How to add a left pointing arrow with annotation to a plotted DEM in MATLAB?

I have been trying to add a left pointing arrow with the label 'Mt Everest' (both in white) to an extracted portion of a DEM. This is the code I have to get to this point, however the arrow and text does not appear when I run the script despite the rest of the properties working.

% extract_mt_everest_dem

%% Load the Mt Everest DEM data

[DEM, DEM_R] = geotiffread('aster_gdem_mosaic_UTM_crop.tif');

DEM_info = geotiffinfo('aster_gdem_mosaic_UTM_crop.tif');

% Manage NoData cells to make then NaN

DEM(DEM<=0) = NaN;

% Extract the lat and lon values for each cell within the data

% Create an index variable

idx = 1:length(DEM(:));

% Get the indices for each cell in the DEM

[I,J] = ind2sub(size(DEM),idx);

clear idx

% Convert the cell indices into projected coordinates

[x,y] = pix2map(DEM_info.RefMatrix, [I(:) J(:)]);

clear I J

% Reshape the x and y vectors to a grid the same size as the data

x_grid = reshape(x,size(DEM));

y_grid = reshape(y,size(DEM));

% Calculate the lat and lon values from the projected coordinates

[lat,lon] = projinv(DEM_info, x(:), y(:));

clear x y

% Reshape the lat and lon vectors to a grid the same size as the data

lon_grid = reshape(lon,size(DEM));

lat_grid = reshape(lat,size(DEM));

clear lat lon

%% Choose the point of interest (POI): Mt Everest

mt_everest = [27.986065 86.922623];

[mt_everest_X, mt_everest_Y] = projfwd(DEM_info, mt_everest(1), mt_everest(2));

%% Calculate distance from Aoraki Mt Cook

x_diff = x_grid - mt_everest_X;

y_diff = y_grid - mt_everest_Y;

% Find the indices of the bounding box of cells within a specified ditance of 20x20 km

max_dist = 10000; % (m)

row_start = find(abs(y_diff(:,1))<=max_dist, 1);

row_end = find(abs(y_diff(:,1))<=max_dist, 1, 'last');

col_start = find(abs(x_diff(1,:))<=max_dist, 1);

col_end = find(abs(x_diff(1,:))<=max_dist, 1, 'last');

% Extract the ROI from whole array using indexing

DEM_ROI = DEM(row_start:row_end,col_start:col_end);

lat_grid_ROI = lat_grid(row_start:row_end,col_start:col_end);

lon_grid_ROI = lon_grid(row_start:row_end,col_start:col_end);

%% Make a map to display the subset data

% Set the font size

font_size = 18;

set(0, 'DefaultAxesFontSize', font_size);

% Make the figure the size of the full screen

figure('units','normalized','outerposition',[0 0 1 1]);

% Set the backrgound colour to white

set(gcf,'color','w');

% Make a projected map axis using worldmap

ax = worldmap([nanmin(lat_grid_ROI(:)) nanmax(lat_grid_ROI(:))],[nanmin(lon_grid_ROI(:)) nanmax(lon_grid_ROI(:))]);

% Set axis font size to 18

setm(ax,'FontSize',font_size);

hold on % to allow for overplotting

pcolorm(lat_grid_ROI, lon_grid_ROI, DEM_ROI);

cmap = landcolor; % get the landcolour colour map

colormap(cmap); % use the landcolour colour map

shadem; % shade the DEM to highlight the relief

hbar = colorbar('northoutside','fontsize',font_size); % create a colourbar

set(get(hbar,'Xlabel'),'String','Elevation (m)'); % add a colourbar label

% Add a scale bar

scalebar('color',[0 0 0],'FontSize',font_size,'location','southeast');

% Add a label showing Mt Everest with a left-pointing arrow

b = textm(mt_everest(1), mt_everest(2), 4000,'\leftarrow Mt. Everest','FontSize', font_size,'Color',[1 1 1]);

uistack(b, 'top');

close

Anyone know how to get my arrow and label to appear?

Answer 1

[Abhinaya Kennedy]

Hi Adele,

The issue with your code for adding the arrow and label is twofold:

1. Color: Currently, you have set the color of the text and arrow to white ('Color', [1 1 1]'). Since your background is also white, the text and arrow become invisible.
2. Third argument in "textm": The "textm" function takes optional arguments. The "4000" you provided is likely causing an issue.

Here's the corrected code snippet for adding the label:

b = textm(mt_everest(1), mt_everest(2), '\leftarrow Mt. Everest','FontSize', font_size,'Color', 'black');
uistack(b, 'top');

You can adjust the position of the label using the first two arguments of "textm". These represent the latitude and longitude coordinates. Consider using a different font style for the arrow using the "FontName" property in the "textm" function (e.g., 'Symbol'). This might provide a clearer left-pointing arrow symbol.

You can also take a look at the documentation for more information on "textm":

https://www.mathworks.com/help/map/ref/textm.html