[Taxacom] Inappropriate accuracy of locality data

Richard Pyle deepreef at bishopmuseum.org
Tue Nov 30 16:22:37 CST 2010

> We wish to cover
> uncertainty in more depth, especially the possibility of using
> uncertainty rather than just the point-radius method most are using now.
> This would use polygons and include things like buffering, roads, etc. 

I generated a system years ago for buffering lines and polygons. Basically,
a line or polygon is defined by a sequence of points, and if each point has
an error radius around it, that radius can be translated into a buffer zone
around the margins of the line or polygon.  By attaching the error distance
to each point (rather than abstracted to the polygon as a whole), there can
be different error/buffer zones on different parts of a polygon. For
example, if a terrestrial plant specimen is vaguely ascribed from some area
that abuts a water body, and you can say with confidence that the plant
would not have occurred over the water body, then the points along the
polygon that border the water body can have relatively small error; whereas
the borders of the same polygon with less distinct boundaries could have
larger error values associated with the relevant points.

I eventually mothballed this, for several reasons:

1) I could never decide whether the error around the border of a polygon
only applied externally to the defined polygon border, or simultaneously
internal and external (error values on points that describe a line would
obviously apply to both sides of the line)

2) I did not have any tools for easily generating or visualizing these
buffered lines & polygons

3) Nobody else seemed very interested.

The value of the point-radius approach is that it's the easiest and least
time-consuming way to translate the existing human-friendly locality
descriptors attached to most of our data content into something that can be
processed by a computer.  The only caveat is that the scale of the error
radius needs to be acknowledged when interpreting the values for analytical

For example, if I have a fish specimen with locality data "Oahu, Hawaiian
Islands", then a point-radius result would put a point at the geographic
center of Oahu (where the fish was surely not captured), and describe a
circle that included the entire island plus enough of the surrounding water
to encompass the likely collecting place for the specimen.  Such information
would be useless for modeling where on Oahu that species occurs.  But it's
reasonably useful for modeling where, within the Hawaiian Archipelago, the
species occurs -- and it would be very precise information on the scale of
modeling where within the Pacific Ocean the species occurs.

I don't fault the point-radius approach taken so far; but I agree it would
be valuable to also develop a standard for describing lines and polygons
that is practical to use within our community.


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