Data Mapping Final Exam

Definition of Accuracy

close to true value

Definition of Precision

Variability (repeatability)

Advantages of Digital Scouting

Continuous monitoring, maximizes time and efficiency

What is Sampling

The selection of a subset of individuals from within a whole population to estimate characteristics of the whole population.

• Good sampling represents the whole population effectively.

• Bad sampling makes a big bias in data.

Why do we sample?

1. Without data, we don’t know what is happening and we can’t fix problem.

2. It is impossible to analyze whole field inch by inch.

3. To get basic information

4. To understand problems

5. To understand field variations

6. To make the best management decisions

What do Soil Samples Measure

chemical (nutrients) and physical (textures, bulk density) characteristics of soils

What do Plant tissue samples measure

chlorophyll, nutrient uptake, biomass of crop plants

What do remote sensing samples measure

physiological or physical information about plant healthiness, soil OM contents, soil moisture content

What does every sample collected need?

geocoordinates

What is the goal of soil sampling?

characterize the nutrient status of a field as accurately and inexpensively as possible.

Why are plant tissue samples needed?

plant nutrient composition varies with age and the portion of the plant sampled

3 sampling methods

Random (1 per field), Grid (1+ sub sample around georeference point), Zone (1 per pre-defined management zone)

Acreage of Grid Sampling Points

For ¼ section field (2,600’ x 2,600’)

1. 100’ grids: 26 x 26 = 676 points (~1/4 ac)

2. 200’ grids: 13 x 13 = 169 points (~1 ac)

3. 300’ grids: 8 x 8 = 64 points (~2 ac)

How to ensure sample accuracy every year?

Samples should:

1. Be taken on same spots and at the same time of years.

2. Be taken at the right and same depth (consistently 6 or 24 inch depths) or right portion of leaf.

3. Contain an adequate number of samples (soil cores or plants) to accurately represent the area.

Grid Point Sampling

collects one or multiple subsamples around a georeferenced point within a grid or at a grid intersection.

Grid Cell Sampling

randomly collects either one or multiple subsamples throughout the cell for a composite sample.

Grid distance (number of samples) is determined by ________ ________

Spacial Dependency

How to estimate values of unsampled areas

1. Make an average value

   1. Use geospacial statistics (Inverse Distance Weighting or Kriging)

Inverse Distance Weighting (IDW) Information

• Uses a weighted average of sample points, with closer points having more influence. The weight depends ONLY on the distance from the prediction location.

• Easier to use and lower computational cost than kriging

• can be oversensitive to outliers and lacks structural information.

Kriging

• Uses weights based on both the distance from the prediction location and the spatial arrangement of the measured points. Is an advanced geostatistical procedure that assumes the data are spatially continuous.

• More robust than IDW

• Can be computationally demanding, have model selection challenges, and be complex to interpret.

Data and Spacial Patterns

Spatial pattern = kriging or IDW works well

NO spatial pattern = kriging or IDW do NOT work well.

Spacial Dependence/Correlation

• Data close together = highly correlated

• Spatial dependence imputes that up to some distance apart from each other, two observations at different locations are statistically dependent

Semivariogram Definition

An autocorrelogram that expresses the degree of spatial variation as a function of distance

Parts of Semivariogram

• Nugget = the y-intercept that represents the semi-variance between two closest points

• Sill = maximum semi-variance

• De-correlation length (range) = measures spatial continuity (range of spatial dependence)

Types of Variation

Temporal (Time - seasons or years) and Spacial (geographical locations)

What are temporal Variations affected by?

Weather and Management

What is spacial variation affected by?

Landscape positions affect the flow of nutrient and water which impact yield