Is is possible to scale a photograph and rely on dimensions?

Hi - I have a photograph of a house and measurements of the porch columns, windows, etc. When I scale it some of the measurements are correct while others are way off. The photo is angled a bit but I am only concerned with the porch. What am I doing wrong?

 

You can rely on dimensions somewhat. Problem is, when you draft in AutoCAD you do so in an orthographical representation. Pictures are perspective representations so it's extremely difficult, and some would say impossible, to get true hard dimensions.

 

There are ways around this, though. You can reference something that is a known dimension (stair, door, etc), align a 3D perspective view, then use Solids to create "masses" that align with the picture. This is a technique most of your advanced 3D modelers are familiar with.

 

Either way, don't expect to get down to the eight inch of accuracy or anything. Even down to the inch will be hard in most if not all areas of the photo.

The window size comes out great but the length of the porch is about 3 feet larger in cad and the porch post 1.5 feet taller. I thought I would be a little closer in scaling. I guess that angle really messes it up.

You are too close to the building when taking the picture.

Move much further back from the building so that the image appears flatter, more 2d like.

Experiment with zoom. (I forget whether you want to use zoom or not.)

You also appear to be below the level of the building - try to get higher to take your shot.

You want to minimize perspective and parallax error.

Of course, in the end the picture will just be reference.

With a digital camera and the right program you could photograph the house from several vantage points and create a 3D model of it. From that model you could extract the required 2D drawings.

Measuring from the outside edge of each post i come up with the following dimensions

2.2780 + 2.0867 + 1.9824 = 6.3471 total across the length of the porch.

Is this anywhere close to what you are getting?

 

I know that this is not correct but was wandering what you came up with

To get dimensions to eighth inch accuracy, you must take a series of photos to build up a stereoscopic image. Using reference markers, then you can perform photo-rectification. THEN you can get all the dimensions.

 

Or get a laser scanner to give you a point cloud.

 

You cannot get any consistent dimensions from a single photo.

These dimensions "2.2780 + 2.0867 + 1.9824 = 6.3471 total across the length of the porch" don't mean much of anything. The porch isn't 6.3 inches or even 6.3 feet wide. I think that would be obvious just by looking at the door.

Kenny,

 

I took your picture and popped it into a small program that I have called ShiftN. This program flattens the plane of the picture taking out any perspective. Perhaps now with this rectified photo you can scale it down in AutoCAD and trace over the columns on the porch.

Maybe this may get you headed in the right direction.

 

BTW...beautiful house!

It will still be next to impossible to get accurate dimensions tracing over a photograph.

It will still be next to impossible to get accurate dimensions tracing over a photograph.

 

Well if he (kenny) has measurements of the porch columns like he posted at first, he should be able to scale the rectified photo down to a scale roughly and trace over that. It's not 100% accurate, but he will come closer to a result than by using his original photograph.

I guess it all depends on what he plans to do with this drawing.

Take a look at Autodesk 123D catch.

Well if he (kenny) has measurements of the porch columns like he posted at first, he should be able to scale the rectified photo down to a scale roughly and trace over that. It's not 100% accurate, but he will come closer to a result than by using his original photograph.

Yes, if he physically took the picture and could accumulate some real-world dimensional benchmarks, he could make out pretty well. Even still, it would be near impossible to get true dimensions. One could definitely recreate the imagine in CAD as an artistic reference.

 

Take a look at Autodesk 123D catch.

 

WOW! That looks impressive!!

The porch is 270 inches in real life and the post 11 ft. Yes the perspective really messed it up as the house is on a hill and difficult to get a straight forward photo. If I scale it to one measurement the other is always off.

 

If I had taken a perfectly straight on photo and scaled it, should the dimension of the porch column and the dimension of the window be accurate - even thought the window is set back? I am confused that it is 2D - yet perspective comes into play.

What are you trying to accomplish with all this?

It won't work, imagine been stood on a football field, directly behind the goal, the far goal is tiny in comparison. I have scaled off photos before, but you have to be aware of the limitations. It's inaccurate but if you have done a survey and forgotten to measure a couple of places then you can get a guesstimate if you have known sizes on the same plane, it all depends on what accuracy you require. For example going back to the football field, if you measured the near goal height and width, but only measured the far gaol width, then you could scale off a photo for the far goal height. Programs like 123D work by using photos from various angles and can be really accurate if done properly.

I am confused that it is 2D - yet perspective comes into play.

Check out some of these Google Images: https://www.google.com/search?q=orthographic+vs+perspective&source=lnms&tbm=isch&sa=X&ei=CRCmUfn1JKHtygGAmYGYBw&ved=0CAoQ_AUoAQ&biw=1126&bih=742

 

Our eyesight and lenses on cameras capture images in a perspective view... which has depth. Notice on perspective views on a box in any of the images I linked above, the farthest most outline is not the same dimension as the closest, even though they are in fact the same distance. Orthographical views take away depth, so that the farthest and nearest outlines dimension exactly the same once a dimensional tool is applied from whatever authoring software you use (in this case, the DISTANCE command in AutoCAD, or a DIMENSION in AutoCAD, etc).

 

The picture you have has captured a perspective view, not an orthographical view of this house. Hope this helps the confusion.

 

WOW! That looks impressive!!

 

I don't think it really does exactly what you are after.

I think it just creates a 3D reference image, you will still have to create geometry.

It is pretty impressive though!