rectangular aspect ratio

[gimp-artist]

I downloaded a picture, which is too big. I selected only what I needed by using rectangular tool. I wanted the selected part to be a square. I adjusted the tool option to aspect ratio. Then I clicked down a certain point with the mouse, and pressed shift button on keyboard while dragging my mouse. I had successfully selected a square. I copied the selection, and pasted it in a new image. I pressed "shift+t" for reducing the pasted floating layer.

Then here is my issue. The scale tool doesn't recognize the initial object that needs to be reduced as a square. It is a rectangular with different ratio. I think I copied a perfect square. Then why do I have a rectangular after I pasted it on another image?

[Ofnuts]

Image>Print size are the horizontal and vertical resolutions the same?

[Blighty]

(06-30-2018, 09:27 AM)gimp-artist Wrote: Then I clicked down a certain point with the mouse, and pressed shift button on keyboard while dragging my mouse. I had successfully selected a square. I copied the selection, and pasted it  in a new image. I pressed "shift+t" for reducing the pasted floating layer.

When you use the rectangle select tool there are 4 modes in the tool options. Replace, Add, Subtract, Intersect. When you Shift-Drag you are setting the mode to Add. ie the current selection will added to any active selections.

With the floating selection there are 2 options: To New Layer, or Anchor (Anchor merges with the current layer)

Shift+t = Unified Transform Tool

[rich2005]

Usually as Ofnuts post, there is a mis-match between vertical and horizontal resolutions in the picture you downloaded.
but
There should be plenty of indications pointing to that Image size. The way the thumbnails look.... 

For Gimp 2.8 Some things to check

Have you been messing with View -> Dot-for-Dot? You can get something like this:

   

With Dot-for-Dot off the image looks correct. Except for the image size which does not. 
The selection looks like a square, aspect ratio is 1:1, fixed is ticked. No good doing this after the selection is made, do it before. However the giveaway is the size in the tool options.
You can check sizes and resolutions in the image properties Image -> Image Properties

Copy then Paste into a different image. 99% probability this will have Dot for Dot ON and a resolution with H and V equal.

   

Shift-T invokes the scale tool in Gimp 2.8 You can break the link between Width and Height and set equal values.

Gimp is a raster editor, a pixel is a pixel and in Gimp copied / pasted pixels take the properties from the destination image.

[gimp-artist]

Sorry guys. This is my mistake. I selected a square, but a little bit of height part was out of the layer. I relied on the size indication in the rectangular tool option that was a square. Therefore, I thought I selected the square, but the selection was rectangular since the selection is only valid inside of the layer.
This confusion was occurred from the difference between the crop tool and rectangular tool. While the crop tool can include the outside part of the layer after cropping, the rectangular tool excludes the outside part of the layer.
Thanks for the dot for dot explanations. I appreciate why you guys were concerning the dot for dot function. Since I don't care about the real world print size, I simply always turn on the dot for dot option.
Thanks guys, you are helping me a lot. Have a nice day~