[Taxacom] How Garmin Units Store Geographic Coordinates, was: Re: GPS units that record the time a waypoint was taken, accuracy, datum

[Tom Schweich]

Karl Magnacca wrote:
> On Mon, March 23, 2009 2:35 pm, Curtis Clark wrote:
>   
>> If your unit records in datum A, etc. 
>>     
>
> Only if you're hand-writing the coordinates; otherwise, the unit
> displays and downloads all points in whatever datum it's currently set
> for (at least mine does, I assume that they all do).  
>   
At least in the case of Garmin mapping units, the unit itself always 
uses WGS1984 internally.  If you set it to display other geographic 
coordinate systems (Such as NAD1927, NAD1983, etc), or a projection 
system, such as UTM, then unit makes a translation for display purposes 
using a simplified algorithm.  While it may appear that the Garmin GPS 
units can translate coordinates from NAD1927 to, say, ETRS89, it 
actually does not; instead it is just using a different translation from 
the internally stored WGS1984. 

Just FWIW, I collect and store all my plant distribution data in 
WGS1984, and then only project it into another system when preparing the 
final map.  This simplifies combining data acquired at different times. 

-- 
Tom "This Must be true, I read it on the Internet" Schweich   tas27 at schweich.com
http://www.schweich.com