help a historian out?

March 16, 2006 12:49 PM

PhotoshopFilter - I need to bring two map images together, one of a map, the other of a map overlay that was traced on top of the map. Is there a trick to somehow automate this in photoshop?

These maps and overlays are at the national archives (WWII stuff), and of course when they arrived there the maps were sent to one section of the building, and the overlays sent to another. So I can't just "place the overlay on top of the map to see what the markings mean." (Ah, the archives...) So I take digital photos of each and then spend an hour or two trying to get them properly lined up and re-sized, transparent-ing the overlay a bit...and it never comes out very well.

I need to do this dozens of times a day for work, and I'm thinking there *has* to be a straightforward way of merging these images. There are hash marks in two places on the overlay that are used to align the two. Is there a way to simply ping these marks on the two maps and align/resize the images to match?

Help me photoshopheads, you're my only hope...

posted by garfy3 to computers & internet (10 comments total)

Fun stuff. Sounds like you're doing exactly what I would have done in that situation. Can you set up the camera a set distance from the map, and the same distance from the overlay so your images are more equally sized?
posted by Alpenglow at 1:07 PM on March 16, 2006

I could...if I had a portable tripod. But I turn out to be pretty good at snapping a shot 3' above the map. But in some cases, it gets even more complicated, where I need to take 6 shots of a large map, photoshop them together, *then* do the overlay-ing. Surely someone has encountered this before and beat photoshop into submission?

(photoshop sticks tongue out at me).
posted by garfy3 at 1:29 PM on March 16, 2006

If this is something you need to do dozens of times a day, maybe you can get your hands on a tripod,

I'm not really sure where the sticking point is here--are you having trouble matching the "panorama" pieces (color issues, distortion)? Do you need to know how to layer things and change their transparency? Is the issue with the piece resizing?

It seems to me that this is a pretty specialized kind of request--you can probably record actions to help with some aspects of it, but mostly it's the kind of grunt work you need to do by hand. Especially if the quality and "matchability" of the source material is going to vary greatly from project to project.

How, more specifically, are you matching up the pieces now? I would measure the alignment marks in pixels and then do the math to figure out the resize percentage to make them match. Then place one picture in a layer, set the transparency to something appropriate, and line them up by dragging. If the overlay were on white(ish) onion skin-type paper, I might use a plugin like Peel Off White to enhance the printed part of the overlay and deemphasize completely the white background. I can't see this taking very long, so I'm a little unsure exactly where you need help. Any clarifications you can offer?
posted by bcwinters at 1:49 PM on March 16, 2006

transparent-ing the overlay a bit

Doesn't help with your alignment issue, but have you tried just setting the blend mode of your overlay layer to Multiply?
posted by designbot at 2:01 PM on March 16, 2006

If these shots were taken freehand, and especially if this involves compositing several shots together to form a larger image, then I suspect that you have have serious perspective correction that needs to be done first. This referrs to making sure that both maps are indeed truly "square" before you even hope to start aligning one feature of one to the other. Panorama software has this feature, so if you are manually stitching images you should definitely switch over to one of these programs for assembling shots.

It would help if you had some frame of reference in each shot that is a straight edge that you can use for calibration. Is there an edge in the shot -- either the edge of the sheet or some kind of object (like a long ruler) -- that you can use for this? The idea being that you would free transform the image before trying to merge the two maps so that the edges are all straight so that you are sure there is no remaining distortion.
posted by Rhomboid at 2:10 PM on March 16, 2006

bcwinters: Peel Off White is new to me, but sounds like a great tip! Also great idea on doing the math on the pixels, that could save some steps in the re-sizing bit. (historians + math = trouble).

So yes, the key issues are the multiple re-sizings, and the getting-the-transparency-to-be-transparent. I think you've pointed out a couple of efficiency-enhancing moves to make. Same with the 'recording actions' bit--I'll need to figure out which steps to record.

(the other thing is getting tripods in/out of security is generally a mess)
posted by garfy3 at 2:12 PM on March 16, 2006

What you really need is a GIS or Remote Sensing program. There are free stuff out there but I don't know there names or how hard they would be to use. The professional software packages are ESRI's Arc products and Lecia's Erdas Imagine.

The absolute ideal would be to have the maps scanned in a large format scanner. Then use the for mentioned software packages to projected the map to the correct map projection and rubber sheet the overlay to match the map. unfortunately, this can be a fairly time consuming project. However there are some firms that specialize in this type of work.
posted by kashmir772 at 7:29 PM on March 16, 2006

On a second thought, Since it sounds like this government stuff... this may have already been done. Ask around, they may have digitzed versions of the data in a corner somewhere....
posted by kashmir772 at 7:35 PM on March 16, 2006

An architecture shop near me will scan large-size b/w images for something like $1.25 for a 2'x3' image. They've even emailed it to me so I don't have to pay the cost of burning a CD. Prevents the perspective issues and also the color shading due to even-slightly-uneven lighting.

Otherwise, yeah, I'd be doing math to automatically resize them too. The counting-pixels idea is pretty cool.
posted by salvia at 10:47 PM on March 16, 2006

Oh, you can't take the maps out of the archives. Sorry, guess scanning is out.

Another thing I might try if I was having trouble aligning two images: make a rectangle of the same size as a new layer in each image (take the Rectangular Marquee tool, upper left of tools palette, set its style to Fixed Size, then in each image, set a separate layer, make a rectangle and stroke it). Then you could resize &/or rotate each map so the hash marks match the corners of the box. This way, you know they're the same size and orientation, and once you put them in the same image, you just have to align them.
posted by salvia at 11:01 PM on March 16, 2006