NAD83 or WGS84

[+Eric K]

Forgive me if I named those wrong but anyone that can answer this should know what I'm referring too. A lot of the benchmarks list NAD83. Is there a difference between that setting and the WGS84 setting that is the default setting on my Garmin Etrex?

 

Thanks,

Eric

[+rdw]

There is a difference, but it is negligible. More often, benchmarks are off because the coordinates are interpolated from a map with an accuracy of +/- 6 seconds. I have personally seen markers off by more than 100ft. Use the coords to get you close, but use the description to actually find it.

 

rdw

[+patw]

quote:

---

Originally posted by rdw:

There is a difference, but it is negligible. More often, benchmarks are off because the coordinates are interpolated from a map with an accuracy of +/- 6 seconds. I have personally seen markers off by more than 100ft. Use the coords to get you close, but use the description to actually find it.

 

rdw

---

 

Most vertical bench marks are set only to +/- six degrees. That yields a circle of confusion with a 1200 foot diameter! The NGS data sheet will state just under the coordinates listing if the resolution is limited to +/- six degrees.

 

I always go to the NGS data sheets themselves before setting out on a search. One hint: read them from the bottom up. Oftentimes the last entry is 'not found' or 'not recovered' so you save a lot of reading in the winnowing process. You also see the most recent description so you do not have to plow previous ones.

 

good luck! . . . . patrick

 

patrick & shirley

[jayhawk999]

As a practica matter, there is no difference between these two coordinate systems at the present time in North America. The difference amounts to about 1.5 meters at this time but the specifics depend on the exact location on the North American tectonic plate. The resolution of most receivers is about 1.8 meters so, you won't see any difference in any datum used.

If you are pathologically curious, you can go to a NGS web site and calculate the exact differences.

See www.ngs.noaa.gov/cgi-bin/htdp/htdp_pl.prl

and enter the data as described.

 

Have fun!!

[+Alan2]

NADCOM states the accuracy of transformations between NAD 27 and NAD 83 (1986) are typically 12-18 cm based on their surveys throughout the country. For the latest in conversion accuracy see NADCON - North American Datum Conversion Utility

 

Alan

[jayhawk999]

The accuracy of a conversion is not the same thing as the offset or shift between lat/long values in NAD 27 vs. NAD 83. The differences can be as much as 100 meters or so. For example here in California, Fullerton Airport Primary Airport Control Station DX3328 has about 83 meters offset from NAD 27 to NAD 83. This value may be accurate to 10 or so cm which is a different number.

[+Alan2]

Jayhawk: Yes that's the point I was making although it might not have been clear. I was refering to accuracy of the transformation number not the actual offset. The interesting thing is that offset as indicated on the 7.5 quads are way off according to the new surveys NADCOM has done so the quad tick marks offset numbers are wrong from what I can tell. Try doing it where you live if you have a qyuad and see if you get the same thing. NADCOM will list the offset amount and it'll be different substantially by the paper map.

 

Alan