How can I assign a specific length to an image?

[sofasurfer]

I load an image of a truck into Gimp. Gimp's 'scale image' setting shows that the image (the entire image, border to border, not just the  actual truck image) is 297 pixels wide or 2.5 inches wide.

In real life this truck is 227.7 inches long. I want to use the Gimp 'measure' tool to measure the truck in the image and I want it to show that the truck is 227.7 inches long. If I zoom in or out of the image I still want to be able to measure the truck at 227.7 inches.

My reason is that I want to design an accessory that is a given length in real life and measure it in the image as being that length and then merge the image of the accessory with the image of the truck and have it fit properly. I want a consistently 227.7 inch long truck.

I did crop the truck image bumper to bumper and then used the 'scale image' tool to resize the truck to 227.7 inches. I then merged it onto a larger canvas. Now the truck measures 2.something inches again.

What is the proper procedure I should use to accomplish this task. I'm pretty new at Gimp so I am guessing this is my problem.

[PixLab]

(12-02-2021, 12:40 AM)sofasurfer Wrote: I load an image of a truck into Gimp. Gimp's 'scale image' setting shows that the image (the entire image, border to border, not just the  actual truck image) is 297 pixels wide or 2.5 inches wide.

In real life this truck is 227.7 inches long. I want to use the Gimp 'measure' tool to measure the truck in the image and I want it to show that the truck is 227.7 inches long. If I zoom in or out of the image I still want to be able to measure the truck at 227.7 inches.

My reason is that I want to design an accessory that is a given length in real life and measure it in the image as being that length and then merge the image of the accessory with the image of the truck and have it fit properly. I want a consistently 227.7 inch long truck.

I did crop the truck image bumper to bumper and then used the 'scale image' tool to resize the truck to 227.7 inches. I then merged it onto a larger canvas. Now the truck measures 2.something inches again.

What is the proper procedure I should use to accomplish this task. I'm pretty new at Gimp so I am guessing this is my problem.

On the top menu Image ➤ Scale image...
Then change pixel to inchesand input your 227.7 inches (keep the link (chain) closed to keep the proportion)

   

Then prepare yourself for few surprises...
Image will be absolutely huge (see yellow arrow), thus your computer might not follow the resources needed -> GIMP can become very slow
The scale will not be as precise as you expect as you also need distance from the object the picture was taken, the lens specification as well as the sensor size which took the picture (but yes you could do it without it)
and certainly more surprises like pixelisation and so

[Blighty]

Gimp is bitmap editor and works in pixels.

You need to understand the relation between pixels, inches and resolution. Take time to read and understand this:
https://www.gimp-forum.net/Thread-Image-size-in-Gimp