Use m_ map draw evenly spaced lines

If an image is divided into 300×300 points, and the data of each point is known. The x-axis and y-axis coordinates of the image are longitude and latitude. How to correspond the points with longitude and latitude, and how to connect the points of the same data. Draw evenly spaced lines at 20 intervals.

Answer 1

[Matt]

Hi @ke,

To create the graph you described, you can use meshgrid to align your grid data with longitude and latitude, and m_contour to draw contours at regular intervals. Here’s a step-by-step outline of the process::

• Generate longitude and lattitude grids

lon_min = ...; % e.g., 100 
    lon_max = ...; % e.g., 110 
    lat_min = ...; % e.g., 20 
    lat_max = ...; % e.g., 30 
    
lon = linspace(lon_min, lon_max, 300); 
lat = linspace(lat_min, lat_max, 300); 
[Lon, Lat] = meshgrid(lon, lat);  % size(Lon) = [300, 300] 

• Setup "m_map" projection

figure 
m_proj('mercator', 'lon', [lon_min lon_max], 'lat', [lat_min lat_max]); 
m_grid('box','fancy','tickdir','in'); 
hold on 

• Draw evenly spaced contour lines

interval = 20; 
min_val = floor(min(data(:))/interval)*interval; 
max_val = ceil(max(data(:))/interval)*interval; 
v = min_val:interval:max_val; 

[~, h] = m_contour(Lon, Lat, data, v, 'k');  % 'k' for black lines 
clabel(_, h);  % Optional: Add contour labels 

Refer to the following documentations to know more about the functions used:

• countour: https://www.mathworks.com/help/matlab/ref/contour.html
• meshgrid: https://www.mathworks.com/help/matlab/ref/meshgrid.html