[tdwg-tag] time and space namespaces in Darwin Core
tuco at berkeley.edu
Tue Aug 24 23:55:09 CEST 2010
On Thu, Aug 12, 2010 at 11:05 AM, Bryan <pbryan.heidorn at gmail.com> wrote:
> If there is likely to be multiple coordinate systems can we have a column
> that defaults to wgs84 but may be reset to other systems?
I don't think it is likely, given phones and GPSs as sources. Printed maps
could be another story, but who uses those any more? ;-)
> Does someone have a table that could be given to data collectors for
> estimating uncertainty? The variables in this case are limited.
The the contributors to uncertainty from the phones and GPSs can be found
in various references, including the "MaNIS/ORNIS/HerpNET Georeferencing
Guidelines" (http://manisnet.org/manis/GeorefGuide.html), GBIF Best
Practices for Georeferencing (http://www2.gbif.org/BioGeomancerGuide.pdf),
and "The point-radius method of georeferencing locality descriptions and
determining associated uncertainty." (Wieczorek et al., 2004; IJGIS 18-8
1) device accuracy - this depends on the device and the conditions at the
time. Best to either measure these empirically or use conservative estimates
(1km for non-GPS phones; 30m for GPS and GPS-based phones in clear,
unobstructed conditions, worse under cover - perhaps 100m in forests, for
2) device precision - this depends on the latitude, the coordinate system
the device is using, and the number of digits of precision it reports. For
decimal degrees to the nearest 0.00001 degrees in Woods Hole, the
contribution would be under 2 meters. For nearest second, the contribution
would be 39 meters.
3) datum - if the datum is unknown for some reason (generally an avoidable
situation), the uncertainty contribution can be determined either with a
conservative, worst-case scenario (1.6 km maximum difference between WGS84
and any other published datum as of 2004), or with a calculated difference
between the most likely alternate datum at that location (NAD27, for the
uncertainty contribution at Woods Hole is 42 m (Georeferencing Calculator;
http://manisnet.org/manis/gci2.html, using Coordinate source="GPS",
Coordinate System="decimal degrees", Latitude="41.5219444",
Longitude="-70.6681833", Datum="datum not recorded", Coordinate
Precision="exact", GPS Accuracy="0", Distance units="m").
So, the best case scenario is a GPS with a known datum using degrees decimal
minutes to three digits of precision under excellent conditions, in which
case the real uncertainty is probably on the order of 5 meters. The most
likely scenario is a GPS with a known datum under normal conditions using
decimal degrees to five decimal places, in which case the real uncertainty
is probably about 15 meters. The worst case scenario is dismal.
For non-GPS phones you'd have to determine their accuracy in the field by
independent measures. I wouldn't recommend them for precision work.
sent from my Droid
> On Aug 12, 2010 2:04 AM, "Javier de la Torre" <jatorre at gmail.com> wrote:
> +1 for bioblitz.
> I will add that because Fusion tables use WGS84 for geometries it is
> important that we don't mix them. We will be relying on the spatial
> capabilities on fusion tables for data visualization.
> On 12/08/2010, at 03:52, joel sachs <jsachs at csee.umbc.edu> wrote:
> > On Mon, 9 Aug 2010, Javier de ...
> tdwg-tag mailing list
> tdwg-tag at lists.tdwg.org
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