[Taxacom] Where is the Texas Capitol When Seen from Tokyo? was : Re: GPS units that record the time a waypoint was taken, accuracy, datum

[Tom Schweich]

Hmmm ...  I dunno ... this is a clever diagram, but I think it 
perpetuates misunderstanding of a datum and what it's good for.

The apparent location of the Texas Capitol Dome Horizontal Benchmark 
when plotted in the Tokyo geographic coordinate system appears to be 
some distance from the  true location of the Star of Texas in the center 
of the diagram. However, the Tokyo datum (and the resulting geographical 
coordinate system) was designed in the early 20th century for use in 
Japan. It was never intended for use in North America.  Plotting a point 
in Texas using Tokyo geographic coordinates is, frankly, nonsense.  The 
representation that the position of the Texas capitol shifts when using 
Tokyo vs. WGS1984 is, therefore, also nonsense.  The position hasn't 
shifted. Instead we have misapplied the Tokyo geographic system.   
Similar statements can be made about some of the other local 
geographical coordinate systems included on this diagram. 

Generally, geographers face a problem of how to define geographic 
coordinate systems to locate positions on a rather oddly shaped planet 
while keeping the mathematics simple.  This is not an easy task, and 
many geographic systems have fallen by the wayside.  For example, the 
Tokyo datum made an example of in the diagram was replaced by JGD2000 in 
2002.  European Datum 1950 (also on the diagram) has been replaced by 
European Terrestrial Reference System 1989 (ETRS89).  However, neither 
JDG2000 nor ETRS89 are designed for use in Texas. Therefore, JDG2000 and 
ETRS89 should not be used in Texas, either.

Perhaps one important point we can salvage from this diagram is that 
when the Texas Capitol Dome Horizontal Benchmark is plotted using the 
geographical coordinate systems that are applicable to Texas: WGS1984, 
WGS1972, and NAD1927, all points plot inside the (Stand up! Place hand 
on heart!) Star of Texas. I assume that the position of the Texas 
capitol would also plot inside the star when using NAD1983.  As we would 
expect then, using any of the these four systems will generally give the 
same results inside the Texas capitol building, the state of Texas, and 
much of North America, because they were designed for use in those 
areas.  WGS1972 and WGS1984, being global geographic coordinate systems, 
are equally applicable in Texas, Japan, and Europe. 

-- 
Tom Schweich   tas27 at schweich.com
http://www.schweich.com



> Here are some references worth looking at for Position shifts for 
> different Geodetic Datums
>
> http://www.colorado.edu/geography/gcraft/notes/datum/gif/shift.gif
>
> Also see the table on page 24 of the Biogeomancer Guide
>
> http://www.gbif.org/prog/digit/data_quality/BioGeomancerGuide
>
>
>   


-- 
Tom Schweich   tas27 at schweich.com
http://www.schweich.com