[R-sig-Geo] GRTS sampling - 2-level design

[Thierry Onkelinx]

Dear John,

Your procedure will create a spatially balanced level 1 sample (10
"regions") and within those regions a spatially balanced level 2 sample.
When you ignore the structure, there is no longer a spatial balance. So
you'll need to incorporate the two level sampling structure in your
analysis. E.g. by using region as random effect.

I presume you are catching fish along rivers and assume that the rivers are
linear features. I'd consider drawing 10 samples using GRTS to define the
regions. Then use that location as the center point of 5 systematic samples
along the river (-2, -1, 0, +1 and +2 km).

You might want to take a look at our grtsdb package. Available at
https://inbo.r-universe.dev/ It generates a full grid of master samples and
stores it in the database. So you can draw multiple samples from the same
master sample. This is useful in case of monitoring with a changing
population. You draw a sample and keep the lowest ranking locations that
are part of the population. If the population changes over time, then the
new sample will keep a proportion of the original sampling location
relative to the proportion of the population that remained stable. This
allows for repeated measures for stable locations while taking into account
the changes in population.

Best regards,

Thierry

ir. Thierry Onkelinx
Statisticus / Statistician

Vlaamse Overheid / Government of Flanders
INSTITUUT VOOR NATUUR- EN BOSONDERZOEK / RESEARCH INSTITUTE FOR NATURE AND
FOREST
Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance
thierry.onkelinx using inbo.be
Havenlaan 88 bus 73, 1000 Brussel
www.inbo.be

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Op do 7 okt. 2021 om 15:54 schreef John Wilson <jhwilson.nb using gmail.com>:

> Oh, sorry - I normally use the grts() function from the spsurvey package.
> My hacky approach was to make 10 balanced points with grts(), followed by
> imposing a 5 km buffer around each one, and either systematic sampling
> within the buffer circle, or running a separate GRTS for the 5 points
> within each 5 km buffer circle. Even writing this makes me cringe though,
> so hoping for something legitimate... I'll contact the authors if I don't
> get any solid leads on here.
>
> On Thu, Oct 7, 2021 at 10:40 AM Roger Bivand <Roger.Bivand using nhh.no> wrote:
>
> > On Thu, 7 Oct 2021, John Wilson wrote:
> >
> > > Hi everyone,
> > >
> > > I'm working on a sampling design using GRTS, but I'm running into a
> > > logistics problem. The field crew can set 5 nets per day, but only
> > within a
> > > 5 km stretch, due to travel time constraints. With 10 sampling days,
> > that's
> > > a total of 50 sites. The overall sampling area is huge, so running a
> > > regular GRTS design for 50 sites results, of course, in much larger
> > > distances between sampling points.
> > >
> > > Is there a legitimate way to create a 2-level GRTS design, where in
> step
> > 1
> > > we choose 10 spatially-balanced sampling points (one "core" point per
> > > sampling day), and then for each of these "core points", we create a
> grid
> > > of 5 sampling points that are constrained to all be within 5 km from
> each
> > > other? I can make that happen code-wise, but am not sure what the
> > > implications on spatial balance are, or if there's a built-in way to do
> > > this.
> >
> > Do you have a code example? Are you using BalancedSampling, SDraw or
> > Spbsampling or packages (probably SDraw)? Have you run any simulations to
> > try to get a first assessment on the impact of constraining your sample?
> > Might approach a package author also help?
> >
> > Roger
> >
> >
> > >
> > > Would appreciate any thoughts...
> > > John
> > >
> > >       [[alternative HTML version deleted]]
> > >
> > > _______________________________________________
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> > > R-sig-Geo using r-project.org
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> > >
> >
> > --
> > Roger Bivand
> > Emeritus Professor
> > Department of Economics, Norwegian School of Economics,
> > Postboks 3490 Ytre Sandviken, 5045 Bergen, Norway.
> > e-mail: Roger.Bivand using nhh.no
> > https://orcid.org/0000-0003-2392-6140
> > https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
> >
>
>         [[alternative HTML version deleted]]
>
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