Creating a grayscale image from ascii file

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hendra kurnia febriawan
评论： hendra kurnia febriawan 2018-9-16
Hi all,
Could anyone please help me how to create a grayscale image (map) from an ascii file (the file contains three columns: X coord, Y coord and intensity value). So the result of grayscale image will have a georeference coord and the grayscale represents the intensity value. Thank you in advance.
Henz.
I attached the small piece of the file as well from the total 300mb. The grid size is 5 cm and I was thinking to re-gridding with 10 cm or 15 cm of grid size, so the processing time would be not too long. But I don't know how to do that? I really appreciate your help. Thank you.
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Stephen23
Stephen23 2018-9-16
@hendra kurnia febriawan: are the data scattered or gridded?:
https://www.mathworks.com/help/matlab/math/interpolating-scattered-data.html
https://www.mathworks.com/help/matlab/math/interpolating-gridded-data.html
It would probably help if you uploaded the data file by clicking the paperclip button.
hendra kurnia febriawan
hendra kurnia febriawan 2018-9-16
编辑：Walter Roberson 2018-9-16
Hi Stephen,
Thanks for the links. The ascii file has spatial spacing 5 cm, so it is a rectangular grid. The total file is very big (217 mb). I tried to assign the 1st row as x, 2nd row as y and 3rd row as z.
M = dlmread('file.txt',',');
x = M(:,1);
y = M(:,2);
z = M(:,3);
[X,Y,Z] = meshgrid(x,y,z);
But it showed following warning.
Error using repmat
Requested 493606x493606x493606 (896048273.5GB) array exceeds
maximum array size preference. Creation of arrays greater
than this limit may take a long time and cause MATLAB to
become unresponsive. See array size limit or preference panel
for more information.
Error in meshgrid (line 77)
xx = repmat(xx, ny, 1, nz);
Any suggestion for that?
Thanks.

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回答（1 个）

Walter Roberson
Walter Roberson 2018-9-16
M = dlmread('file.txt',',');
x = M(:,1);
y = M(:,2);
z = M(:,3);
ux = uniquetol(x);
uy = uniquetol(y);
Z = reshape(z, length(ux), length(uy));
You might need
Z = reshape(z, length(uy), length(ux)) .';
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Stephen23
Stephen23 2018-9-16
编辑：Stephen23 2018-9-16
@hendra kurnia febriawan: some of the values are missing. You can check the sizes yourself:
  • ux has size 429x1
  • uy has size 1253x1
Now lets calculate how many Z values that would give:
>> 429*1253
ans = 537537
Does M have 537537 rows? No. Actually you have 493606 data values for each of X, Y, and Z. There is no way to reshape a 493606 element vector into a 537537 element matrix. I suspect that you will have to resample/interpolate your data, to get the gridded data that you require. I would love to help you, but your sample file simply repeats the same X and Y values like this (I copied the data verbatim):
4.063e+05,6.474e+06,75.828
4.063e+05,6.474e+06,75.982
4.063e+05,6.474e+06,75.443
4.063e+05,6.474e+06,75.029
4.063e+05,6.474e+06,74.431
4.063e+05,6.474e+06,74.651
4.063e+05,6.474e+06,74.873
4.063e+05,6.474e+06,76.198
...
and I doubt that that represents the real data values (are they really only stored to four/five significant figures?). Please:
  • create a file suitable for uploading,
  • check the file has useful values (not just 4 sigfig),
  • edit your question,
  • upload the file by clicking the paperclip button.
hendra kurnia febriawan
hendra kurnia febriawan 2018-9-16
Hi Stephen,
Thanks for the comments. I apologize for the previous data since I only simply extracted and took a piece of row from the original file without checking first. I have re-uploaded the data. Actually, the grid resolution is 5cm so you can notice the values are only different after the comma since it shows in decimal meter.
Thank you.

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