[Taxacom] Where is the Texas Capitol When Seen from Tokyo? was : Re: GPS units that record the time a waypoint was taken, accuracy, datum
taxacom3 at achapman.org
Tue Mar 24 01:00:21 CDT 2009
Tom, you are right to some extent - at least if you remain in the Centre
of the US, or where the NAD1927 was centred. The further you move away
from that point (but staying in North America) the more you move away
from the two points coinciding until eventually are at the end of the
Aleutian Islands where you may be up to about 235 meters different
between the two systems. If you use NAD27 in Hawaii for example, the
difference is close to 500 meters.
The difference between NAD 1983 and WGS 84 is less of a problem and the
difference anywhere in the USA is less than 1 meter.
One issue that often arises is that many older maps that people use will
be using an older Datum. In Australia, I know that a lot of people
still use old military maps that were developed using AGD66 or earlier
leading to a difference of up to 200 meters (or maybe more).
If one extrapolates beyond the coast onto islands where one may not know
a better or more relavant Datum (which often happens - I have seen
people extrapolate AGD66 out to some very distance off-shore islands),
the difference/uncertainty can be much larger.
As many have stated - it really doesn't matter a lot what Datum you use
as long as you record and document what you have used - BUT - how many
users extract that information when they pull the latitude and longitude
out of a database - more often than not, they will extract species name,
latitude, longitude and ignore other fields such as datum,
uncertainty/accuracy etc. So, to save many of these problems, I
recommend that unless there is a good reason not to, to use WGS84, or
one that may be very close to it (GAD94 in Australia which differs only
by about 10cm), NAD 83 (<1 meter), etc.
Tom Schweich wrote:
> 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
> 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.
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