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Site Selection for Survey Design

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tonyolsen edited this page Apr 27, 2020 · 2 revisions

Site Selection

Site selection in 2-dimensional space is based on the creation of a hierarchical grid within the unit square. The steps are:

  • Create unit square.
  • The sample frame, whether points, lines or polygons are transformed to fit in the lower left quadrant (0, 0.5) by (0, 0.5).
  • The sample frame is then randomly located within the unit square by randomly selecting a point in lower quadrant and translating the sample frame based on that random point.
  • The unit square must be subdivided into a hierarchical grid where the inclusion probability from the sample frame is less than 1.
  • Determine the hierarchical level: This can be achieved by determining the maximum inclusion probability in the sample frame given the survey design, includes knowing the sample size "n" and the unequal inclusion probability for each element in the sample frame.
    • Determine inclusion probabilities for elements in the sample frame
    • Ensure the maximum inclusion probability for all elements is less than 1. If not less than 1, then the elements must be sub-divided into small elements (applies to linear and area sample frames only as points can not be sub-divided).
    • Determine maximum size of a grid cell within unit cell. Process for doing this depends on if point, linear or polygon sample frame.
  • Knowing the grid cell size, determine the hierarchical level required to have cell size be less than the maximum grid cell size.
  • Assign hierarchical addresses to the cells
  • Hierarchically randomize the addresses
  • Calculate inclusion probability for each cell
  • Create line of length "n", the sample size comprised of segments in hierarchical order where length of each segment is the sum of the inclusion probabilities within each cell.
  • Select a random number between 0 and 1, say k. Select a systematic sample of the cells by selecting cells at k, k+1, k+2,..., k+(n-1).
  • For each selected cell, select a "site" from the elements of the sample frame within the cell. This process differs for points, lines and polygons. In each case, will know the sample frame element and the x,y coordinates for the point. This is the GRTS spatially-balanced sample of size "n" specified by the survey design.
  • Place the points in the GRTS sample in reverse hierarchical order.
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