How to get accurate dimensions from an image? (fix distortion)

Question

As a sign maker, for estimating I can usually interpolate sizes from an image, the sizes are enought to get rough costing for materials etc and then if the job goes ahead then I take precise measurements. Most of the time I don't carry a ladder nor does my measuring tape hold still enough to get the dimensions I need. So I usually make proxy measurements and interpolate them.

I wondered if there is a way to make this process more accurate.

For example in the image below, The red dimensions are 550mm wide, and come out correct on the lens corrected image (give or take 5mm).

The blue dimensions should be 1785mm but the perspective distortion in the image is showing at as ~1900mm, meaning the green dimensions are also distorted.

Is there a way to lens correct or perspective correct in image, something that maybe stretches the image more towards the edges than in the middle? I am using photoshop, but if there is another tool/software/plugin please advise.

I have attached the original, unedited image as well.

Answer 1

This isn't actually a graphic design issue as such.

First things first, cameras are not absolute measuring devices. This means that cameras do not know the scale of what you see. So to measure from a camera image you must know the size of something on your measurement.

This can be as easy as using a ruler from one measurement to another and write it down. To putting something known in the scene, like a ruler or grid that is on your measurement plane.

Image 1: Picture in a camera can not account for scale. As a smaller scene just looks same.

Second thing is to remove camera distortion. While adobe raw knows some lenses generic distortions characteristics. Its better to measure this, all you really need is a picture of a checkerboard.

Anyway. This is where i will veer of in a different direction and say that you don't want to rely on a picture like this for measurement. Why? Well for nearly the same effort you can do a 3D track/photogrammetry. That is ultimately way better.

Reasons being:

• They are designed to be used for measurement by people who do measurement. While image editors are made to to make good looking images and measurement is a secondary purpose if at all.

• Due to their nature distortion removing is baked into the software. So its well documented how to do it and the solver can deduce the distortion even without knowing it.

• It can completely eliminate the estimate now measure accurately later. With a little practice you can get a better measurement than you can by hand the first time.

• It measures reality, not just a 2D slice of it. So you later figure out that youd need to know this one distance in depth. No problem you captured it allready.

  Maybe the wall isnt on one plane? Uneven or curved? No problem.

• Sometimes theres not enough space to do the photograph in one go because there is not enough room to place the camera, you need to go around a corner etc. This is no problem for a track or photogrammetry they rely on multiple sources and can accurately stich sources together.

• Plus it allows you to do kickass mockups in a way most other methods can not.

• Since this kind of stuff is also used in real measurement infrastructure the software may also calculate the error. So not only is the measurement accurate but you actually know how accurate.