Layering maps

[quoddy]

I would like to take 2 maps and line them up with each other.

A detailed topo map or road map as first layer and (cadastral) property lines on a second. The only map I have to begin with is the property lines map which is just black lines on a white background, stored as a pdf file. I can export the single page to tiff or png (200 MB) and want to make background transparent over a more detailed map so I have some concept of roads, rivers, etc. My first attempt comes close but after I identify a common point on both maps and attempt to line them up there is too much misalignment.

I worked with other software in business setting where I choose several common points on both maps and software will morph/contort one image to match the other. Is something like this possible in Photoshop (CS4 Extended)?

Thanks

quoddy

[mistermonday]

If the contents of each map by itself are to scale (their content) but the maps are not the same size, you should be able to align one corner then using Edit>Transform>Scale and holding down the Shift key, you should be able to drag the diagnally opposite corner to make the that layer larger or smaller until it is the same size. As for the overlay, if the top layer is white with black lines, try changing the Blend Mode of that layer to Darken or Multiply. You could also double click the layer to bring up the Layer Style dialog box and use the Blend-If sliders.
Regards, Murray

[quoddy]

Hi Murray

That was my thought too but there are problems with highways and streams not lining up. I'm not a cartographer so I can't explain the declination, assymetry and so on. I know that some maps figure magnetic north differently and others use faulty algorithms for plotting the surface of a sphere to a flat map. But I guess the cause of the problem isn't the issue but instead how to work with it. I'm sure the map with the accuracy issues is the cadastral map of plot descriptions. Fortunately I don't need it to be exact (I'm not an architect building a shopping mall), I merely trying to locate properties.

At a summer's volunteer job years ago at the BLM, they had me stitching maps of various origins together and it dealt with these very issues. The BLM had proprietary hardware and software that would have you choose static locations on each map (mountain peaks, forks in rivers, features on shoreline) and after you had 15 or 20 pair of related points, the software would fuse the two together. I use something like that in photoshop when I create a panorama using photomerge. It also blends colors but the feature I am looking for is more like the transform (skew and warp). I just haven't played enough to know how to do this yet.

quoddy

[4personnen]

I had to do the same with French maps for a friend, and we discovered that, among the position of the north and some others problems likely to find in those issues, the IGN maps (from the French geographical state body) were always very accurate, but the cadastral (is it an English word?) ones quite never...So We made our choice on important things to align and let the others as is...And it was not just transform , we had to do some perspective???!! retouchings too.