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Converting stem locations in a square plot to X Y metre coordinates

DATE: 2018-05-01

AUTHOR: John L. Godlee

I'm working on a large dataset of woodland plots with in the SEOSAW network[1]. As part of this work I want to compare the spatial distribution of tree stems within the plots. Some of the plots have their stem locations recorded as decimal latitude/longitude co-ordinates, presumably done with a handheld GPS, while other plots have their stems recorded as metre coordinates from the plot edges, presumably these plots were done with a set of tape measures.

1: https://seosaw.github.io

I can't easily compare plots with these different methods and it's good to be consistent, so I set about writing a function in R that can take stem locations and plot corner loctions in latitude/longitude coordinates and convert them to rought X Y metre coordinates. The function can be found below, and also here[2]. This is what I came up with:

2: /files/latlong_xy/latlong_xy_function.R

# Function allowing lat long to x y coordinate conversion
# John Godlee (johngodlee@gmail.com)
# 2018_04_20

# Packages ----
library(dplyr)  # Data manip.
library(rgdal)  # CRS stuff
library(raster)  # spLines()
library(rgeos)  # gDistance()

# Function ----

# stem_id = a unique ID string for each stem
# stem_lon, stem_lat = latitude longitude coordinates for each stem
# corner_id = unique ID string for each plot corner
# corner_lon, corner_lat = latitude longitude coordinates for each plot corner

latlong_xy <- function(stem_id, stem_lon, stem_lat, corner_id, corner_lon, corner_lat){

long_2_utm <- function(x,y) {
    paste("UTM zone ",
    	(floor((x + 180)/6) %% 60) + 1,
    	ifelse(y < 0, "S", "N"), 
    	sep = "")
}

epsg <- make_EPSG()  # Create list of EPSG dataset to search for CRS
wgs84 <- epsg[grep("WGS 84", epsg$note, ignore.case=TRUE),]  # Search for wgs84
wgs84[grep("longlat", wgs84$prj4, ignore.case=TRUE),]  # grep proj4string to check
wgs84_crs <- CRS(wgs84[grep("longlat", wgs84$prj4, ignore.case=TRUE),]$prj4[2])  # Store string as vector

# Must change the UTM zone to match location of your plot.
utm_id <- long_2_utm(mean(corner_lon), 
    mean(corner_lat))

utm_zone_crs <- CRS(wgs84[grep(utm_id, wgs84$note, ignore.case=TRUE),]$prj4[1])  # grep for UTM zone and store

# Convert stem data to utm
stems_points <- SpatialPointsDataFrame(as.matrix(cbind(stem_lon, stem_lat)),  # extract only long lat coords
    proj4string = wgs84_crs,
    data = data.frame(stem_id))

# Transform SPDF to utm
stems_points_sp_utm <- spTransform(stems_points, utm_zone_crs)

# Convert back to dataframe
stems_points_df_utm <- as.data.frame(stems_points_sp_utm)

# Give column names
colnames(stems_points_df_utm) <- c("id", "x_utm", "y_utm")

# Convert plot corners to utm ----
plot_corners_clean <- data.frame(corner_id, "x" = corner_lon, "y" = corner_lat)

plot_corners_points <- SpatialPointsDataFrame(plot_corners_clean[,2:3], 
    proj4string = wgs84_crs, 
    data = data.frame(plot_corners_clean[,1]))

# Transform SPDF to utm
plot_corners_points_utm <- spTransform(plot_corners_points, utm_zone_crs)

# Convert back to dataframe
plot_corners_df_utm <- as.data.frame(plot_corners_points_utm)

# Give column names
colnames(plot_corners_df_utm) <- c("id", "x_coord", "y_coord")

# Make spatial lines from corners ----

# Get corner locations
corner_nw <- plot_corners_df_utm[1,]

corner_ne <- plot_corners_df_utm[2,]

corner_sw <- plot_corners_df_utm[4,]

# Create x axis line
x_line <- rbind(corner_nw, corner_ne) %>%
    dplyr::select(x_coord, y_coord) %>%
    as.matrix(.) %>%
    spLines(., crs = utm_zone_crs)

# Create y axis line 
y_line <- rbind(corner_nw, corner_sw) %>%
    dplyr::select(x_coord, y_coord) %>%
    as.matrix(.) %>%
    spLines(., crs = utm_zone_crs)

# Calculate x y distances from line to point for each stem and append to data frame ----
stems_points_df_utm$x_coord <- as.vector(gDistance(stems_points_sp_utm, y_line, byid = T))
stems_points_df_utm$y_coord <- as.vector(gDistance(stems_points_sp_utm, x_line, byid = T))

# Clean up old and unnecessary columns
stems_loc_df <- stems_points_df_utm %>%
    dplyr::select("id", "x_coord", "y_coord")

stems_loc_df
}