I had been referencing stops from the original north shore geology field trip guide, until I found an updated version of the guide in a book (embedded below p.132-144)!
In this chapter, he lists the military grid reference system (MGRS) coordinates (Green 2011) so I entered them into a spreadsheet. I then downloaded it as a comma separated values (CSV) sheet to convert it to a spatial object. The main highlight of this script is how I dealt with different MGRS zones for points in the same data frame:
library(sf)
plot_locations_HARV <- read.csv("geology.csv")
co <- st_read(dsn="counties_in_minnesota", layer="mn_county_boundaries")
pts <- lapply(1:nrow(plot_locations_HARV),function(x){
point <- plot_locations_HARV[x,]
epsg <- ifelse(as.integer(gsub("[^[:digit:]]","",point$utm))==15,2027,2028)
pt <- st_as_sf(point, coords = c("E", "N"))
this_crs <- st_crs(paste0("+init=epsg:",epsg))
st_crs(pt) <- this_crs
pt_project <- st_transform(pt, st_crs(co))
return(pt_project)})
df <- do.call(rbind, pts)
st_write(df, dsn="geology", layer="geology", driver= "ESRI Shapefile")
I assigned the numeric part of the MGRS zone as the Universal Transverse Mercator (UTM) zone. From there, I did my best assigning a coordinate reference system (CRS) to the points accordingly. I ultimately wanted it in the same projection as the MN county layer.
I’ll resume my efforts to “ground truth” the locations as soon as the conditions are right, and to add some additional points corresponding to details in the stop descriptions. If next weekend doesn’t allow the ice to thaw, that will probably mean in the spring!
Literature Cited
Green, John C., et al. “The North Shore Volcanic Group: Mesoproterozoic plateau volcanic rocks of the Midcontinent Rift system in northeastern Minnesota.” Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America: Geological Society of America Field Guide 24 (2011): 121-146.