[time-nuts] Averaging Location for Position Hold

Michael Perrett mkperrett at gmail.com
Wed Sep 14 15:05:18 UTC 2011


When GPS was first developed (Late 70's) the DATUM used was the World
Geodetic System, 1972 (WGS-72).  The next release was indeed in 1984
(WGS-84). The early GPS receivers had over 200 datums stored in permanent
memory. In its most basic form a datum defines the center of the earth and
the equation of the earths ellipsoid.

>From Wikipedia:

A *geodetic datum* (plural *datums*, not *data*) is a reference from which
measurements are made. In surveying <http://en.wikipedia.org/wiki/Surveying>and
geodesy <http://en.wikipedia.org/wiki/Geodesy>, a *datum* is a set of
reference points on the Earth's surface against which position measurements
are made, and (often) an associated model of the shape of the earth (reference
ellipsoid <http://en.wikipedia.org/wiki/Reference_ellipsoid>) to
define a geographic
coordinate system<http://en.wikipedia.org/wiki/Geographic_coordinate_system>
.


The user, mostly military or marine, would choose the datum he/she wanted to
use. This would match up the local map with the GPS derived position. The
difference could be quite large (hundreds of feet), especially important in
the Z (vertical) direction!

Michael / K7HIL

On Tue, Sep 13, 2011 at 2:23 PM, jmfranke <jmfranke at cox.net> wrote:

> I believe that should read WGS 84 not WPS84.
>
> John  WA4WDL
>
> ------------------------------**--------------------
> From: "Chris Albertson" <albertson.chris at gmail.com>
> Sent: Tuesday, September 13, 2011 5:03 PM
> To: "Discussion of precise time and frequency measurement" <
> time-nuts at febo.com>
> Subject: Re: [time-nuts] Averaging Location for Position Hold
>
>  2011/9/13 Miguel Gonçalves <mail at miguelgoncalves.com>:
>>
>>> Hi all!
>>>
>>> I am installing a timing GPS unit over a new location where I already
>>> have a
>>> NMEA GPS with PPS (let's call it unit A). The NMEA GPS is logging every
>>> 16
>>> seconds its GPGGA string.
>>>
>>
>> The Oncore UT+ can does it's own site survey automatically.  That's
>> the best way.
>>
>> You can only compare the GPS location with Google if both are using
>> the same "system".  The most common one today is WPS84 but you need to
>> check.
>>
>> The problem is that the Earth is not a Sphere and different systems
>> assume non-sphere shapes.  Getting this wrongs gives about the size
>> error you observed, more or less.
>>
>> Also, can you really trust Google Earth as an authoritative source?
>> I'm not sure.    An interesting test would be to go find a USGS
>> benchmark or a section marker near you then enter it's location into
>> Google.  See if Google hits the marker.
>> --
>>
>> Chris Albertson
>> Redondo Beach, California
>>
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