GPS datum setting

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robkanraft
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GPS datum setting

Post by robkanraft » Wed Jun 04, 2008 9:50 pm

I changed a setting on my gps and can't remember where I started. Of the many choices, which data setting (for punching in the cords) is the most common or default for use in our area?

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Steve S
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Re: GPS datum setting

Post by Steve S » Fri Jun 06, 2008 3:24 pm

I usually set my GPS to the data settings specified on the map that I carry in addition to the GPS. For map datum, I usually select WGS84. For coordinate system I mostly use UTM.
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robkanraft
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Re: GPS datum setting

Post by robkanraft » Fri Jun 06, 2008 5:23 pm

Thanks Steve..

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Cowper
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Re: GPS datum setting

Post by Cowper » Mon Jun 09, 2008 1:38 pm

Sorry I didn't see this post sooner; hopefully you'll check back again.

Two settings are of primary interest, the first and less important being the map datum (almost always WGS 84, or NAD27 CONUS). The WGS 84 is used by most electronic maps, is the default on Google Earth, etc, while the North American Datum of 1927, Continental US (thus NAD27 CONUS) is the one that most USGS Topos use. But the truth is, in Arkansas, even if you get this setting wrong and enter "WGS 84" coordinates into your unit when it is set to NAD 27 CONUS, you'll only miss by somewhere around 60 to 75 feet, so this one isn't really worth worrying about in practical terms, because it has only a very minor accuracy impact. Also, any waypoint you mark with your GPS is automatically adjusted for whichever datum you are displaying, so it is only the manually entered points that have any impact at all.

The second and more important issue is the coordinate setting, also called "grid" setting in the Garmin software. For Topos, like Steve said, UTM is very useful. Here are the settings coordinates for Woolum in the most common settings:
UTM 15 S 510192 3980647
hddd.ddd N35.97018 W92.88696 (degrees, decimal degrees)
hddd mm.mmm N35 58.211 W92 53.218 (degrees, minutes, decimal minutes)
hddd mm ss.s N35 58 12.7 W92 53 13.1 (degrees, minutes, seconds, decimal seconds)
The first one is pretty obvious, if you get a number BEFORE the letters N or W, you'll know you're on the wrong system. But look how minor the differences are in the next three - basically just the appearance of spaces in-between some of the numbers, and slight movement of the decimal point. Unlike the “map datum” issue, this grid system is very important even to casual users. Two examples show how important:

Example #1, Say someone sends you the coordinates above for Woolum in the hddd.ddddd format, as N35.97018 W92.88696, but your unit is set on hddd mm.mmm format. When you try to manually enter the numbers, the North 35. will enter normally, but you won’t even be able to enter the 9, because the units knows that minute only go up to 59.999, because after that, the “degrees” would just roll to 36 degrees. So you’ll keep punching keys, and the unit won’t accept what you’re trying to enter.

Example #2, say they send you the same coordinates, but this time in the hddd mm.mmm format, only your unit is set in hddd.ddddd format.
So they send you N35 58.211 W92 53.218, but you enter it as N35.58211 W92.53218. This time your data will appear to enter normally, BUT, your waypoint will be off by 33.5 miles, a totally unacceptable error.

In printed material, you’ll often see the degree symbol, the minute symbol (‘) or the seconds symbol (“), but often these are omitted and you see just a space instead. Once you know to look carefully at the spacings you can get it right, but if you are not sure, ask the person who sent the coordinates to clarify. Also, there is no universally accepted "standard" here - all of these systems are commonly seen in use. You just have to switch back and forth among them depending on which one was used by your information source.
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