Two colormaps for same worldmap axis

Hi,

I'm trying to attribute two datasets with the same range to two different colormaps onto one worldmap figure. I have tried every solution suggested on MathWorks, and none of them seem to work either 1) because I'm plotting onto a worldmap or 2) because the axes and ranges are the same. I saw one worldmap-specific solution, but it requires the image processing toolbox which I unfortunately don't have disc space for. Thank you in advance!

Answer 1

[dpb]

You can only set one colormap for one graphic object with a colormap property --

colormap(target,colormap)
...
  target — Target
    Figure object | Axes object | PolarAxes object | GeographicAxes object | graphics object
...

You will have to create two maps overlaying to do this and then you may (will?) have problems of opacity, quite possibly.

If we had any idea at all of the effect you're trying to create, somebody could possibly add more.

Answer 2

[Image Analyst]

Try this demo and adapt as needed:

% Make ramps so we have an image to work with.
rows = 300;
columns = 200;
values = linspace(1, 100, rows*columns);
img1 = repmat((1:rows)', [1, columns]);
% Make another one with the same range of values.
img2 = img1;

% Put both datasets into the same image.
% We need to make sure they don't share the same value range
% or else they will share the same colormap.
img = [img1, img2 + max(img1(:))];
imshow(img, []);
axis('on', 'image');

% Make colormap for the first half of the values
map1 = jet(128);
% Make colormap for the first half of the values
map2 = parula(128);
% Combine them
cmap = [map1;map2];
colormap(cmap);
colorbar;
title('Jet Color Map               Parula Color Map');

Note that the left half of the image uses the jet colormap while the right half of the image uses the parula colormap.

[Tiger]

I appreciate the response

The specific issue I've run into (if it helps) is I'd like to use demcmap to color topography in the background of a worldmap, then plot points according to a different colormap on top. Any colormap I define afterwards changes the topography colormap as well. Thanks!

[dpb]

Again, yes, unless you have two separate graphics objects, you can have only one colormap.

Or, the trick IA uses above is to scale the second set of data so it doesn't overlap the range of the first and then the colormap is computed based on those varying ranges. That's probably doable for a map dataset but somewhat of a pain I'd think.

There really is only one colormap in the above:

>> map1 = jet(128);
% Make colormap for the first half of the values
map2 = parula(128);
% Combine them
cmap = [map1;map2];
>> whos map1 map2 cmap
  Name        Size            Bytes  Class     Attributes

  cmap      256x3              6144  double              
  map1      128x3              3072  double              
  map2      128x3              3072  double              
>>

it's just comprised of two pieces; it's the scaling of the image data over two disparate ranges that results in the effect.

I don't have the mapping toolbox with which to play, specifically, about trying to overlay two axes.