Matlab Image and Plot with Unexpected Flip

Question

In the examples below, there are some obvious code differences in each case, but I don't understand how they would change the orientation of the image or the plot, as shown in each case.

The original image has a size of 407 x 813 x 3.

For the data in the contour, X ranges from -180 to +175, Y ranges from -85 to +85

For the image, I am trying to fit it so that X ranges from -180 to +180, Y ranges from -90 to +90. So the image covers a slightly wider geographic range than the image does.

I have a contour plot of some geographic data. It plots like this, and looks like I expect it to look:

The code to plot this looks is

figure (1);
contour(X, Y, dec0Mat,contoursAt, 'ShowText','on', 'LineWidth', 2);
colormap('Gray');   

I can plot the image by itself, and it looks ok:

using this code: figure (99) imagesc([-180 180], [-90 90], worldMap);

But when I try to combine the two, I get this:

Using this code:

figure (1);
image([-180 180], [-90 90], worldMap);
hold on
title('Declination at elev = 0 km');
contour(X, Y, dec0Mat,contoursAt, 'ShowText','on', 'LineWidth', 2);
colormap('Gray');

In this case the map looks right, but the contour is flipped vertically.

In another case, I get this:

Using this code:

figure (3);
hold on
title('Declination at elev = 30 km');
imagesc([-180 180], [-90 90], worldMap);
contour(X, Y, dec30Mat,contoursAt, 'ShowText','on', 'LineWidth', 2);
colormap('Gray');

In this case the map is flipped vertically and the contour is plotted right.

Going by the Matlab documentation for imagesc, it says:

imagesc(x,y,C) displays C as an image and specifies the bounds of the x- and y-axis with vectors x and y. If x(1) > x(2) or y(1) > y(2), the image is flipped left-right or up-down, respectively. If x and y are scalars, the image is translated to the specified location (x,y) such that the upper left corner of the image starts at (x,y).

I highlighted what might be relevant for an image being flipped, but neither seems to be the case here and wouldn't explain the inconsistency where is is flipped in one case and not the other.

Answer 1

The explanation is that the imagesc command, like image, sets the 'Ydir' axis property to 'reverse'.

When called with C or X,Y,C, image sets the axes limits to tightly enclose the image, sets the axes YDir property to 'reverse', and sets the axes View property to [0 90].

This means that the vertical axis values increase from the top of the axis to the bottom.

Now you can compare the two cases:

1. If you run the contourf command by itself, you have the "normal" axis mode, with vertical axis values increasing from the bottom of the axis to the top. The vertical axis labelling in your first figure reflects that.

2. If you plot the image with imagesc and run contour on the same figure, the vertical axis is first flipped by imagesc. The subsequent contour command operates on a flipped vertical axis, and plots accordingly. That's why the contour lines are vertically flipped with respect to case 1.

Note that the combined figure obtained in case 2 is correct. If you "visually" combined the first two images of your question (which were obtained from calling imagesc and countour independently) it would be wrong, because they have different vertical axes.

Answer 2

This is not exactly a problem of rotation, but more a problem of vertical flip.

If you look carefully at your first two plots, you'll see that the vertical scales are flipped, so if you combine your two plots directly (whatever the way) you will end up with what you observe, i.e. one plot that is flipped with respect to the other.

I would suggest to flip the contour plot before superposition:

hold on
image([-180 180], [-90 90], worldMap);
title('Declination at elev = 0 km');
contour(X, Y, flipud(dec0Mat), contoursAt, 'ShowText','on', 'LineWidth', 2);
colormap('Gray');

or

hold on
image([-180 180], [-90 90], worldMap);
title('Declination at elev = 0 km');
contour(X, -Y, dec0Mat, contoursAt, 'ShowText','on', 'LineWidth', 2);
colormap('Gray');

Best,