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Vacation + Sailboat + GPS + GRASS + RSubmitted by dylan on Fri, 2007-08-03 20:41.
Two short excursions (approx. 5 miles each) were measured with a Garmin GPS 12, which was set to record a track point every 5 seconds. With an open horizon, and such a short tracking interval it was possible to accurately map out changes in velocity and direction over the duration of the excursions. GRASS was used to extract line segment start and end nodes, calculate line segment length, and assign a velocity to each line segment. These data were dumped to CSV file for analysis in R. Simple plots of velocity vs. bearing from east were created. All code and GPS logs (see attached files) are included on this page. These examples demonstrate several simple GIS and plotting techniques which can be performed in GRASS & R with very few keystrokes.
## download track data as points with gpstrans gpstrans -dt > output.raw # trim top couple of garbage lines off with text editor... # convert into GRASS line segments with AWK awk ' BEGIN{print "VERTI:"; n = 0} { vertex = $6" "$7 if( (n > 0 && n < 3) ) {print "L 2\n"old_vertex"\n"vertex} else if(n >2 ) {print "L 2\n"old_vertex"\n"vertex} old_vertex = vertex; n++ } ' output.raw > output.ascii # need to pre-make the hobie.ascii file first! # these were generated with a combination of gpstrans and awk # this is the prefix i.e. 'hobie1' infile=$1 ## import and make table v.in.ascii --o in=$infile.ascii out=$infile format=standard ## add cat to line features v.category in=$infile out=htemp option=add type=line ## rename, and overwrite the original vector g.remove vect=$infile g.rename vect=htemp,$infile v.db.addtable map=$infile columns="cat int, len double, velocity double, start_x double, start_y double, end_x double, end_y double" ## upload data by cat -- for each line segment # v.to.db map=$infile column=cat option=cat v.to.db map=$infile column=len option=length v.to.db map=$infile column=start_x,start_y option=start v.to.db map=$infile column=end_x,end_y option=end ## calculate velocity in m/s (assuming 5 second interval) echo "update $infile set velocity = len / 5" | db.execute db.select $infile fs="," > $infile.csv # read in some data: # these were dumped from GRASS x <- read.csv('hobie1.csv') y <- read.csv('hobie2.csv') # some diagnostics: plot(cumsum(x$len), type='l') plot(cumsum(y$len), type='l') plot( density(x$velocity[x$velocity > 1])) plot( density(y$velocity[y$velocity > 1])) # compute angle from x-axis, with quadrant correction # this is the same as atan(dx/dy) + quadrant test # i.e. this is the angle (neg = CW / pos = CCW) from East (E = 0 Deg) # # we can check this with a combination of d.grid and d.vect for # specific lines # d.grid -t -b size=10 origin=306791,4123865 # d.vect hobie2 disp=shape,cat,dir cat=120-130 # matches y[122,] ---> angle = 135 Deg # dx <- with(x, (end_x - start_x)) dy <- with(x, (end_y - start_y)) x$angle <- atan2(dy, dx) * 180/pi dx <- with(y, (end_x - start_x)) dy <- with(y, (end_y - start_y)) y$angle <- atan2(dy, dx) * 180/pi # combine into a single df # remove bogus values z <- rbind(x,y) z <- na.omit(z) # load required packages library(plotrix) ## angle vs velocity on linear plot par(mfcol=c(1,2)) plot(velocity ~ angle, data=z, xlim=c(-180,180), xlab="Angle from East (positive angles are CCW)", ylab="Velocity (m/s)", main="Velocity Field", col=color.scale(velocity,c(0,1,1),c(1,1,0),0), pch=16, cex=1.5) ## polar plot: polar.plot(z$velocity, z$angle ,main="Sailing Direction from East (0 Deg)", line.col='blue', lwd=2)
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With my normal crew and better wind we were able to get the boat up to almost 17 knots (20 mph)!