calculating Net Squared Displacement and repeating at 0 when target is reached - r

Question

Apologies if a similar query has been posted - couldn't find it.
I have GPS locations (UTM) for multiple individuals.
X Y AnimalID DATE
1 550466 4789843 10 1/25/2008
2 550820 4790544 10 1/26/2008
3 551071 4791230 10 1/26/2008
4 550462 4789292 10 1/26/2008
5 550390 4789934 10 1/27/2008
6 550543 4790085 10 1/27/2008
I am attempting to calculate Net Squared Displacement and once NSD has reached at least 800m, I'd like to repeat the formula starting at 0 at the next row.
Desired output is this:
XLOC YLOC ANIMALID DATETIME Xdist Ydist NSD GROUP
1 550466 4789843 10 1/25/2008 17:00 354 701 785 1
2 550820 4790544 10 1/26/2008 1:00 605 1387 1513 1
3 551071 4791230 10 1/26/2008 9:00 609 1938 2031 2
4 550462 4789292 10 1/26/2008 17:00 72 642 646 3
5 550390 4789934 10 1/27/2008 1:00 81 793 797 3
6 550543 4790085 10 1/27/2008 9:00 82 149 170 3
7 550380 4789441 10 1/27/2008 17:00 178 192 262 3
8 550284 4789484 10 1/28/2008 1:00 559 426 703 3
9 549903 4789718 10 1/28/2008 9:00 0 35 35 3
10 550462 4789327 10 1/28/2008 17:00 574 275 636 3
11 549888 4789567 10 1/29/2008 1:00 532 263 593 3
12 549930 4789555 10 1/29/2008 9:00 65 4 65 3
13 550397 4789288 10 1/29/2008 17:00 124 140 187 3
14 550338 4789432 10 1/30/2008 1:00 554 339 649 3
15 549908 4789631 10 1/30/2008 9:00 84 75 113 3
16 550378 4789367 10 1/30/2008 17:00 657 1876 1988 3
17 550414 4789354 10 1/31/2008 1:00 531 91 539 4
18 549883 4789445 10 1/31/2008 9:00 188 136 232 4
19 550226 4789490 10 1/31/2008 17:00 126 141 189 4
20 550288 4789495 10 2/1/2008 1:00 176 187 257 4
I added the 'Group' column to indicate when 800 NSD was attained.
I'm really struggling with how exactly to code for this particular approach mainly because the first UTM has to be identical until 800m has been reached.
In other words, I can't do this:
xdist<-abs(diff(X)
ydist<-abs(diff(Y)
nsd<-sqrt(xdist^2+ydist^2)
I need to do this until the target of 800m was reached:
xdist <- abs(X in row 2 - 550446)
ydist <- abs(Y in row 2 - 4789843)
Then the unique UTMs will need to be from rows 3, 4, 17 and so on.
I hope this makes sense and I'd appreciate any help!

Answer 1

I think this is what you are looking for:
data$GROUP[1] <- 1
data$Xdist[1] <- data$XLOC[2] - data$XLOC[1]
data$Ydist[1] <- data$YLOC[2] - data$YLOC[1]
data$NSD[1] <- as.integer(sqrt(data$Xdist[1]^2+data$Ydist[1]^2))
for ( i in 2:(nrow(data)-1)) {
if ( data$NSD[i-1] > 800) {
data$Xdist[i] <- data$XLOC[i+1] - data$XLOC[i]
data$Ydist[i] <- data$YLOC[i+1] - data$YLOC[i]
data$NSD[i] <- as.integer(sqrt(data$Xdist[i]^2+data$Ydist[i]^2))
data$GROUP[i] <- (data$GROUP[i-1] + 1)
} else {
data$Xdist[i] <- data$XLOC[i+1] - data$XLOC[i] + data$Xdist[i-1]
data$Ydist[i] <- data$YLOC[i+1] - data$YLOC[i] + data$Ydist[i-1]
data$NSD[i] <- as.integer(sqrt(data$Xdist[i]^2+data$Ydist[i]^2))
data$GROUP[i] <- (data$GROUP[i-1])
}
}
output:
> data
XLOC YLOC ANIMALID DATE TIME Xdist Ydist NSD GROUP
1 550466 4789843 10 1/25/20081 7:00 354 701 785 1
2 550820 4790544 10 1/26/2008 1:00 605 1387 1513 1
3 551071 4791230 10 1/26/2008 9:00 -609 -1938 2031 2
4 550462 4789292 10 1/26/2008 17:00 -72 642 646 3
5 550390 4789934 10 1/27/2008 1:00 81 793 797 3
6 550543 4790085 10 1/27/2008 9:00 -82 149 170 3
7 550380 4789441 10 1/27/2008 17:00 -178 192 261 3
8 550284 4789484 10 1/28/2008 1:00 -559 426 702 3
9 549903 4789718 10 1/28/2008 9:00 0 35 35 3
10 550462 4789327 10 1/28/2008 17:00 -574 275 636 3
11 549888 4789567 10 1/29/2008 1:00 -532 263 593 3
12 549930 4789555 10 1/29/2008 9:00 -65 -4 65 3
13 550397 4789288 10 1/29/2008 17:00 -124 140 187 3
14 550338 4789432 10 1/30/2008 1:00 -554 339 649 3
15 549908 4789631 10 1/30/2008 9:00 -84 75 112 3
16 550378 4789367 10 1/30/2008 17:00 -48 62 78 3
17 550414 4789354 10 1/31/2008 1:00 -579 153 598 3
18 549883 4789445 10 1/31/2008 9:00 -236 198 308 3
19 550226 4789490 10 1/31/2008 17:00 -174 203 267 3
20 550288 4789495 10 2/1/2008 1:00 NA NA NA NA
Also I think you made a mistake above at xdist16 because for xlocline17 - xlocline16 + xdistline15 = 550414 - 550378 + (-84) = -48 and not 657 as you specified. Unless I missed something at your formula.
Hope this helps!