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Author Topic:   Lowrance HDS Coordinate Systems
jimh posted 04-13-2013 10:15 AM ET (US)   Profile for jimh   Send Email to jimh  
The features and sophistication of the Lowrance HDS series of chart plotters continue to impress me. I recently noticed that the HDS devices can adapt to many coordinate systems other than Latitude and Longitude. For example, the HDS can shift to the Universal Transverse Mercator coordinate system (UTM). Many maps produced by the United States Geological Survey (USGS) are provided with coordinate grids in the Universal Transverse Mercator system. While UTM maps are not a primary interest for mariners, there may be some lakes or rivers which are mapped only on USGS maps using UTM. The Lowrance HDS series can shift its coordinate system to UTM and present its position with UTM coordinates. This may be helpful in locating positions found on the UTM map.

To change the HDS coordinate system, follow the menu options shown in

http://continuouswave.com/whaler/reference/HDS_Menu.html#CoordsystemMenu

As a test of the precision of the conversion of the coordinate system in the HDS to UTM from Latitude and Longitude, I compared the conversion of the HDS to the conversion from an academic website that provides a conversion calculator. The conversion calculator is found at

http://www.uwgb.edu/dutchs/UsefulData/ConvertUTMNoOZ.HTM

Rather than just test a random position, I downloaded a USGS map of my neighborhood and found a close-by position where two major grid lines crossed. This position in UTM was:

Zone 17 T
Easting 319000
Northing 471000

Using my HDS chart plotter with the coordinate system set for UTM, I moved the cursor to those coordinates and created a waypoint. Then I changed the coordinate system to Lat-Long, and checked the waypoint location. The waypoint in Lat-Long was

42° 31 ' 16.3'' North
083° 12' 12.8'' West

I then used these same arguments as inputs for the Java Script calculator mentioned and linked above to convert this same location. Entering the UTM, the calculator calculated

42° 31' 16.317'' North
083° 12' 12.782'' West

These values are in remarkable agreement with the HDS conversion values. The difference is mainly in rounding and amounts to an error of less than 0.02'' of Latitude. That is a distance of less than an inch.

To further explore this UTM geo-location data, I took my GNSS receiver to the driveway of the home nearest the UTM grid crosspoint. There I obtained a deduced position with an error of only a few feet. Of course, I really did not know the actual position of the UTM coordinates, but I estimated them from the USGS map. The exercise was sufficient to show me that there was extremely good correlation between the USGS charted location and the Latitude and Longitude obtained by my GNSS as predicted by two different conversion processes.

jimh posted 04-13-2013 10:28 AM ET (US)     Profile for jimh  Send Email to jimh     
I also located the nearby UTM coordinate intersection on GOOGLE EARTH. In that application you can also select the coordinate system (in the Preferences menu). The UTM coordinate intersection on GOOGLE EARTH appears about 25 feet from my location with the GNSS receiver. I actually can't quite reach the UTM coordinate location because it is inside a neighbor's home.

Using GOOGLE EARTH with the preferences set to UTM may be another option for conversion, but I find the calculator (see link above) to be handier if you already know the UTM coordinates.

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