The itslive_flowline function calculates ice flowlines using ITS_LIVE mosaic velocity data.
Note: This function may take a few seconds to run.
[x,y] = itslive_flowline(region,xi,yi)
[lat,lon] = itslive_flowline(region,lati,loni)
[...] = itslive_flowline(...,spacing)
[...,...,d,v] = itslive_flowline(...)[x,y] = itslive_flowline(region,xi,yi) calculates flow path(s) from seed locations given in projected map coordinates xi,yi. If multiple starting points are specified, output xi and yi will be cell arrays. Input region is a number from 1 to 19 corresponding to the mosaic region. For a map of ITS_LIVE regions, type itslive_regions. Flowlines are calculated upstream and downstream of seed location(s).
[lat,lon] = itslive_flowline(region,lati,loni) as above, but if inputs are geographic coordinates, outputs are too.
[...,...,d,v] = itslive_flowline(...) also returns distance d in meters along the flowline. Negative numbers are upstream of the seed location and positive numbers are downstream. If four outputs are requested, the velocity v (m/yr) is the linearly interpolated velocity along the flowline(s).
[...] = itslive_flowline(...,spacing) specifies spacing along the flowline in meters. Default spacing is 10 m.
Consider the location (51.5313°N,125.9555°W) in British Columbia, Canada, which is in RGI & ITS_LIVE Region 2. Using the geo2itslive function, we can convert the coordinates to projected meters in the map coordinates for Region 2:
>> [xi, yi] = geo2itslive(2, 51.5313, -125.9555)
xi =
295011.70
yi =
5713051.22The itslive_flowline function allows inputs as geo coordinates or projected map coordinates, so since we'll be plotting the results on a map, let's enter the seed location as projected coordinates xi,yi. Below, we're plotting velocity using itslive_imagesc for Region 2 after setting the axis limits of the map. We plot the seed location as a yellow pentagram, and for a little extra context we're adding a background image with plot_basemap and setting the colormap with cmocean:
% Define a seed location:
xi = 295012;
yi = 5713051;
figure
axis([260076 329851 5671516 5741291])
itslive_imagesc(2)
hold on
clim([0 200])
plot(xi,yi,'yp','markerfacecolor','y') % yellow pentagram
plot_basemap('epsg',32610) % optional basemap
cmocean amp % optional colormap
exportgraphics(gcf,'itslive_flowline_documentation_BC_1.jpg')
From the seed location marked as a yellow star above, get the flowline that passes through that location:
% Calculate flowline from seed location in Region 2:
[x,y] = itslive_flowline(2,xi,yi);
% Plot the flowline:
plot(x,y,'y','linewidth',2)
exportgraphics(gcf,'itslive_flowline_documentation_BC_2.jpg')
It is possible to generate multiple flowlines at once. Consider this 7x6 grid of seed locations in northern Greenland, which are plotted on top of an itslive_imagesc map of velocity for Greenland (Region 5):
% Create a grid of points in projected meters:
[Xi,Yi] = meshgrid(-359399:20e3:-256068,-1076060:-20e3:-1197793);
figure
axis([-483846.00 -160640 -1376958 -892150])
itslive_imagesc(5);
set(gca,'colorscale','log')
hold on
plot(Xi,Yi,'ko')
exportgraphics(gcf,'itslive_flowline_documentation_greenland_grid_1.jpg')
With the black circles as seed locations, generate a flowline for each point:
[X,Y] = itslive_flowline(5, Xi, Yi);
whos
Name Size Bytes Class Attributes
X 7x6 8331016 cell
Xi 7x6 336 double
Y 7x6 8331016 cell
Yi 7x6 336 double Above, the whos call lists variables and their sizes. It tells us that Xi and Yi are cell arrays whose 7x6 dimensions match the dimensions of X and Y. Each cell contains an array of the coordinates of the flowlines. Here's an easy way to plot them all in a loop:
for k = 1:numel(X)
plot(X{k},Y{k},'k')
end
exportgraphics(gcf,'itslive_flowline_documentation_greenland_grid_2.jpg')
This example uses mapzoomps and scalebarps from Antarctic Mapping Tools, modismoaps from my MODIS Mosaic of Antarctica repo, and bedmachine and bedmachine_profile from my BedMachine repo.
I started this example by manually clicking around on a polar stereographic map of the Amundsen Sea Embayment to find a seed location along the grounding line. The seed location is arbitrary, but I selected a spot on the grounding line because it's a helpful reference point for many types of analysis.
x_seed = -1590539; % m easting
y_seed = -256071; % m northing
figure
mapzoomps('pine island glacier','mapwidth',200) % initialize 200 km wide map
modismoaps('contrast','low') % plots background image
hold on
h = itslive_imagesc(19, 'alpha',0.75); % semitransparent velocity
set(gca,'colorscale','log')
clim([1 3e3]) % color axis limits
bedmachine % plots grounding line
p=plot(x_seed,y_seed,'wo','markersize',8,... % plots seed location
'markerfacecolor','r'); % as white-ringed red dot
exportgraphics(gcf,'itslive_flowline_documentation_pig_map_1.jpg')
Now calculate a flowline from the seed location (takes a few seconds on my laptop). Below, I'm plotting the flowline as a red and white line of different linewidths to give the effect of white-outlined red line.
[x,y,d,v] = itslive_flowline(19,x_seed,y_seed);
p(2) = plot(x,y,'w','linewidth',2);
p(3) = plot(x,y,'r','linewidth',1);
exportgraphics(gcf,'itslive_flowline_documentation_pig_map_2.jpg')
Zoom out to see the entire flowline:
delete(h) % deletes old velocity map
axis tight off % resets axis limits to all data
modismoaps('contrast','low') % fills extents with background image
itslive_imagesc(19, 'alpha',0.75) % recreates velocity map
bedmachine % re-plots grounding line
uistack(p(2),'top') % places flowline on top of graphics
uistack(p(3),'top') % places flowline on top of graphics
uistack(p(1),'top') % places marker on top of graphics
scalebarps('location','se') % scalebar in lower right corner
exportgraphics(gcf,'itslive_flowline_documentation_pig_map_3.jpg')
And here's what the velocity looks like along the profile:
figure
subplot(2,1,1)
plot(d/1000,v)
box off
axis tight
ylabel("Velocity (m yr^{-1})")
subplot(2,1,2)
bedmachine_profile(x,y,'horiz',d/1000)
axis tight
xlabel("Distance along flowline (km)")
ylabel("Elevation (m)")
exportgraphics(gcf,'itslive_flowline_documentation_pig_profile.jpg')
Notice above that the 0 distance along the flowline corresponds to the grounding line, because that's where we defined the seed location.
The MATLAB functions in this repo and this documentation were written by Chad A. Greene of NASA/JPL. The NASA MEaSUREs ITS_LIVE project is by Alex S. Gardner and the ITS_LIVE team.