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Specific Design Information - Estuary Example

This section provides detailed information on creating GRTS Designs.  In addition, a presentation; Monitoring Design: Component Details, by A.R. Olsen is available as a pdf file (2.5 MB)

The survey design is a plan for selecting the sample appropriately so that it provides valid data for developing accurate estimates for the entire population or area of interest.  At this point in the design process it is assumed that the folloing have been completed:

  1. Established the monitoring objectives
  2. Identified Resource Characteristics
  3. Identified Institutional constraints requiring consideration
  4. Defined the target population
  5. Specified Subpopulations, Domains and Stratification
  6. Created a sampling frame for all units of the target population from which to select the sample
  7. Selected a GRTS survey design for the selection of sample of units from this frame
  8. Selected a strategy for allocation of sampling efforts in space and time
  9. Established sample size(s)

Additional information on each of these items is available on Design Specifics

Oregon Estuaries Survey Design Example

Purpose: Illustrate area sample for an estuarine resource.

This information is presented as an example and recommended format for documenting a GRTS design. It is recommended that this example be followed, prior to initiating efforts to produce a design to meet a particular objective. Due to the randomization scheme within psurvey.design, each execution of the R code will produce a different set of sampling points and output files and will not exactly reproduce the example files.

Materials needed for this example:

Example includes:

Designs Description Files produced
1 UnEqual Sites – three estuarine area size categories, two panels (50 sites), and over sample of 50 sites. OREstuaryUnequalSites.dbf

An ArcGis map is included, or_est_map.pmf, for the resulting designs for this example (requires ArcReader software to view, explore and print).

Design File Documentation would routinely begin here:


address of person requesting the design

Description of Sample Design

Target population: All Oregon coastal estuaries of less than 100 km2 in area.

Sample Frame: GIS coastal estuaries.

Survey Design: A Generalized Random Tessellation Stratified (GRTS) survey design for an area resource. The GRTS design includes reverse hierarchical ordering of the selected sites.

Multi-density categories: Small (<1 km2), medium (1-10 km2), large (10-100 km2)

Stratification: None

Panels: Two panels of equal size (25 each, total 50)

Expected sample size: Approximately 17 sites in small estuaries, 17 sites medium estuaries, and 16 sites in large estuaries.

Oversample: 50 sites, approximately equal numbers for each category.

Site Use: The base design has 50 sites in estuaries. Sites are listed in SiteID order and must be used in that order. All sites that occur prior to the last site used must have been evaluated for use and then either sampled or reason documented why that site was not used. As an example, if 30 sites are to be sampled, then the first 30 sites in SiteID order would be used.

Sample Frame Summary

The total estuarine area is 179.6 km2

Estuary Area Number Total Area (km2)
Small 34 2.056918
Medium 32 85.073934
Large 3 92.462461
Total 69 179.593314

Site Selection Summary

Unequal Sites Small Medium Large Sum
Panel 1 9 11 5 25
Panel 2 10 7 9 25
Sum 19 18 13 50
Over Sample 15 16 19 50

Maps of Estuarine Sites: Oregon Estuaries, OverSample Sites

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Description of Sample Design Output:

The sites are provided as a shapefile that can be read directly by ArcMap. The dbf file associated with the shapefile may be read by Excel.

The dbf file has the following variable definitions:

Variable Name



Unique site identification (character)


Internal identification number


Albers x-coordinate


Albers y-coordinate


Multi-density categories used for unequal probability selection


Weight (in meters), inverse of inclusion probability, to be used in statistical analyses


Strata used in the survey design


Identifies base sample by panel name and Oversample by OverSamp

auxiliary variables

Remaining columns are from the sample frame provided

Oregon_Estuaries is an ArcMap that displays the estuaries and sampling sites for the design.

Projection Information

PROJCS[" NAD­_1927_Albers "
GEOGCS[" GCS_North_American_1927 ",
DATUM[" D__North_American_1927 ",
PRIMEM[" Greenwich",0.0],

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Evaluation Process

The survey design weights that are given in the design file assume that the survey design is implemented as designed. That is, only the sites that are in the base sample (not in the over sample) are used, and all of the base sites are used. This may not occur due to (1) sites not being a member of the target population, (2) landowners deny access to a site, (3) a site is physically inaccessible (safety reasons), or (4) site not sampled for other reasons. Typically, users prefer to replace sites that can not be sampled with other sites to achieve the sample size planned. The site replacement process is described above. When sites are replaced, the survey design weights are no longer correct and must be adjusted. The weight adjustment requires knowing what happened to each site in the base design and the over sample sites. EvalStatus is initially set to "NotEval" to indicate that the site has yet to be evaluated for sampling. When a site is evaluated for sampling, then the EvalStatus for the site must be changed. Recommended codes are:

EvalStatus Code




Target Sampled

site is a member of the target population and was sampled


Landowner Denial

landowner denied access to the site


Physical Barrier

physical barrier prevented access to the site



site is not a member of the target population


Not Needed

site is a member of the over sample and was not evaluated for sampling

Other codes


Many times useful to have other codes. For example, rather than use NT, may use specific codes indicating why the site was non-target.

Statistical Analysis

Any statistical analysis of data must incorporate information about the monitoring survey design. In particular, when estimates of characteristics for the entire target population are computed, the statistical analysis must account for any stratification or unequal probability selection in the design. Procedures for doing this are available from the Aquatic Resource Monitoring web page given in the bibliography. A statistical analysis library of functions is available from the Aquatic Resource Monitoring web page to do common population estimates in the statistical software environment R (psurvey.analysis)

ARM Web Page: http://www.epa.gov/nheerl/arm

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For further information, contact

Anthony (Tony) R. Olsen
Western Ecology Division
200 S.W. 35th Street
Corvallis , OR 97333
Voice: (541) 754-4790
Fax: (541) 754-4716
email: Olsen.Tony@epa.gov


Diaz-Ramos, S., D. L. Stevens, Jr, and A. R. Olsen. 1996. EMAP Statistical Methods Manual. EPA/620/R-96/002, U.S. Environmental Protection Agency, Office of Research and Development, NHEERL-Western Ecology Division, Corvallis, Oregon.

Stevens, D.L., Jr. 1997. Variable density grid-based sampling designs for continuous spatial populations. Environmetrics, 8:167-95.

Stevens, D.L., Jr. and Olsen, A.R. 1999. Spatially restricted surveys over time for aquatic resources. Journal of Agricultural, Biological, and Environmental Statistics, 4:415-428

Stevens, D. L., Jr., and A. R. Olsen. 2003. Variance estimation for spatially balanced samples of environmental resources. Environmetrics 14:593-610.

Stevens, D. L., Jr., and A. R. Olsen. 2004. Spatially-balanced sampling of natural resources in the presence of frame imperfections. Journal of American Statistical Association:99:262-278.

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