How to mask data points outside a border using geoshow?

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peterhack on 11 Nov 2016
Answered: Srishti Gaur on 2 Aug 2018
I am using the presented code of this thread to mask the data points outside the border so that only data points within are displayed. Works fine so far. However, I want to use the DisplayType surface. Thus I am using geoshow to plot the data. Unfortunately it does not fill the whole map within the borders as a buffer is left.
Any ideas on this?
Here is an example:
S = shaperead('landareas', 'UseGeoCoords', true,...
'Selector',{@(name) strcmp(name,'Australia'), 'Name'});
x = linspace(min(S.Lon), max(S.Lon), 100);
y = linspace(min(S.Lat), max(S.Lat), 100);
[x,y] = meshgrid(x,y);
a = 1;
b = 9;
z = a + (b-a).*rand(100,100);
isin = inpolygon(x,y,S.Lon,S.Lat);
z2 = z;
z2(~isin) = NaN;
lnlim = [min(S.Lon) max(S.Lon)];
ltlim = [min(S.Lat) max(S.Lat)];
lt = linspace(ltlim(1), ltlim(2), 3);
ln = linspace(lnlim(1), lnlim(2), 3);
for ii = 1:2
ltbox{ii} = lt([1 2 2 1 1]'+(ii-1));
lnbox{ii} = ln([1 1 2 2 1]'+(ii-1));
[lnmask, ltmask] = deal(cell(2));
for ii = 1:2
for jj = 1:2
[lnmask{ii,jj}, ltmask{ii,jj}] = polybool('-', ...
lnbox{ii}, ltbox{jj}, S.Lon, S.Lat);
ltboxall = ltlim([1 2 2 1 1]);
lnboxall = lnlim([1 1 2 2 1]);
[lnmaskall, ltmaskall] = polybool('-', lnboxall, ltboxall, S.Lon, S.Lat);
geoshow(y, x, z2, 'DisplayType','surface')
for ii = 1:4
patchm(ltmask{ii}, lnmask{ii}, 'w', 'edgecolor', 'none');
plotm(ltmaskall, lnmaskall, 'k');

Accepted Answer

Kelly Kearney
Kelly Kearney on 15 Nov 2016
You seem to have combined both of my suggested methods into one in this example. Are you trying to do the mask-with-NaNs option (which is easier, but may leave you with some jagged edges near the country border)? Or the patch overlay?
Either way, I think the "not filling" issue you're seeing is an artifact of how worldmap defines Australia versus how the shapefile does. The former includes a wider longitude range, probably accounting for a small island or two off the east coast somewhere (my Australian geography leaves something to be desired). If instead you call worldmap with your data limits, you should see better results:
worldmap(ltlim, lnlim);
Kelly Kearney
Kelly Kearney on 16 Nov 2016
Here's a patch overlay example. I decided to plot the patch in pre-projected coordinates for two reasons:
  1. patchm can't handle face/vertex syntax, which is the easiest way to plot patches with holes.
  2. This geographic region is large enough that the curvature introduced by the map projection is important. If we ignore that and try to create a mask just using straight lines in lat/lon space, then we won't completely mask the edges of the data.
% Australia border, and limits around it
S = shaperead('landareas', 'UseGeoCoords', true,...
'Selector',{@(name) strcmp(name,'Australia'), 'Name'});
ltlim = [min(S.Lat) max(S.Lat)] + [-1 1];
lnlim = [min(S.Lon) max(S.Lon)] + [-1 1];
% Some coarse fake data
nx = 20;
ny = 20;
x = linspace(lnlim(1), lnlim(2), nx);
y = linspace(ltlim(1), ltlim(2), ny);
[x,y] = meshgrid(x,y);
a = 1;
b = 9;
z = a + (b-a).*rand(ny,nx);
% Plot data as surface
geoshow(y, x, z, 'DisplayType','surface', ...
'zdata', ones(size(x))*-1, ... % keep below gridlines
'cdata', z, ...
'facecolor', 'flat'); % to show coarseness
geoshow(S, 'edgecolor', 'k', 'facecolor', 'none');
% Create mask overlay
ltbox = ltlim([1 2 2 1 1]);
lnbox = lnlim([1 1 2 2 1]);
[ltbox, lnbox] = interpm(ltbox, lnbox, 1);
[xbox, ybox] = mfwdtran(ltbox, lnbox); % Box around data
[xaus, yaus] = mfwdtran(S.Lat, S.Lon); % Australia polygon
[xmask, ymask] = polybool('-', xbox, ybox, xaus, yaus); % box w/ Aus. hole
[f,v] = poly2fv(xmask, ymask);
hmask = patch('faces', f, 'vertices', v, ...
'facecolor', 'w', 'edgecolor', 'none');

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More Answers (2)

peterhack on 17 Nov 2016
Great work, thank you so much! Only issue left is that printing it as a native chart (.eps, .pdf) leaves a lot of stripes around the borders. I do not now whether this is due to the polygon or to some printing properties?

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Srishti Gaur
Srishti Gaur on 2 Aug 2018
Dear Kelly Kearney I have a doubt, I am making a spatial plot (see image)
</matlabcentral/answers/uploaded_files/127390/Spatialplot.jpg>, Can I do something that this plot will cover my basin boundary only?

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