[R-sig-Geo] auto-calcuations for Spatial objects

[Michael Sumner]

Hello,

I've been experimenting with additions to the trip class to
automatically calculate metrics such as distance, bearing and
turning angle. It's pretty simple since the only function that
needs to know about this is the constructor, and it updates
automatically for subsetting and so on. The problem is in
decisions regarding which methods to calculate. If the data is
longitude / latitude then ellipsoid measurements can be applied, and
if projected, apply Euclidean methods. But this is not so obvious for
bearings and angles, since those calculations do really need to
be spherical. If the projection metadata is not set it seems
wrong to assume one way or the other, but if it is set then we
can always transform to long/lat in order to be consistent. The
other issue is that these things could be unit-based, especially
important for speed (we naturally use metres/second in most map
projections and R's base time class, but kilometres and hours?
when working in long/lat).

I'm tempted to enforce the need for projection metadata - without it
there are just too many ambiguities.

I don't have any clearer thoughts than that, just wanted to put
it out there for discussion. It's mostly relevant to trip objects
that can be subsetted or updated in different ways as a sort of
hybrid between a "point" and "line segment" interpretation, but
could be useful for SpatialPolygons/Lines - especially when rgeos
brings more capability for manipulating these objects?

Another problem is the "specialness" of metrics, whether they are
protected from the general attribute data as coordinates are. This is
not so bad as long as it's clear to the user that any edits on these
attributes are likely to be lost!  I've long thought that I should similarly
protect the Time and ID attributes for trips, perhaps treat time as a
coordinate
and ID as a simple index identifier. I've tried this but haven't
committed anything
or investigated how disruptive it would be to the existing methods.

A minor question regards whether to take a "to next" or "from
previous" line segment model.

So, in summary:

 - detect projection metadata and calculate metrics if this is set
 - if CRS is NA, do no calculations
 - consider ellipsoid distance, spherical bearing/angle, speed in m/s
   and options for user control over defaults
 - add distPrev, aziPrev, trackAngle, speedPrev with appropriate
  missing start/end values

Thoughts?

Cheers, Mike




-- 
Michael Sumner
Institute for Marine and Antarctic Studies, University of Tasmania
Hobart, Australia
e-mail: mdsumner at gmail.com