# [tlocoh-info] Calculating temporally overlapping hulls with s=0, can it be done?

Andy Lyons lyons.andy at gmail.com
Mon May 31 02:05:13 CEST 2021

```Hi Michelle,

Good question. The short answer is that yes all hulls have a time stamp
associated with them (specifically the timestamp of the 'parent point'),
regardless of the value of s (which only affects which other points are
considered nearest neighbors).

Whether it makes sense or not to pair-up hulls from different
individuals based on the time stamp of the parent point, when hull
construction does not take time into consideration, is a good question.
When s>0, one can say with their head held high that the points that
compose the hull are in some sense local in both time in space. In that
case it's reasonable to compare two space-time hulls from different
individuals at the same time, and take their physical distance apart as
some kind of association metric at that moment in time.

When time is not included in hull construction, the meaning of a hull is
more akin to density of usage over the entire sampling interval. In that
case, the time stamp of the parent point doesn't signify any particular
characteristic at that moment in time except maybe to say at that
specific moment the individual was in a high density hull or a low
density hull. That might have some significance if you see high density
hulls occurring during a certain time of the day or part of the season.
Pairing space-only hulls from two individuals at the same time might
allow you to say whether or not the two individuals tended to be in high
density or low density hull at the same time, but it wouldn't tell you
much about their association at that particular time (I think).

That being said, you could incorporate another hull metric 'tspan' (time
span of the nearest neighbors) to measure the time span of the nearest
neighbors (regardless of s), and use tspan to compare the window of time
that two two time-matched hulls represent. If the hulls for individual A
and individual B have a similar distribution of tspan, you might be able
to argue that time-paired hulls are a measure of dynamic association at
the scale of tspan.

Cheers,
Andy

Subject:
Calculating temporally overlapping hulls with s=0, can it be done?
From:
Michelle Pretorius <PRTMIC015 at myuct.ac.za>
Date:
5/7/2021, 3:38 AM

To:
"tlocoh-info at lists.r-forge.r-project.org"
<tlocoh-info at lists.r-forge.r-project.org>

Hi all,

I would like to calculate hull metrics for pairs of temporally
overlapping hulls of two individuals (using `lhs.to.add` from the
`tlocoh.dev` package). However, I have used s=0 for both individuals
(after following the cross-validation methods set out by Dougherty et
al. (2017)
<https://movementecologyjournal.biomedcentral.com/articles/10.1186/s40462-017-0110-4>),
and thus the hulls were constructed from the closest points in space
only. Do hulls still have times associated with them even if they are
formed with s = 0?

Using the `lhs.to.add` and `lhs.to.stats` functions I do /technically/
get a result (i.e., there are no errors), but I am wondering if this is
theoretically sound?

Best,
Michelle

Disclaimer - University of Cape Town This email is subject to UCT
policies and email disclaimer published on our website at
http://www.uct.ac.za/main/email-disclaimer or obtainable from +27 21 650
9111. If this email is not related to the business of UCT, it is sent by
the sender in an individual capacity. Please report security incidents
or abuse via https://csirt.uct.ac.za/page/report-an-incident.php.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/tlocoh-info/attachments/20210530/94ea172b/attachment.html>
```