Datum Conversions

[DEMAN]

A NGS Data Sheet shows "...horizontal coordinates were scaled from a topographical map and have an estimated accuracy of +/- 6 seconds." I use NAD27 datum and UTM grids. How can I determine how many meters are equivalent to 6 seconds?

[ArtMan]

How can I determine how many meters are equivalent to 6 seconds?

The earth's circumference is about 40,075 km. (The meter was originally defined as one ten-millionth of the length of a line through Paris from the equator to the North Pole, and the circumference is thus four times this distance, about 40 million meters, or 40,000 km.)

 

Divide 40,075 by 360 degrees = 111.32 km/degree

Divide 111.32 by 60 minutes = 1.86 km/minute

Divide 1.86 by 10 = 186 meters/6 seconds of latitude.

 

For Metric (or SI) challenged individuals, this is equivalent to about 610 feet.

 

The +/- 6 seconds of longitude is more tricky, since the linear distance decreases as you travel from the equator to the poles. At temperate latitudes, however, it will, in very round figures, correspond to half the latitude error.

 

The practical bottom line is this: if your GPSr says you are exactly at the scaled coordinates, the actual place is probably somewhere within 186 meters (610 feet) of where you are.

 

In my experience, the accuracy is usually much better than that, typically within 10-20 meters. (Once in a while, however - perhaps due to a typographical or transcription error - it's way off.

[+seventhings]

My experience is consistent with ArtMan's regarding the magnitude of error for marks with SCALED coords. My GPS' GOTO will generally gets me to within 50-100 feet of a mark with SCALED coords, although the error will occasionally be in the 200-300 feet range.

 

Most experienced hunters have learned NOT to rely on the GPS GOTO or coords to find a mark with SCALED coords. I use the GOTO or coords to tell me when I'm in the ballpark and its time to start looking for a good place to park. But the only reliable way to find a mark with SCALED coords is to use the datasheet description. Then (if I find the mark), I include my GPS' coords in the post to help the next hunter.

 

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[DEMAN]

My thanks to Artman & Seventhings for your excellent help. The only benchmark I have attempted using my GPS got me within +11 meters in the easterly direction and +27 meters in the northing direction so I guess I have experienced pretty much the same thing you have found in the past.