Specific Design Information - Stream 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:
- Established the monitoring objectives
- Identified Resource Characteristics
- Identified Institutional constraints requiring consideration
- Defined the target population
- Specified Subpopulations, Domains and Stratification
- Created a sampling frame for all units of the target population from which to select the sample
- Selected a GRTS survey design for the selection of sample of units from this frame
- Selected a strategy for allocation of sampling efforts in space and time
- Established sample size(s)
Additional information on each of these items is available on Design Specifics
Luckiamute Watershed Stream Example
Purpose: Example linear GRTS survey design for Luckiamute Watershed streams
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 these examples:
- R version 2.2.1
- psurvey.design package (Version 2.2.1)
- R script or text file: luckiamutedesign.R
- Frame materials as ArcGis files:
- luck-ash.dbf
- luck-ash.prj
- luck-ash.sbn
- luck-ash.sbx
- luck-ash.shp
- luck-ash.shx
- Download Example Files
Examples includes:
Designs | Description | Files produced |
1 | Equal Sites– 50 sites, equal wts | Equal Sites.dbf Equal Sites.prj Equal Sites.shp Equal Sites.shx |
2 | Stratified Sites – Perennial 50 sites, Oversample 50 sites, Intermittent 50 sites, Oversample 50 sites. | Stratified Sites.dbf Stratified Sites.prj Stratified Sites.shp Stratified Sites.shx |
3 | Unequal Sites– Perennial w/ 3 Strahler categories (25,25,25 sites) plus Oversample (36) – Intermittent w/ 3 Strahler categories (17,5,3 sites) | Unequal Sites.dbf Unequal Sites.prj Unequal Sites.shp Unequal Sites.shx |
4 | Panel Over Time Sites – Perennial w/ 3 Strahler categories (17,17,16), Oversample (50) – Intermittent w/ 3 Strahler (17,5,3) | Panel Sites.dbf Panel Sites.prj Panel Sites.shp Panel Sites.shx |
An ArcGis map is included, Luckiamute Design.pmf, for the resulting designs for this example (requires ArcReader software to view, explore and print).
Design File Documentation would routinely begin here:
Contact:
Nameaddress of person requesting the design
Voice:
Fax:
email:
Description of Sample Design
Survey Design: A Generalized Random Tessellation Stratified (GRTS) survey design for a linear stream resource is used for this example. The GRTS design includes reverse hierarchical ordering of the selected sites.
Target population: All perennial and intermittent streams/rivers in the Luckiamute Watershed Council basin. Watershed boundaries defined by Luckiamute Watershed Council.
Sample Frame: GIS stream network coverage from PNW portion of RF3. Watershed Map
Sample Frame Summary
The total stream length (km) is 822.4882 km (including intermittent).
Strahler Order | Perennial (km) | Intermittent (km) |
0 | 10.64198 | 10.825986 |
1 | 189.88902 | 294.699772 |
2 | 133.10086 | 20.512487 |
3 | 60.84859 | 3.027805 |
4 | 68.24550 | 0 |
5 | 30.69616 | 0 |
Multi-density categories: Strahler order categories: 1 st, 2 nd, and 3 rd+.
mdcaty | Perennial (km) | Intermittent (km) |
1 (0+1) | 200.52218 | 305.52758 |
2 | 133.10086 | 20.512487 |
3+ (3+4+5) | 159.79025 | 3.027805 |
I. Equal Probability GRTS Survey Design
Stratification: None
Expected sample size: 50 sites
Oversample: None
Site Use: The base design has 50 sites. 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.
Site Selection Summary
Equal Probability | Strahler Order 1 | Strahler Order 2 | Strahler Order 3+ |
Perennial | 11 | 7 | 14 |
Intermittent | 16 | 2 | 0 |
Sum | 27 | 9 | 14 |
Map of Equal Probability Sites
II. Stratified GRTS Survey Design with Over Sample
Stratification: Stratify by Perennial and Intermittent streams .
Expected sample size: 50 sites for Perennial stratum and 50 sites for Intermittent stratum, plus in each stratum, sites selected with equal probability within stratum.
Oversample: 50 sites for Perennial stratum and 50 sites for Intermittent stratum.
Site Use: The stratified design has 50 sites, plus an over sample of 50 sites, in each stratum. 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.
