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

[Edzer Pebesma]

On 09/29/2010 04:27 AM, Michael Sumner wrote:
> 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 would not hesitate to enforce them. It is regrettable that R has,
afaik, no semantics for units of measurements. The alternative would be
to issue a warning that coordinates are assumed to be in metric space
(or whatever you assume).

> 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?

Subsetting is another interesting issue. When you compute distance,
direction, speed etc from a trip of four points, what would you return,
or what would the returned object mean when one selects the second and
fourth point?

> 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

this requires you to check this and throw errors in any downstream
function/method that uses a trip object - not convenient.

>  - 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?

Have you looked at ljtraj objects in package adehabitat? It seems to be
under active development. Clément Calenge sent me this reference:

Calenge et al. 2009. The concept of animals' trajectories from a data
analysis perspective - Ecological Informatics, 4, 34-41.
-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de