GRTS sampling - 2-level design
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 /////////////////////////////////////////////////////////////////////////////////////////// To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of. ~ Sir Ronald Aylmer Fisher The plural of anecdote is not data. ~ Roger Brinner The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. ~ John Tukey /////////////////////////////////////////////////////////////////////////////////////////// <https://www.inbo.be> 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
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
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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 at nhh.no https://orcid.org/0000-0003-2392-6140 https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
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