rtop-package: A package providing methods for analysis and spatial...

In rtop: Interpolation of Data with Variable Spatial Support

A package providing methods for analysis and spatial interpolation of data with an irregular support

Description

This package provides geostatistical methods for analysis and interpolation of data that has an irregular support, such as as runoff characteristics or population health data. The methods in this package are based on the top-kriging approach suggested in Skoien et al (2006), with some extensions from Gottschalk (1993). This package can be used as an add-on package for the automatic interpolation package developed within the intamap project (www.intamap.org).

Workflow

The work flow within the package suggests that the user is interested in a prediction of a process at a series of locations where observations have not been made. The example below shows a regionalization of mean annual runoff in Austria.

Although it is possible to perform each step with all necessary arguments, the easiest interface to the method is to store all variables (such as observations, prediction locations and parameters) in an rtop-object, which is created by a call to createRtopObject. The element params below consists of changes to the default parameters. A further description can be found in getRtopParams. The changes below means that the functions will use geostatistical distance instead of full regularization, and that the variogram model will be fitted to the variogram cloud. Most other functions in the rtop-package can take this object as an argument, and will add the results as one or more new element(s) to this object.

The data in the example below are stored as shape-files in the extdata-directory of the rtop-pacakge, use the directory of your own data instead. The observations consist of mean summer runoff from 138 catchments in Upper Austria. The predictionLocations are 863 catchments in the same region. observations and predictionLocations can either be stored as SpatialPolygonsDataFrame-objects, or as sf-polygons.

rpath = system.file("extdata",package="rtop")
library(sf)
observations = st_read(rpath, "observations")
predictionLocations = st_read(rpath,"predictionLocations")


# Create a column with the specific runoff:
observations$obs = observations$QSUMMER_OB/observations$AREASQKM
params = list(gDist = TRUE, cloud = TRUE)
rtopObj = createRtopObject(observations, predictionLocations, 
          params = params)

There are help-methods available in cases when data are not available as shape-files, or when the observations are not part of the shape-files. See readAreaInfo and readAreas.

A call to rtopVariogram adds the sample variogram to the object, whereas
rtopFitVariogram fits a variogram model. The last function will call rtopVariogram if rtopObj does not contain a sample variogram.

rtopObj = rtopVariogram(rtopObj)
rtopObj = rtopFitVariogram(rtopObj, maxn = 2000)

The function checkVario is useful to produce some diagnostic plots for the sample variogram and the fitted variogram model.

checkVario(rtopObj)

The interpolation function (rtopKrige) solves the kriging system based on the computed regularized semivariances. The covariance matrices are created in a separate regularization function (varMat), and are stored in the rtop-object for easier access if it is necessary to redo parts of the analysis, as this is the computationally expensive part of the interpolation. Cross-validation can be called with the argument cv=TRUE, either in params or in the call to rtopKrige.

rtopObj = rtopKrige(rtopObj)
if (is(rtopObj$predictions, "Spatial")) {
  spplot(rtopObj$predictions, col.regions = bpy.colors(), c("var1.pred"))
} else {
# the plotting order to get small polygons on top is not automatic with sf, 
# but here is a method that works without modifying the predictions
  library(dplyr)
# Arrange according to areas attribute in descending order
  rtopObj$predictions |> arrange(desc(AREASQKM)) |>
  # Make ggplot and set fill color to var1.pred
  ggplot(aes(fill = var1.pred)) + geom_sf() 
}
rtopObj = rtopKrige(rtopObj, cv = TRUE)

if (is(rtopObj$predictions, "Spatial")) {
  spplot(rtopObj$predictions, col.regions = bpy.colors(), c("var1.pred","var1.var"))
} else {
# Here is an alternative method for plotting small polygons on top of the larger ones, 
# modifying the predictions 
  rtopObj$predictions = rtopObj$predictions[order(rtopObj$predictions$AREASQ, decreasing = TRUE), ]
# It is also possible to change the order of the polygons 
  ggplot(rtopObj$predictions) + aes(fill = var1.pred) + geom_sf() + scale_fill_distiller(palette = "YlOrRd")
}

References

L. Gottschalk. Interpolation of runoff applying objective methods. Stochastic Hydrology and Hydraulics, 7:269-281, 1993.

Skoien J. O., R. Merz, and G. Bloschl. Top-kriging - geostatistics on stream networks. Hydrology and Earth System Sciences, 10:277-287, 2006.

Skoien, J. O., Bloschl, G., Laaha, G., Pebesma, E., Parajka, J., Viglione, A., 2014. Rtop: An R package for interpolation of data with a variable spatial support, with an example from river networks. Computers & Geosciences, 67.

rtop documentation built on July 4, 2024, 9:09 a.m.