Dear Thierry,
Thank you so much for your reply. Yes, the loss of the spatial balance
once the two-tiered approach is not accounted for was what was worrying me.
The incorporation of region as a random effect has two issues - 1) the
overall sampling area is a lake, and "regions" don't make sense in that
context. 2) The analysis is a mark-recapture for fish (using the MARK
software); I've never seen the incorporation of random effects in the
Jolly-Seber / Cormark-Jolly-Seber models outside of Bayesian framework and
outside of individual random effects... but even if I could do that - the
regions just don't really make sense anyway (that I can see, anyway - maybe
I'm not thinking about it the right way?)
Thank you for the grtsdb suggestion. Do you have any examples of how this
works? I couldn't find any vignettes or worked examples...
Thank you so much,
John
On Fri, Oct 8, 2021 at 10:39 AM Thierry Onkelinx <thierry.onkelinx at inbo.be>
wrote:
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 at inbo.be
Havenlaan 88 bus 73, 1000 Brussel
www.inbo.be
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ensure that a reasonable answer can be extracted from a given body of data.
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Op do 7 okt. 2021 om 15:54 schreef John Wilson <jhwilson.nb at 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 at 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
5 km stretch, due to travel time constraints. With 10 sampling days,
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
we choose 10 spatially-balanced sampling points (one "core" point per
sampling day), and then for each of these "core points", we create a
of 5 sampling points that are constrained to all be within 5 km from
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 you have a code example? Are you using BalancedSampling, SDraw or
Spbsampling or packages (probably SDraw)? Have you run any simulations
try to get a first assessment on the impact of constraining your
Might approach a package author also help?
Roger
Would appreciate any thoughts...
John
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