28.4. Appendix D: sintaksis skrip R dalam QGIS

Contributed by Matteo Ghetta - funded by Scuola Superiore Sant'Anna

Penulisan skrip R dalam Processing memiliki kiat dengan adanya sintaksis khusus.

Pemrosesan skrip R dimulai dengan mendefinisikan Inputs (masukan) dan Outputs (keluaran), dimana dimulai dengan 2 (dua) karakter pagar (##).

Sebelum masukan, grup untuk menempatkan algoritma dapat didefinisikan. Jika grup sudah didefinisikan sebelumnya, algoritma akan ditambahkan ke dalamnya, jika belum kelompok akan dibuat. Dalam contoh di bawah, nama grup adalah My Group.

##My Group=group

28.4.1. Masukan

Semua data masukan dan parameter harus lengkap. Terdapat beberapa tipe masukan:

• vector: ##Layer = vector

• vector field: ##F = Field Layer (where Layer is the name of an input vector layer the field belongs to)

• raster: ##r = raster

• table: ##t = table

• number: ##Num = number

• string: ##Str = string

• boolean: ##Bol = boolean

• elements in a dropdown menu. The items must be separated with semicolons ;: ##type=selection point;lines;point+lines

28.4.2. Keluaran

Seperti pada masukan, masing-masing keluaran harus didefinisikan pada awal skrip:

• vector: ##output= output vector

• raster: ##output= output raster

• table: ##output= output table

• plots: ##output_plots_to_html (##showplots in earlier versions)

• To show R output in the Result Viewer, put > in front of the command whose output you would like to show.

28.4.3. Syntax Summary for QGIS R scripts

A number of input and output parameter types are offered.

28.4.3.1. Input parameter types

Parameter	Syntax example	Returning objects
vector	Layer = vector	sf object (or SpatialDataFrame object, if ##load_vector_using_rgdal is specified)
vector point	Layer = vector point	sf object (or SpatialDataFrame object, if ##load_vector_using_rgdal is specified)
vector line	Layer = vector line	sf object (or SpatialDataFrame object, if ##load_vector_using_rgdal is specified)
vector polygon	Layer = vector polygon	sf object (or SpatialPolygonsDataFrame object, if ##load_vector_using_rgdal is used)
multiple vector	Layer = multiple vector	sf object (or SpatialDataFrame objects if ##load_vector_using_rgdal is specified)
table	Layer = table	dataframe conversion from csv, default object of read.csv function
field	Field = Field Layer	name of the Field selected, e.g. "Area"
raster	Layer = raster	RasterBrick object, default object of raster package
multiple raster	Layer = multiple raster	RasterBrick objects, default object of raster package
number	N = number	integer or floating number chosen
string	S = string	string added in the box
longstring	LS = longstring	string added in the box, could be longer then the normal string
selection	S = selection first;second;third	string of the selected item chosen in the dropdown menu
crs	C = crs	string of the resulting CRS chosen, in the format: "EPSG:4326"
extent	E = extent	Extent object of the raster package, you can extract values as E@xmin
point	P = point	when clicked on the map, you have the coordinates of the point
file	F = file	path of the file chosen, e.g. "/home/matteo/file.txt"
folder	F = folder	path of the folder chosen, e.g. "/home/matteo/Downloads"

A parameter can be OPTIONAL, meaning that it can be ignored.

In order to set an input as optional, you add the string optional before the input, e.g:

##Layer = vector
##Field1 = Field Layer
##Field2 = optional Field Layer

28.4.3.2. Output parameter types

Parameter	Syntax example
vector	Output = output vector
raster	Output = output raster
table	Output = output table
file	Output = output file

Catatan

You can save plots as png from the Processing Result Viewer, or you can choose to save the plot directly from the algorithm interface.

28.4.3.3. Badan skrip

The script body follows R syntax and the Log panel can help you if there is something wrong with your script.

