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Guest peejay

Innacuracy in eTrek?

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Guest peejay

I logged my home location (reported accuracy was 20 feet) then checked the data on a map, 1:50 000. The plot was out by about 1/2 mile. Is this about normal?

Thanks

Pat

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Guest Ultralight

have you check to see if the Map datum your etrex is using is the same as the map you used to check your position? most common i believe is WSG84

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Guest jeremy

On the eTrex there's a function to convert to NAD27. Most older maps are in NAD27 and will cause an error like you're describing. If it's a good map it will say somewhere what datum it is using.

 

Jeremy

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Guest peejay

Hmm, the map I am using is issued by the New Zealand Lands and Survey Dept. It has the following info...

Geodetic Datum 1949 International Spheroid

New Zealand Map Grid Projection

Date of compilation June 1986

 

I am using WGS 84 map datum, is this corect?

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Guest jeremy

Probably not. The WGS84 datum was created in 1984, While NAD27 was created in 1927. So it is possible that the Geodetic Datum 1949 is closer to NAD27. Never heard of that one though.

 

Jeremy

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Guest fryed

I just did a quick conversion from Geodetic Datum 1949 New Zealand to WGS 84:

 

Geo1949 - 40:00:00.0S 175:00:00.0E

WGS84 - 39:59:53.7S 175:00:00.6E

 

I guess this doesn't really help much except to show that your datum and WGS84 are definitely different.

 

I don't think you can really compare it to the NAD27 datum either becuase that datum is not valid over the same coordinates as teh Geodetic Datum 1949 NZ.

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Guest OregonSurveyor

Hey Peejay,

Some rough calculations show that the difference in the two datums would have created an error of about 640 feet. I would suspect that the shift in datum had you plotting onto your map too far north by 638 feet and too far west by 47 feet. Hopefully you can find a datum in the etrex that closely matches your NZ datum. Some GPS units allow a "user defined" datum, with some research this may be a possiblity for you also.

 

------------------

Lat. N 45d-30.6875'

Long. W 122d-37.2055'

Elev.=172.97'

king6kids@hotmail.com

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Guest peejay

Thanks for the help guys, Ill keep researching.

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