Introduction
Definitions of Gulf Stream location sometimes centre on thermal signature, but it might make sense to work with dynamic height instead. This is illustrated here, using a model for , with the distance along the transect. The idea is to select , the halfway point in the function, where the slope is maximum and where therefore the inferred geostrophic velocity peaks.
Methods and results

library(oce)

## Loading required package: methods
## Loading required package: mapproj
## Loading required package: maps

data(section)
## Extract Gulf Stream (and reverse station order)
GS < subset(section, 109<=stationId & stationId<=129)
GS < sectionSort(GS, by="longitude")
GS < sectionGrid(GS)
## Compute and plot normalized dynamic height
dh < swDynamicHeight(GS)
h < dh$height
x < dh$distance
par(mfrow=c(1, 3), mar=c(3, 3, 1, 1), mgp=c(2, 0.7, 0))
plot(x, h, xlab="Distance [km]", ylab="Dynamic Height [m]")
## Fit to tanh, with x0 line
m < nls(h~a+b*(1+tanh((xx0)/L)), start=list(a=0,b=1,x0=100,L=100))
hp < predict(m)
lines(x, hp, col='blue')
x0 < coef(m)[["x0"]]
abline(v=x0, col='blue')
# Temperature section, again with x0 line
plot(GS, which="temperature")
abline(v=x0, col='blue')
## Show lon and lat of x0, on a map
lon < GS[["longitude", "byStation"]]
lat < GS[["latitude", "byStation"]]
distance < geodDist(lon, lat, alongPath=TRUE)
lat0 < approxfun(lat~distance)(x0)
lon0 < approxfun(lon~distance)(x0)
plot(GS, which="map",
map.xlim=lon0+c(6,6), map.ylim=lat0+c(6, 6))
points(lon0, lat0, pch=1, cex=2, col='blue')
data(topoWorld)
## Show isobaths
depth < topoWorld[["z"]]
contour(topoWorld[["longitude"]]360, topoWorld[["latitude"]], depth,
level=1000*1:5, add=TRUE, col=gray(0.4))
## Show Drinkwater September climatological North Wall of Gulf Stream.
data("gs", package="ocedata")
lines(gs$longitude, gs$latitude[,9], col='blue', lwd=2, lty='dotted')

Exercises
From the map, work out a scale factor for correcting geostrophic velocity from crosssection to alongstream, assuming the Drinkwater (1994) climatology to be relevant.
Resources

Source code: 20140622gulfstreamcenter.R

K. F. Drinkwater, R. A Myers, R. G. Pettipas and T. L. Wright, 1994.
Climatic data for the northwest Atlantic: the position of the shelf/slope
front and the northern boundary of the Gulf Stream between 50W and 75W,
19731992. Canadian Data Report of Fisheries and Ocean Sciences 125.
Department of Fisheries and Oceans, Canada.
Rbloggers.com offers daily email updates about R news and tutorials on topics such as: Data science, Big Data, R jobs, visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...