[Proj] Re: New Member - Question

[Strebe at aol.com]

Robert:

You understand correctly.

Regards,

daan Strebe
Geocart author
http://www.mapthematics.com/


In a message dated 4/1/2005 11:29:40 AM 太平洋標準時, robert at dgi.com writes:

> Hello, I'm a new member to the list. My name is Robert McFaul and
> I work for a geologic modeling software company. 
> 
> Got a question for you all regarding datums and surveys and the
> earth-centered ellipsoids. Does a given point on the ground have 
> the same latlong value for all surveys using the same earth-centered 
> ellipsoid (GRS80, WGS84 etc.) regardless of the projection system used? 
> 
> In the past a surveyor would 'pin' the ellipsoid to the earth 
> at a position of their choice creating their datum. Another surveyor
> collecting the very same locations might use the same ellipsoid and 
> projection system but the ellipsoid is 'pinned' to another location 
> thus changing the latlong (and projection system) values of its survey 
> points. These changes might be very slight or could be quite different
> (hundreds of feet or meters difference). 
> 
> Am I correct in thinking that the earth-centered ellipsoids remove this
> variation in datums?
> 
> Hope this is clear. If I've confused you a little email me and I'll 
> finish the job:-)
> 
> Thanks.
> 
> robertm.
> robert at dgi.com
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