Clary Lake base map. Around the perimeter of Clary Lake they drew a 150′ contour (the heavy blue line) that for mapping purposes satisfactorily approximates the High Water Mark of Clary Lake, which also happens to be within 1/2 foot of the actual elevation of the top of the dam. What’s more, they also digitized 148′ and 146′ contours corresponding to water depths of 2′ and 4′ respectively. How convenient!

    

6. I also added tax map information to the map also obtained from the Maine Office of GIS to help people figure out where their property is. 

    

7. You’ll notice 2 small red triangles, one on the south shore near my property and one over at the end of Hodsdon Lane in David Hodsdon’s yard; these are 2 of the GPS control points from the GPS elevation survey I performed back in December 2011. More control points will be added later this fall.

This depth map will become part of the Clary Lake Water Level Survey we’ll be conducting later this fall.