I have a set of 1200 historical aerial photos covering North Carolina outer banks area from 1974, 1984 and 1992. I need to georeference them and produce seamless color balanced dataset that will be utilized for compilation of NC outer banks atlas. Except for scale information, flight number and date on photo paper copies, no other information regarding camera positions or ground control points is available. No matter how well I pick control points for image-to-image rectification (I use ArcGIS base map for that), the photos still don't align properly when I display them together in ArcMap. I learnt that analytical aerial triangulation may significantly improve the results, but it seems like it needs GCPs that I don't possess. So, my question is how can I triangulate my photos (or fix my alignment problem in any other way) without GCPs and preferable using ArcGIS, Agisoft Photoscan or other open source software.
In my work I encounter this situation often. The method that I use is next:
- Prepare the images in Photoshop: crop, contrast, brightness, whatever is needed to improved image quality.
- Also in Photoshop use the Photomerge tool from File->Automate->Photomerge. Keep "Blend images" checked. After testing it seems that the "Auto" method yields the best results when it comes to georeferencing. Here is a short explanation on the merging methods. If you are going to use many images at once you might get and error with "scratch disk full"; just add another partition with a lot of free space, because Photoshop is out of TEMP memory.
- Next step is georeferencing; if the place is completely reworked, you'll have a tough time finding common points. If you are lucky and the settlements still exist, then the roof corners, road intersection, any kind of building will help a lot and will make the georeferencing processes easier. Regarding the RMS, I try to keep it under 2 meters overall and the most often used transformation is the 2nd Polynomial. First Polynomial it's just a matter of luck and third Polynomial, when I have a lot of points on which I am confident, but the error is still high and the image does not want to align.
As a rule of thumb I merge images where the area has been almost completely reworked and having to georeference multiple small images is impossible as there are places where nothing is left from 60-70 years ago. Where the landscape remained almost the same I prefer to keep the images simple because on a smaller image, the error will be smaller and if one is careful while doing the georef the overlap will be almost perfect.
Another solution is to use ERDAS IMAGINE and from toolbox use the Autosync Workstation. It's a nice tool, the only drawback is that you have to convert to the .IMG file format for the best results.
Now it all depends what kind of work do you intend to further do on the old imagery. I work on UXO(unexploded bombs from WW2) where precision is important, the lower the RMS, the better.
Years ago when I did Photogrammetry you looked for tie points. These were just points that were identified on two or more photos, to build a photo model. We used points that were on sharp features like a roof corner or any sharp edge. Once we had points to hold our model together, we use GCP to tie the model to the earth/map. Basically these were easily identified points that were surveyed or the coordinates were given to us. Usually white X marks for us on the photos.
Now with GPS you can get GCP of any feature, if not pull coordinates off a basemap ortho, understanding the ortho errors will influence your errors. But the georeferencing is the same, find good features on both the base map and the aerial you want to register, You may need many points depending on the scale, camera errors, and elevation. Realize the ortho already has the camera and elevation error adjusted. You need to adjust/warp your images to an ortho to make them look/fit the basemap aerials better. Play with the different type of transformations in ArcMAP and you should end up with a usable product.