I have googled for a while and I can't seem to find the correct term for this.

Imagine a tunnel for trains, cars, or anything really. Along the tunnel there are various cracks, fissures, weak spots, etc. To visualize this on paper, the 3D structure must be projected in 2D. I'm looking for a word to refer to this kind of map.

Example diagram showing flaws in tunnel wall

The closest I've come up with is "projection map". Is there a more suitable term for this?

In Japanese it can be called tenkaizu (展開図, てんかいず), but its dictionary or Google translations don't fit since the term is also used for templates for folding paper.

  • You’re going to have to describe this picture much more clearly. – Jim Jul 24 '17 at 6:57
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    This is quite a specialised question, referring to an architectural, or surveyor's diagram. You perhaps need to consult a different site to this one. – WS2 Jul 24 '17 at 7:16
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    I agree with @WS2. We could make some suggestions, but the best word to use is the one that is actually used within the domain/industry in question. Convention is king in such cases. – Max Williams Jul 24 '17 at 7:55
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    Civil Engineering is a tag on engineering.stackexchange.com: engineering.stackexchange.com/?tags=civil-engineering – Max Williams Jul 24 '17 at 7:56
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    I'm voting to close this question as off-topic because it is outside our level of expertise and belongs to engineering.stackexchange.com, which it has apparently been asked in. – marcellothearcane Jul 24 '17 at 9:13

I got a response from the Engineering Stack Overflow site.

Here is the answer I got:

An engineering drawing that shows a curved surface "unwrapped" onto a plane is called a development.

This is commonly used in sheet metal working, where making the object follows the reverse process - you cut out the correct shape(s) on a flat sheet of material and then bend them into the correct 3-D form(s) and join them together.

See http://nptel.ac.in/courses/112103019/36 for some examples - google will find plenty more.

In engineering drawing, a projection is "what you would see if you looked at the object from a long distance away, through a telescope" which is not what your example shows.


  • The quote is wonky as a stand-alone statement, and not an answer to the question. – Phil Sweet Jul 25 '17 at 3:30
  • How else can I link it? I just copy pasted the same answer I got from the more specialized stack overflow site. Plus I added the question link as well. – Pochi Jul 25 '17 at 6:01
  • I think if you add the bolding from the original answer back in, that would make it clearer. If you can find a definition for "development" used in this way, that would help, too (I think there's one at the same website that is linked). – 1006a Jul 25 '17 at 6:57
  • @1006a ah, sorry i didn't notice the bolding had disappeared. Thanks! – Pochi Jul 25 '17 at 7:33
  • @Pochi My earlier comment wasn't meant as a critique of your post, only as a critique of the quote itself . Sorry for being unclear. – Phil Sweet Jul 25 '17 at 21:37

Notice that your example is using two separate 2D images to convey 3D information to the reader.

This very much reminds me of European projection:

enter image description here

Instinctively, I would have called it "a case of European projection".

However, Wikipedia lists a more generalized name:

Oblique projection

An oblique projection is a simple type of graphical projection used for producing pictorial, two-dimensional images of three-dimensional objects:

  • it projects an image by intersecting parallel rays (projectors)
  • from the three-dimensional source object with the drawing surface (projection plan).

In both oblique projection and orthographic projection, parallel lines of the source object produce parallel lines in the projected image.

Pedantically, the lines that are drawn on your example image are not parallel. However, please note that the page I linked refers to engineering drawing, which are often used as blueprints and need to be pedantically precise.
Your example image is precise on the left image (it notes dimensions etc.), but the right image seems to be a visualization, not a blueprint. Therefore, there is less need for parallel lines as the right image is not intended to convey precision.

If the image you linked is not representative of the whole tunnel, but rather a specific segment of it, the same Wikipedia page lists a second option:

Section views

Projected views (either Auxiliary or Multiview) which show a cross section of the source object along the specified cut plane. These views are commonly used to show internal features with more clarity than may be available using regular projections or hidden lines.

I think the image you linked exactly fits the definition of a section view, but I would instinctively refer to it as an oblique projection.

The reason for this is the meaning of the terms.

  • "Section view" reveals that it is a "slice" from a larger part.
  • "Oblique projection" focuses on the projection method that is used, rather than whether the shown object is a slice of a larger object or not.

What do you call a 2D map that shows the characteristics of a 3D train tunnel?

The most accurate answer to this question would be "oblique projection", as your generalized question also allows for images that are not section views (but are still oblique projections!)

  • You seem to have misunderstood. The right hand image is a section or elevation. The non-parallel lines are to show how the left hand image (which is where the detail is) lines up with the real features. Your "European/Oblique" projection is completely wrong: that is a series of 2D views from different directions; OP's drawing is specifically not an actual view that can be seen from any actual direction, it is a representation. – AndyT Jul 24 '17 at 14:47
  • @AndyT: Then the OP's own example of the Japanese is not correct. When googling the japanese 展開図, these images consistently pop up, which are examples of oblique projection. (I omitted the other results as OP explicitly excluded the folding of paper in regards to 展開図). – Flater Jul 24 '17 at 15:13

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