R spatial aggregate, IDs out of order

Question

HUC12_intersects is a SpatialPolygonsDataFrame object with over 5000 polygons

library(rgdal)
library(sp)
library(rgeos)
library(raster)
library(maptools)

HUC12_intersects@data
AREA_ACR_1 GEOL_CODE new_soil HRU_ID
22093.3         2        2     10
22093.3         2        2     12
22093.3         2        2     10
22093.3         2        2     10
 1033.8         1        1      3
22093.3         2        2     10
22093.3         2        2      9
22093.3         2        2     10
22093.3         2        2     10
22093.3         2        2     10

HUC12_intersects@polygons[[1]]

An object of class "Polygons"
Slot "Polygons":
[[1]]
An object of class "Polygon"
Slot "labpt":
[1] 2007122.2  840584.2

Slot "area":
[1] 12538.28

Slot "hole":
[1] FALSE

Slot "ringDir":
[1] 1

Slot "coords":
         x        y
 [1,] 2007095 840527.3
 [2,] 2007049 840514.4
 [3,] 2007028 840547.0
 [4,] 2007061 840587.5
 [5,] 2007132 840631.9
 [6,] 2007174 840658.8
 [7,] 2007195 840639.8
 [8,] 2007207 840625.4
 [9,] 2007208 840594.1
 [10,] 2007151 840559.9
 [11,] 2007095 840527.3



 Slot "plotOrder":
 [1] 1

 Slot "labpt":
 [1] 2007122.2  840584.2

 Slot "ID":
 [1] "109"

 Slot "area":
 [1] 12538.28

The output from HUC12_dissolve is out of order

HUC12_dissolve<-aggregate(HUC12_intersects,by="HRU_ID")

HUC12_dissolve@data
  HRU_ID
1       1
2      10
3     100
4      12
5      14
6      15
7       2
8     202
9       3
10      5
11      7
12      8
13      9

I've tried sorting the data.frame and sorting the polygons to match the sorted data.frame but this does not preserve the spatial aspect of the polygons

order_HRUs<-order(HUC12_dissolve@data[,1],decreasing=F)
HUC12_dissolve2<-HUC12_dissolve
cnt<-0
for(i in seq(1,length(order_HRUs))){
  cnt<-cnt+1
  HUC12_dissolve2@polygons[[cnt]]<-HUC12_dissolve2@polygons[[which(order_HRUs==cnt)]]
}

This creates polygons with gaps and they are not labeled with the correct "HRU_ID"

I want to sort the data.frame, and I want the sorted data.frame "HRU_ID" to match the polygons

Answer 1

The aggregation is tracked using the row ids which is why the attribute id is out of order. If you want to reorder the data, which is honestly a bit arbitrary, operate on the row names not just the attributes.

You can use a vector of values as an index to resort the rows of a data.frame. If we create a data.frame and then pass a random vector of values that are the same length, within the bounds of n, we can resort the data.frame.

( x = data.frame(IDS=1:10, y=runif(10)) )
x[sample(1:10),]

Spatial objects honor this type of index sorting and subsetting. Here is a simple example where we resort a spatial object using the rownames. First, add packages and data as well as add a new IDS column to the sp meuse object.

library(sp)
data(meuse)
coordinates(meuse) <- ~x+y
( meuse@data <- data.frame(meuse@data, IDS=sample(1:nrow(meuse))) )

Here we create an object where the rownames and IDS are subset to a data.frame and then the spatial object is reordered using a bracket index with the new order of the rownames based on the sorting of the IDS.

x <- data.frame(r=row.names(meuse@data), IDS=meuse@data$IDS)
meuse <- meuse[match(x[order(x$IDS),]$r, row.names(meuse@data)),]
head(meuse@data)