Site Selection Summary
Stratified |
Strahler 0 | Strahler 1 | Strahler 2 | Strahler 3+ | Total |
Perennial | 1 | 17 | 17 | 15 | 50 |
Intermittent | 1 | 43 | 6 | 0 | 50 |
Over Sample |
|
||||
Perennial | 1 | 20 | 10 | 19 | 50 |
Intermittent | 3 | 42 | 5 | 0 | 50 |
Map of Stratified Sites
III. Unequal Probability GRTS Survey Design with Over Sample and Stratification
Stratification: Stratify by Perennial and Intermittent streams .
Expected sample size: 75 sites with approximately an equal number within each Strahler order category for Perennial stratum. For Intermittent stratum, 25 sites with an expected sample size of 17, 5, and 3 for Strahler order categories 1 st, 2 nd, and 3 rd+, respectively.
Oversample: 36 sites for Perennial stratum and none for Intermittent stratum.
Site Use: The unequal probability design has 75 sites, plus an over sample of 36 sites, for the Perennial stratum and 25 sites for the Intermittent stratum. 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.
Site Selection Summary
Unequal |
Strahler 0+1 |
Strahler 2 |
Strahler 3+ |
Total |
Perennial |
18 |
32 |
25 |
75 |
Intermittent |
16 |
5 |
4 |
25 |
Over Sample |
|
|
|
|
Perennial |
12 |
11 |
13 |
36 |
Map of Unequal Sites
IV. Panels for Surveys Over Time with Unequal Probability GRTS Design with Over Sample and Stratification.
Stratification: Stratify by Perennial and Intermittent streams .
Panels: For Perennial stratum, three panels. Year1 sites will be visited in year 1 and then year 3, year 5, etc. Year2 sites will be visited in year 2, year 4, year 6, etc. YearAll sites will be visited every year. Intermittent stratum has a single panel of sites that will be visited only once.
Expected sample size: 50 sites with approximately an equal number within each Strahler order category for Perennial stratum. For Intermittent stratum, 25 sites with an expected sample size of 17, 5, and 3 for Strahler order categories 1 st, 2 nd, and 3 rd+, respectively.
Oversample: 50 sites for Perennial stratum and none for Intermittent stratum.
Site Use: The base design has 50 sites in Perennial stratum. 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 in the watershed, then the first 30 sites in SiteID order would be used.
Site Selection Summary
Perennial Sites
Panel |
Strahler 0+1 |
Strahler 2 |
Strahler 3+ |
Total |
Year1 |
7 |
4 |
6 |
17 |
Year2 |
7 |
4 |
6 |
17 |
YearAll |
6 |
4 |
6 |
16 |
OverSamp |
15 |
17 |
18 |
50 |
Sum |
35 |
29 |
36 |
100 |
Intermittent Sites
Panel |
Strahler 0+1 |
Strahler 2 |
Strahler 3+ |
Total |
YearOnce |
17 |
5 |
3 |
25 |
Map of Panel Sites
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 |
Description |
SiteID |
Unique site identification (character) |
arcid |
Internal identification number |
x |
Albers x-coordinate |
y |
Albers y-coordinate |
mdcaty |
Multi-density categories used for unequal probability selection |
weight |
Weight (in meters), inverse of inclusion probability, to be used in statistical analyses |
stratum |
Strata used in the survey design |
panel |
Identifies base sample by panel name and Oversample by OverSamp |
auxiliary variables |
Remaining columns are from the sample frame provided |
Luckiamute_Streams is an ArcMap that displays the stream network and sampling sites for the 4 example designs.
Projection Information
PROJCS["Clarke_1866_Albers"
GEOGCS["GCS_Clarke_1866",
DATUM["D_Clarke_1866",
SPHEROID["Clarke_1866",6378206.4,294.9786982]],
PRIMEM[" Greenwich",0.0],
UNIT["Degree",0.0174532925199433]],
PROJECTION["Albers"],
PARAMETER["False_Easting",0.0],
PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-96.0],
PARAMETER["Standard_Parallel_1",29.5],
PARAMETER["Standard_Parallel_2",45.5],
PARAMETER["Latitude_Of_Origin",23.0],
UNIT["Meter",1.0]]
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 |
Name |
Meaning |
TS |
Target Sampled |
site is a member of the target population and was sampled |
LD |
Landowner Denial |
landowner denied access to the site |
PB |
Physical Barrier |
physical barrier prevented access to the site |
NT |
Non-Target |
site is not a member of the target population |
NN |
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 environments R and S-PLUS (psurvey.analysis)
For further information, contact
Anthony (Tony) R. Olsen
USEPA NHEERL
Western Ecology Division
200 S.W. 35th Street
Corvallis , OR 97333
Voice: (541) 754-4790
Fax: (541) 754-4716
email: Olsen.Tony@epa.gov
Bibliography:
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.