Remember that you have to load all additional libraries in the script:

library(sp)

28.4.4. Examples

28.4.4.1. Contoh pada keluaran vektor

Mari ambil sebuah algoritma dari koleksi daring untuk membuat titik-titik acak dari luasan lapis masukan:

##Point pattern analysis=group
##Layer=vector polygon
##Size=number 10
##Output=output vector
library(sp)
spatpoly = as(Layer, "Spatial")
pts=spsample(spatpoly,Size,type="random")
spdf=SpatialPointsDataFrame(pts, as.data.frame(pts))
Output=st_as_sf(spdf)

Explanation (per line in the script):

1. Analisis pola titik merupakan kelompok algoritma

2. Lapis merupakan lapis vektor masukan

3. Size is a numerical parameter with a default value of 10

4. Keluaran merupakan lapis vektor yang akan dibuat oleh algoritma

5. library(sp) loads the sp library

6. spatpoly = as(Layer, "Spatial") translate to an sp object

7. Call the spsample function of the sp library and run it using the input defined above (Layer and Size)

8. Create a SpatialPointsDataFrame object using the SpatialPointsDataFrame function

9. Create the output vector layer using the st_as_sf function

That's it! Just run the algorithm with a vector layer you have in the QGIS Legend, choose the number of random point. The resulting layer will be added to your map.

28.4.4.2. Contoh dengan keluaran raster

The following script will perform basic ordinary kriging to create a raster map of interpolated values from a specified field of the input point vector layer by using the autoKrige function of the automap R package. It will first calculate the kriging model and then create a raster. The raster is created with the raster function of the raster R package:

##Basic statistics=group
##Layer=vector point
##Field=Field Layer
##Output=output raster
##load_vector_using_rgdal
require("automap")
require("sp")
require("raster")
table=as.data.frame(Layer)
coordinates(table)= ~coords.x1+coords.x2
c = Layer[[Field]]
kriging_result = autoKrige(c~1, table)
prediction = raster(kriging_result$krige_output)
Output<-prediction

By using ##load_vector_using_rgdal, the input vector layer will be made available as a SpatialDataFrame objects, so we avoid having to translate it from an sf object.

28.4.4.3. Contoh dengan keluaran tabel

Mari sunting algoritma Summary Statistics sehingga keluarannya menjadi sebuah berkas tabel (csv).

Badan skrip seperti berikut:

##Basic statistics=group
##Layer=vector
##Field=Field Layer
##Stat=Output table
Summary_statistics<-data.frame(rbind(
    sum(Layer[[Field]]),
    length(Layer[[Field]]),
    length(unique(Layer[[Field]])),
    min(Layer[[Field]]),
    max(Layer[[Field]]),
    max(Layer[[Field]])-min(Layer[[Field]]),
    mean(Layer[[Field]]),
    median(Layer[[Field]]),
    sd(Layer[[Field]])),
  row.names=c("Sum:","Count:","Unique values:","Minimum value:","Maximum value:","Range:","Mean value:","Median value:","Standard deviation:"))
colnames(Summary_statistics)<-c(Field)
Stat<-Summary_statistics

Baris ketiga menentukan Vector Field pada masukan dan baris keempat menjelaskan algoritma yang hasilnya haruslah sebuah tabel.

Baris terakhir akan mengambil objek Stat yang dibuat dalam skrip dan mengonversikannya ke dalam sebuah tabel csv.

28.4.4.4. Contoh dengan keluaran konsol

We can use the previous example and instead of creating a table, print the result in the Result Viewer:

##Basic statistics=group
##Layer=vector
##Field=Field Layer
Summary_statistics<-data.frame(rbind(
sum(Layer[[Field]]),
length(Layer[[Field]]),
length(unique(Layer[[Field]])),
min(Layer[[Field]]),
max(Layer[[Field]]),
max(Layer[[Field]])-min(Layer[[Field]]),
mean(Layer[[Field]]),
median(Layer[[Field]]),
sd(Layer[[Field]])),row.names=c("Sum:","Count:","Unique values:","Minimum value:","Maximum value:","Range:","Mean value:","Median value:","Standard deviation:"))
colnames(Summary_statistics)<-c(Field)
>Summary_statistics

The script is exactly the same as the one above except for two edits:

1. no output specified (the fourth line has been removed)

2. the last line begins with >, telling Processing to make the object available through the result viewer

28.4.4.5. Contoh dengan plot

To create plots, you have to use the ##output_plots_to_html parameter as in the following script:

##Basic statistics=group
##Layer=vector
##Field=Field Layer
##output_plots_to_html
####output_plots_to_html
qqnorm(Layer[[Field]])
qqline(Layer[[Field]])

The script uses a field (Field) of a vector layer (Layer) as input, and creates a QQ Plot (to test the normality of the distribution).

The plot is automatically added to the Processing Result Viewer.