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PLOT/VS with map projections

Question:

All I want to do is plot a ship track with a particular map projection.

Example:

Explanation:

The mp_line.jnl script should only be used to overlay a line plot on top of a SHADE, FILL or CONTOUR plot. When run by itself, without the /OVER qualifier, it does not work as expected.

An easy way to create a blank underlay for the mp_line script is to shade data from one of the etopo datasets with various qualifiers so that you end up with a blank plot.

Solution:

The plot above shows a ship track in Puget Sound, Washington. The script to create it is given below and is commented with several tricks we used to get the plot just right.

! How to draw a ship track in a map projection.
!
! This script uses map projection scripts to get the
! correct aspect ratio for a ship track in Puget Sound.

set win/size=.2

! We'll need to create an underlay with some dataset.
! ETOPO60 is one everyone should have.
use etopo60

set region/x=237.0:238.0/y=47.4N:48.25N

! The region of interest is smaller then the grid size
! of etopo60 so we'll need to regrid it to higher
! resolution.
define axis/x=237.0:238.0:0.01/units=degree x_ax
define axis/y=47.4:48.25:0.01/units=degree y_ax
define grid/x=x_ax/y=y_ax map_grid

! Set the grid as required by the map projection scripts
set grid map_grid
go mp_grid map_grid

! Now issue the map projection commands
go mp_mercator
go mp_aspect

! Create our blank underlay
shade/noaxis/pal=white/nokey/nolabs rose[g=map_grid], x_page, y_page

! Get the ship track lat and lon
file/var="ship_lat,ship_lon" mp_ship_track.dat

! Ferret hates negative numbers so convert these longitudes
! (in the range -123:-122) to be greater than zero.
let new_lon = 360 + ship_lon

! Plot the ship track
go mp_line plot/vs/line/noaxis/nolab/over new_lon,ship_lat

! Note that we defined the axis and the region to have
! a lower latitude of 47.4. This is to make sure that
! the bottom graticule line of 47.5 is drawn.
go mp_graticule 237.0 238.0 0.25 47.5 48.25 0.25

go mp_label 236.9 47.50 0 0 .15 "@AS47.50 N"
go mp_label 236.9 47.75 0 0 .15 "@AS47.75 N"
go mp_label 236.9 48.00 0 0 .15 "@AS48.00 N"
go mp_label 236.9 48.25 0 0 .15 "@AS48.25 N"

go mp_label 237.00 47.46 0 0 .15 "@AS123.00W"
go mp_label 237.25 47.46 0 0 .15 "@AS122.25W"
go mp_label 237.50 47.46 0 0 .15 "@AS122.50W"
go mp_label 237.75 47.46 0 0 .15 "@AS122.75W"
go mp_label 238.00 47.46 0 0 .15 "@AS122.00W"

go mp_label 237.50 48.30 0 0 .2 "@TRShip Track in Puget Sound"