How select/crop geographic region on the basis of lat/lon values?

I want to select/crop region on the basis of lat/lon.

I have geographical data with dimensions of lat,lon and time from which I want to crop a region using four lat/lon(e.g. LAT: 13,18; LON: 73,76) points like a rectangle.

I have attached my sample data for reference.

Can anyone help me with this problem?

Answer 1

[Pranavkumar Mallela]

Hi,

As per my understanding, you are trying to crop a region that is part of the sample data, which includes the 'rf' value over a 100x100 area and 48 time instances.

First use 'ncread' to read the 'rf' variable from the above attached file in the following way:

rf = ncread("test.nc", 'rf');

The variable 'rf' is a 100x100x48 double matrix. To crop a certain geographical region at a given time 't', you can use the following code:

cropped_region = rf(13:18, 73:76, t); % at a given time 't'

For more information regarding selection of data from a matrix, you can refer the following documentation: https://www.mathworks.com/help/matlab/math/array-indexing.html

Hope this helps! Thanks!

[Kamran Afroz]

Hi Pranav,

Thanks for your reply.

Actually, what you are suggesting is on the basis of index and I want on the basis of geographical grid points (Latitude and Logitude). The variable rf has only rainfall data on the dimensions of Latitude, longitude and time but what You have suggested is only giving me on the basis of these dimensions indexing.

To check Values of LAT/LON please check "lon" and "lat" variable in the data

[Pranavkumar Mallela]

Hey, thanks for clarifying that.

You can utilize the 'find' function to find the indices of the required latitude and longitude ranges.

The code is as follows:

lat = [1 2 3 4 5 6 7 8 9 10]; % assume this is your latitude data
lon = [1 2 3 4 5 6 7 8 9 10]; % assume this is your longitude data

% Set the required limits here
req_lat_ll = 13; req_lat_ul = 18;
req_lon_ll = 73; req_lon_ul = 76;

lat_ll  = find(lat <= req_lat_ll ,1,'last'); % ll = lower limit
lat_ul = find(lat >= req_lat_ul,1,'first'); % ul = upper limit

lon_ll  = find(lon <= req_lon_ll ,1,'last');
lon_ul = find(lon >= req_lon_ul,1,'first');

cropped_region = rf(lat_ll:lat_ul, lon_ll:lon_ul, t); % at a given time 't'

Now you should be able to access rainfall using latitude and longitude ranges.

For more information regarding the 'find' function, please refer to this documentation:

https://www.mathworks.com/help/matlab/matlab_prog/find-array-elements-that-meet-a-condition.html

Thanks! Hope this helps!

[Kamran Afroz]

Hi Pranav,

Thank you for your time and help!

It worked.