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Why a GPS may not be accurate

Izaak Walton Report

April 23, 2013
Blaine Kloppenborg , The Daily Freeman Journal

Years ago, when I was young, carefree and had money in my pockets, I was attending an Outdoor Writers Convention in Montana. One afternoon, I went fly fishing with the late Joe Brooks who was then the fishing editor of Outdoor Life magazine, and Ray Bergman who was the eastern field for Outdoor Life. Three of us were trout fishing on Georgetown Lake near Anacona, Mont., which was crammed with big, healthy rainbows.

Georgetown had fine strands of grass and reeds in the shallower parts of the lake, and the big rainbows like to hang around them. We were double hauling casts and mending drifting lines and catching a ton of fish. Long story short, two days later, we went back to the lake and none of us could find our hot spot. And I remember Bergman - always the visionary - wryly saying: "I predict that someday somebody will invent an electronic device that will lead us right back to our hot spot."

Well, that was back in the 1960s and guess what? Today, we have that little device. It's called a GPS unit. That little GPS might not be as accurate as you might think - well, at least many of them aren't. Here's something you should know: The Iowa Department of Natural Resources now shows on many of its topographic maps of lakes in Iowa, the GPS coordinates of brush piles, rock piles, artificial humps and other fish-holding structures. The coordinates provide remarkable accuracy if anglers and boaters set their GPS units to work with Universal Transverse Mercator coordinates.

Now here's the rub: The scientific world uses several systems to accurately measure the earth's surface. GPS units are accurate only if there settings are in agreement with the GPS "language" provided by map coordinates. This includes most units now being used by hunters and fishermen. Most factories set the defaults on their GPS units to latitude and longitude because those settings are global in nature. However, because the latitude and longitude lines on a globe change (they are closest together at the poles and furtherest apart at the equator), they do not represent the most accurate information that can be presented for a particular location. The Iowa DNR began collecting and distributing GPS coordinates related to lakes in Iowa more than five years ago, and chose to use UTM coordinates because that system provided better accuracy than conventional latitude and longitude GPS settings. Moreover, consumer-grade GPS units aren't always accurate to within 01" latitude and longitude, you're often hard pressed to find that secret fishing spot once you thought you had it located. Compared to older and even present day GPS units that use longitude and latitude coordinates, UTM coordinates are more accurate because each unit of change is a 3-foot change on the ground. Fortunately, modern GPS units generally have the ability to use multiple types of coordinate settings. By setting the unit to use UTM settings as default each time it's turned on, anglers can have pinpoint accuracy using coordinates provided on topographic maps for Iowa lakes that are free at the Iowa DNR website.

Strange look-alikes

Had a nice phone chat this week with George Maze, a local Webster City resident. George called me to verify what he thought was a Eurasian Collard Dove. He was dead right. That's what it was. I'm a bit more than surprised that a lot more folks haven't discovered this bird. They're all over Webster City this spring. They're a dove, actually. They look exactly like a mourning dove, except for a few minor things.

To be truthful, the "mourning" doves you've been seeing lately, probably were not mourning doves. Look again. Look closer. Mourning doves are darker brown. The Eurasian dove is a light creamy-chocolate color. The mourning dove has a tiny black dot on its neck. The Eurasian has what appears to be a semi-ringed black collar. The mourning dove has dark spots on its wings. The Eurasian has no markings. When flying, the mourning dove has a long pointed tail. The collard dove has a squarish or rounded looking tail. They're an Old World native and got established in Florida in the 1980s and rapidly - very rapidly - started expanding it's range. Like house sparrows, they prefer to live in towns rather that in the wild. And like sparrows, they are overly abundant. I've seen them all over town. And yes, they'll hang around your bird feeders.

Water temperatures

As a rule, I generally don't get too excited about early spring fishing, especially right after ice-out. Just hitting the water with high hopes and a rod in your hand seldom leads to early-season fishing success. Spring lakes and rivers are often cold and discolored. This week, I had the good fortune to hob-knob with some DNR Fisheries personnel and visit several local fishing holes to check current water temperatures. All temperatures were taken at a six-inch depth, and 20 feet out from shore. Little Wall Lake at Jewell was 44.5 degrees off the north boat ramp. Briggs Woods Lake gave a reading of 44 degrees off the south boat dock. The Boone River north of Nokomis Park (upstream) was considerably colder, giving us a reading of 41 degrees. The Des Moines River at Fort Dodge and Dolliver State Park gave a reading of 40 degrees. I'm sure all the cold rains we've had no doubt bumped those readings back down a degree or two.

And now have a good weekend.

P.S. One quick reminder: Boat license renewals are due April 30. After the April 30 deadline, there is a $5 penalty.

Correction: A type-gremlin got into my column last week. The column read "Flatworms could put arch-rival eagles and ospreys at odds." Actually, that should have read "Platforms" could be placing them at odds.

 
 

 

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