How to get cursor position in geodetic format on terrain map?

I have following code to draw terrain map

[A,R] = readgeoraster("n39_w106_3arc_v2.dt1", "OutputType", "double");

latlim = R.LatitudeLimits;

lonlim = R.LongitudeLimits;

usamap(latlim,lonlim)

geoshow(A,R,"DisplayType", "surface", "ButtonDownFcn", @get_cursor_position)

demcmap(A)

where

function get_cursor_position(varargin)

XYZ = get(gca, "CurrentPoint");

pos = [XYZ(1, 1), XYZ(1, 2), XYZ(1, 3)] % cursor position

assignin("base", "CursorPosition", pos); % save variable to workspace

end

I want to get the cursor position position in geodetic format whenever I click on the figure.

2 problems I have:

1. When I click any point on the map, I get a cursor position value, but not in geodetic format.
2. I get same height value at different locations where I am sure they have different height. get(gca, "CurrentPoint") does not work well.

Answer 1

[Suraj]

Hello Said

I understand that you are using Mapping Toolbox to read and display elevation data. You are trying to create a callback function that saves the latitude, longitude, and elevation from the last clicked point on the map to a variable in the base workspace.

Let me address the issues you have pointed out -

1. “When I click any point on the map, I get a cursor position value, but not in geodetic format.”

• The “CurrentPoint” property of “Axes” object returned by “gca” function contains X-Y-Z values relative to the axes of the plot.
• You can use the “gcpmap” function to retrieve the current point (the location of last button click) of the current “axesm”-based map in the form “[latitude longitude z-altitude]”.

2. “I get same height value at different locations where I am sure they have different height. get(gca, "CurrentPoint") does not work well.”

• The example you have attached plots the data as a surface map (2 dimensional) with the elevation data represented by a colormap, applied using the “demcmap” function.
• This means that “geoshow” is essentially rendering a 2-dimensional representation of the 3-dimensional data from the “.dt1” file.
• This is why “gcpmap” and “gca” functions return the same height value at all the points on the map.
• I would recommend you query the DEM data directly using “geointerp” function with the latitude and longitude points obtained from “gcpmap”.

Here is a code snippet that accomplishes the task using the above workarounds –

[A,R] = readgeoraster("n39_w106_3arc_v2.dt1", "OutputType", "double");
latlim = R.LatitudeLimits;
lonlim = R.LongitudeLimits;
usamap(latlim,lonlim)
geoshow(A,R,"DisplayType", "surface", "ButtonDownFcn",{@get_cursor_position, A, R})
demcmap(A)

function get_cursor_position(varargin)
     pt = gcpmap;
     lat = pt(1,1);
     lon = pt(1,2);
     z_altitude = geointerp(varargin{3}, varargin{4}, lat, lon);
     pos = [lat, lon, z_altitude];
     assignin("base", "CursorPosition", pos);
end

Please refer to the following documentation for more information –

1. “gcpmap” - https://www.mathworks.com/help/map/ref/gcpmap.html
2. “geointerp” - https://www.mathworks.com/help/map/ref/geointerp.html

I hope this helps.

Best regards,

Suraj.