maps and isolines on the map of the continents

after having visualized the boundaries of the continents in the form of a map, I would like to create isolines within this map which identify and connect points with the same characteristics to each other. example: considering Naples as the center, I would like to create a line that connects the points that are located at a distance of 100 km from Naples, but only those on the continents not on the sea. an example would be lines 1-2-3, etc. in this figure

how can I do?

[darova]

Please attach the data. DO you have any attempts?

[ELISABETTA BILLOTTA]

I calculated the data to create the isolines .. they are a simple Excel table with latitude, longitude of the point and value at that point.

I thought about contour but I don't know how to enter the data.

Answer 1

[darova]

What about this?

t = 0:0.02:2*pi;            % angle
r = 1 + 0.1*cos(10*t);      % radius
[x0,y0] = pol2cart(t,r);    % calculate cartesian coordinates
u0 = diff(x0);              % component of x tangent vector
v0 = diff(y0);              % component of y tangent vetor
u1 = u0./hypot(u0,v0);      % normlizing
v1 = v0./hypot(u0,v0);      % normalazing
u1(end+1) = u1(end);        % copy last value (to make the size as 'X')
v1(end+1) = v1(end);        % copy last value (to make the size as 'X')
x1 = x0 + 0.3*v1;           % equidistant curve
y1 = y0 - 0.3*u1;           % equidistan curve
plot(x0,y0,'r')
line(x1,y1)
legend('Original curve','equidistant curve')

See also this: LINK