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I work with many old civil engineering documents, and occasionally I'll need to georeference one which has coordinates pairs listed at many given points but does not list which coordinate system these coordinate pairs refer to. These are typically scans of the original drawing lacking any metadata except for what has been scanned. I've always used the "brute force" methods as described by brenth here: .shp with unknown coordinate system

My question though, is if I already know the approximate location and can easily find say lat,lon and I also have the original coordinate pairs, is there a tool out there that can eliminate the need for the "brute force" method, and speed things up for me?

Thanks

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I know you've already accepted an answer but I'll ask anyway... are you sure it's in a common georeference? I've seen construction documents that are referenced only to a nearby landmark. In that case it might be best to rubber sheet it if you have know points. –  Sean May 2 '11 at 19:35
    
So far most are in a common coordinate system (although I do have to work off of an old site specific system periodically) This is more of a general question since the coordinates often change between sets of drawings. We do rubber sheet drawings but they need to be in a common coordinate system so that they can be easily shared. –  Paul May 3 '11 at 15:36

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As far as I know, there's no tool that can directly figure out the projection of a given map, although one would be very useful. What I have done in the past is a bit like the brute-force method, but perhaps a bit more refined.

Assuming you know roughly where in the world the map represents, use a site like spatialreference.org (which seems to be down at the moment), or a program like QGIS to find a list of potential projections. I recently had georef a map of a mine in Wyoming somewhere, so I managed to narrow it down to a handful of potentials that had Wyoming in the title, and I noted down the EPSG code.

Then create a text file with the list of known reference points, and use gdaltransform:

gdaltransform -s_srs <your EPSG code or proj4 string here> -t_srs EPSG:4326 < listofcoords.txt

This will display a list of coordinates in lon,lat form. If they are close to what you were expecting, then you have your projection.

Having thought about it, would there be a reasonable number of people looking for a semi-automatic program that does this? In a few weeks, when my time is more my own, I could possibly write a Python script that simply compares actual results with expected results, and produces a list of near matches.

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Thanks for the quick reply MerseyViking (great name by the way). I like the idea of using gdaltransform. Just to clarify, could I put any number of EPSG codes/ proj4 strings in that line of code? Are they comma separated? I'm all for a Python script that accomplishes this if you're willing. –  Paul May 2 '11 at 17:54
    
Thanks :) No, sadly you'll still have to do it one at a time. I've had to do this a few times, and doubtless I'll have to do it again in the future, so there is some motivation for me to have a stab at it. –  MerseyViking May 2 '11 at 18:04

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