MATLAB Answers


Changing the colour intensity of a 2D plot

Asked by John
on 25 Dec 2012


I'm plotting lat long data on a map like this.

I'm using the plotm() function:


I'm trying to change the colours so that areas with more data are a different colour. I would like the intensity of the colour to change with the amount of data points.

Would anybody know how I could achieve this?




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2 Answers

Answer by Walter Roberson
on 26 Dec 2012

plotm() cannot have different colors within any one line (column of data).

There are tricks for using patch() for drawing very narrow lines.

It appears to me that your data is sufficiently dense that you could use scatterm() and have the output look relatively continuous. scatterm() accepts a matrix of colors, one RGB row per point.


Hello Walter and Image Analyst,

Walter, thanks for your help, the change you suggested to rgbarray changed the color intensity slightly but the majority of the points are black with a bit more dark blue. I was hoping to get the colors more variable as it is difficult to distinguish between he two.

Image Analyst, thanks for your suggestion, I will try your code and will post later with the results.

All the best

What I think I would suggest at this point is to use interp1() on relative_counts, using a look-up table that emphasized the low values but still had high values. For example,

mapval = interp1( 0:.1:1, [0 .3 .5 .7 .725 .75 .775 .8 .825 .9 1], relative_counts);
rgbarray = [zeros(length(x), 2), mapval(:)];

You would adjust the first parameter (possibly unevenly) according to where you wanted the intensity boundaries to be, and would adjust the second parameter according to the percentage blue you wanted each range to show up with.

Hello Walter,

I tried your new suggestion. Unfortunately the points were all black, similar to the image posted below on Image Analyst's post. When you say 'adjust the first parameter', do mean, 0:.1:1? What would you suggest I change it to? Apologies for all the questions but I don't really understand the code and I'm not sure on how to proceed.

Thank you

Answer by Image Analyst
on 26 Dec 2012
Edited by Image Analyst
on 13 Jan 2013

If you treat it as an image you could do that, with some work. You could run the thing through a convolution, conv2(). This would give you a higher signal where the roads are more dense. Essentially that is an indexed image. Then you apply a colormap to it, and call ind2rgb to turn it into a color image. Then you use the roads as a mask over the color image so that roads in concentrated areas show up as a different color than roads in sparse areas. Hopefully that described it enough for you to carry out the code. Though I think this representation would be very confusing and distracting to look at.

for k = 1 : length(x)
  yourImage(round(y(k)), round(x(k)) = 1;
densityImage = conv2(yourImage, ones(5)/25, 'same');
imshow(densityImage, []);


Can you upload the image? The fig file is no good to me. But I'm not sure how the x,y would correspond to pixels. I don't have the mapping toolbox so I'd probably have to calibrate the x,y to pixel row,column locations.

Hello Image Analyst,

I am not using the mapping toolbox to source the image. I am using this file on the file exchange to retrieve the map and then I just overlay the data on it. I code I use is below:

 latlim = [53.0 53.6];
 lonlim = [-6.5 -6];
  axis([lonlim, latlim])
  axis image
  hold on

The x and y data (GPS) is in this file.

Would your method be possible with this setup?

Thank you

An example:

Hello Walter and Image Analyst,

Just wondering if you have had any further thoughts on this?


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