Evaluation of sample points involves screening them to see if they are part of the target population and are feasible to sample. Point evaluation may be conducted by the project manager or the crew lead. Office point evaluation should be completed before the start of the field season or immediately before the start of a scheduled field trip (i.e. hitch). Additional point evaluation occurs in the field before sampling a point.
Evaluating sample sites against rejection criteria
Before data collection occurs at a sample point, each sample point is evaluated to determine if it is part of the target population and if it can be sampled. Point rejection criteria are used during this process so that points can be evaluated in a systematic and unbiased way. Rejection criteria set limits beyond which failing to sample a point is considered acceptable and clearly define the reasons for sample point rejection. When consistently applied, the use of rejection criteria will preserve the ability to make statistical inferences from the data while also maximizing efficiency and promoting safety during field sampling. However, the rejection criteria that are used to implement a specific design must be carefully considered during analysis and reporting because they can limit the inferences that can be drawn from the data. For example in Terrestrial AIM, if all plots on slopes greater than 50% are rejected, then the monitoring data only tell you about resource status on slopes less than 50%.
By using the rejection criteria to determine whether or not a randomly selected point can be sampled, the unbiased nature of the monitoring design is preserved. Sample points should be reviewed against rejection criteria in the office using ancillary data sources (e.g., ownership maps, topographic maps, and aerial or satellite imagery) and the same GIS data used to produce the monitoring design. If a point is accepted in the office, the field sampling team should review the rejection criteria again once they have arrived at the point in the field. If a point is rejected, it is important to document the reason(s) for rejection as this information is incorporated into data analysis.
Terrestrial Rejection Criteria
A terrestrial sample point is rejected as non-target if:
- The sample point is >3 miles walking distance from the closest point accessible by a vehicle
- Sample point is not on BLM land
- The slope of the land where the sample point falls exceeds 50% (26.57 degrees)
- (Optional and determined by the monitoring objectives): The transect established at the sample point crosses a boundary between different management units (e.g., in an allotment-scale monitoring project, the sample point intersects two allotments)
- (Optional, only recommended if forested areas are already covered by a monitoring effort) Plot center (point), and at least a one acre area contiguous to the plot center, has canopy cover of woody vegetation of greater than 25%. Pinyon-juniper should not be excluded from sampling using this criteria.
Aquatic Rejection Criteria
An aquatic sample point is rejected as non-target if the reach can not be slid (according to the field protocol) to avoid the following circumstances:
- The stream is actually a man-made ditch or channel and no stream feature would have otherwise been present. Note: Streams that have been altered to be ditches are still included in the target population.
- The BLM does not own any portion of land within the selected reach.
- There is no evidence that a waterbody or stream channel was ever present at the site.
- ≥5 transects of the reach would not be safe to wade
- The sample point is a wetland, impoundment, beaver pond network, etc.
Click here for more information on the Aquatic Scouting and Monitoring Design Management Guidance. For an example of the Aquatic Design Management Excel file, please visit here.
Helpful Documents and Links
Terrestrial Guidance on Documenting Rejection and Sampling Status