Thanks Rolf,
I see your point, and I admit that I've waffled a bit on this issue.  I'm
hesitant to drive down the kriging path (although indicators might be
suitable), because there are many known (and unknown) covariates that affect
the contaminant burden, and few of these are quantified at the scale or
resolution we would require to make such an approach useful (at least based
on my experience).
Regarding the data; the sites were not "chosen" as one would choose
locations to sample, rather they are a mix of quasi - synoptic and
randomized sampling designs from ~ 50 or so different
investigators/agencies. We've simply mashed them into one large database for
our purposes. The habitat of the organisms is fixed in space (lakes), but
given that there is no real selection protocol for sites, I wondered if this
fit into the "grey" area.


On Thu, Feb 10, 2011 at 3:42 PM, Rolf Turner <r.turner@auckland.ac.nz>wrote:

>
> On 11/02/2011, at 6:00 AM, david depew wrote:
>
>  Dear list,
>> A brief and (hopefully) simplistic question regarding point pattern
>> analysis.
>>
>> We have compiled a large, continental database of chemical burdens in a
>> model organism. Currently, the data span 40 years and covers the entire
>> country of Canada (including the high arctic). We have categorized the
>> numeric data into categorical data (i.e. categorical marks) based on risk
>> thresholds. We'd like to assess whether or not there are interesting
>> spatial
>> patterns ( i.e. clusters of levels of risk (high vs low)), much like a
>> case/control approach. My question is as follows;
>>
>> Is there a "best" geographic projection for this approach? Currently all
>> data are in Latitude/Longitude. My inclination is to use something along
>> the
>> lines of an equal area projection to maintain a reasonable representation
>> of
>> spatial dispersion.
>>
>
> This doesn't sound to me like ***point pattern*** data.  It sounds like
> you have taken measurements of ``chemical burdens'' in a particular
> organism, at a number of ***chosen*** sites over a number (40) of years.
>
> Thus the observation points are deterministic, not random, and so point
> pattern analysis doesn't come into it.
>
> It may be the case that some sort of combination of kriging and time series
> or repeated measures analysis might be called for.  But here I speak of
> that of which I know nothing.
>
>        cheers,
>
>                Rolf Turner
>
>


-- 
David Depew
Postdoctoral Fellow
School of Environmental Studies
Queen's University
Kingston, Ontario
K7L 3N6

david.depew@queensu.ca
P: (613) 533-6000 x77831

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